B6: Timber frame
B 6.1 TIMBER FRAME - COMPONENT ELEMENTS
External wall - platform construction
Details of a typical platform method of timber frame construction are shown below in detail 6.1.1.
Sheathing, insulation, cladding, etc., and outer masonry leaf omitted for clarity.
Detail 6.1.1 Timber frame component elements (57Kb)
B 6.2 TIMBER FRAME - ACCURATE SETTING OUT
Accurate setting out of the sub-structure on which the timber frame sits is vital to ensure that the wall panels bears properly on the sub-structure.
It is important that panels are squared and plumbed. panels should not be out of plumb more than 10mm in any floor and not by more than 10mm over the full building height (these are maximum figures, the aim should be to have effectively a tolerance of 0mm). if the panels are out of plumb by more than 10mm the timber frame manufacturer should be contacted for advice.
If wall cavities run off (whether from poor setting out or panels being out of plumb) this can affect the appearance of the building, the ability of the cavity to provide an adequate barrier against wind driven rain and also affect the performance of cavity barriers and wall ties.
Wall ties and cavity barriers are readily available for cavity widths of 50, 75 and 100mm, the usual cavity barriers and wall ties delivered to site are normally suitable for 50mm cavities. most wall ties, anchor straps, cavity barriers and fire stops have a range of effective cavity widths; where cavities are outside the normal 50mm then contact the timber frame manufacturer for advice and if needed alternative products can then be supplied.
Sole plate (when used)
Where no separate soleplates are used, the bottom rail of the panel will act as the soleplate. Ievel the sole plate before fixing wall frames; if the sole plate is not level it may be supported on a mortar bed (and slate if necessary). The mortar bed should be not more than 10mm thick and should extend the full width of the sole plate, where this is not the case the timber frame manufacturer should be contacted for advice.
The packing should be uniform under the sole plate rather than intermittent; it is important that any gaps in the external wall and in particular the party wall be sealed. there should be no need for packing at first floor or roof level if the base is properly levelled.
The fixing of the soleplate to the rising walls or substructure should be specified by the design engineer in the Site fixing Schedule (supplied by the timber frame manufacturer). The type and thickness of packing can affect the performance of the fixings and it is recommended that the timber frame manufacturer be consulted where the packing is in excess of 10mm.
Detail B 6.2.1 Set out/Detail B 6.2.2 Level sole plate/Detail B 6.2.3 Overhangs (41Kb)
Care should be taken to ensure the level of the concrete slab as any rise in the concrete slab may result in nonload bearing walls becoming load bearing.
The sub-structure and concrete slab should be level to within +/- 6mm and a good builder should aim for half this value.
Sole plate overhangs
The sole plate should not overhang the sub-structure on which it sits by more than 12mm. On the cavity side, where ledges cannot be avoided they should be protected by the breather membrane; the membrane should extend below the DPC by at least 50mm.
Where the soleplate bearing on the sub-structure varies by more than 12mm the timber frame manufacturer should be contacted.
B 6.3 TIMBER FRAME - INSULATION DETAILS
Therm al insulation
The most common type of thermal insulation used in timber frame external walls is glass fibre. The glass fibre comes in rolls and is often compressed in the roll; care should be taken that the insulation is `fluffed out' to its proper thickness before it is used. The rolled type must be supported and is normally stapled to the studs to hold it in place and to prevent it from sagging. Care should be taken to ensure that there are no gaps in the insulation (to avoid cold bridging) and if the insulation is fitted in small sections that each section is supported. it is recommended that each space between the studs is filled with a single piece.
Semi-rigid insulation may be tightly wedged between studs (filling any gaps with foam). In floor construction, quilt insulation may be supported by netting and semi-rigid insulation supported by battens. The insulation should fit neatly between the joists without leaving any gaps.
Detail B 6.3.1 (36Kb)
B 6.4 TIMBER FRAME - FOUNDATIONS/SUBSTRUCTURE
The most common type of foundation is the continuous concrete strip formed centrally under load-bearing and external walls depth, thickness and width of all foundations are dependent on the nature of the ground that the foundation bears on.
The sole (or the panel bottom rail) can be fixed to the rising wall by means of stainless steel fixing clips located at 1200mm centres; the clips should also be fixed in place with stainless steel nails. The function of the sole plate is to:
* Provide an accurate positioned base on which the timber frame and any timber suspended ground floor are can then be fixed.
* Provide a nailing plate for the timber frame wall panels or ground floor joists.
* Secure and protect the DPC, DPM and/or radon barrier.
The soleplate may be fixed to the rising wall with appropriate nailing. The fixing of the sole plate (whether stainless steel nails or fixing clips) should be in accordance with the Site Fixing Schedule supplied by the timber frame manufacturer.
Sole plates should be pressure treated with preservatives. See Designing for Durability in Section A4.
The cavity void in timber frame construction must be ventilated. This can be achieved by providing proprietary ventilators at 1500mm horizontal centres below DPC level in the external masonry leaf. These ventilators must be kept free of debris as should the wall cavity in general. It is also usual to provide vents at eaves level.
Detail B 6.4.1 Foundations/Detail B 6.4.2 Rising walls/Detail B 6.4.3 Sole plate fixing (54Kb)
Detail B 6.4.4 Rising walls/Detail B 6.4.5 Floor types (40Kb)
Details B 6.4.6 Holding down strap (10Kb)
The cavity void in timber frame construction must be drained and ventilated. This can be achieved by providing the equivalent of 1 open brick perpend (about 650mm2 normally by the use of proprietary ventilators providing both functions) at 1500mm horizontal centres below dpc level in the external masonry leaf. These ventilators must be kept free of debris such as mortar droppings as should the wall cavity in general. It is also usual to provide wall vents at eaves level and if specified by the building designer on either of a cavity barrier at compartment floor level. The centres of the vents can be in proportion to the area of the vents but in general weep holes should not exceed 1500mm.
Timber-frame dwellings can be built with all the common types of ground floor.
The selection of appropriate floor type will depend on a number of factors including:
* End user requirements.
* Site conditions.
* Insulation standards.
* Contractor's preference.
B 6.5 TIMBER FRAME - EXTERNAL WALL COMPONENTS
The external walls consists of three elements, the load bearing timber frame inner leaf, the ventilated cavity and the external non-load bearing masonry leaf. As an alternative to masonry cladding appropriate timber cladding fixed to vertical battens, or render on stainless steel mesh , can be used, (see B8.1 to 8.4 cladding). Any proprietary rain screen should have been assessed by an appropriate body such as the Agrément Board.
The function of the stud framework is to:
* act as the vertical load bearing skeleton of the external wall.
* resist lateral wind loads.
* provide a framework for the fixing of sheathing, internal linings, etc.
Vertical loads on the wall panel such as those from floor joists or roof trusses should occur over the studs or be offset from the centre-line of the stud by no more than the stud thickness; Where walls are made up of a series of smaller a head binder should be used to connect the panels together.; head binders should generally be positioned and fixed on site. A top/head rail and head binder allows generally loads from floor joists, rafters etc., to be borne between studs. Heavier loads from trimmers, beams etc., may require additional support.
Generally are not required for structural purposes. Where they are used their function is to:
* provide support for partitions and/or plasterboard sheet edges.
* provide support for fixings and fittings.
* resist buckling of studs.
* support joints in external sheathing.
The function of sheathing is to :
* provide the necessary stiffness to resist lateral loads.
* resist wind penetration of the structure.
* enclose and support wall insulation.
* provide a solid background onto which the breather membrane is fixed.
Appropriate sheathing materials include sheathing grade plywood and oriented strand board (OSB3). Sheathing materials should have an appropriate certificate of approval from a body such as the Agrément Board or be included in BS 5268-2; other methods of proving their suitability such as CE marking may be acceptable.
Detail B 6.5.1 Wall panel components/Detail B 6.5.2 Noggings (44Kb)
Improved `U' values may be obtained by using a more efficient thermal insulation as well as by increasing the stud depth.
By fitting a vapour check between the internal wall and warm side of the insulation the amount of water vapour passing through the wall and the likelihood of condensation occurring in the timber frame structure will be reduced.
Suitable vapour checks include 500-gauge virgin polythene. The use of vapour check plasterboard is not recommended unless a condensation risk analysis is carried out.
The external face of the sheathing material must be covered with a breather membrane. Its function is to protect the frame until the cladding is complete and to provide a second line of defence against wind driven rain or moisture which may penetrate the outer cladding. The breather membrane must be waterproof but permeable to allow water vapour passing through the inner leaf to enter the ventilated cavity. The breather membrane should comply with BS 4016.
The stud locations are usually identifiable on the outer surface by a tape of a different colour to that of the breather membrane to assist in fixing wall ties to studs.
Proprietary stainless steel wall ties should be nailed to the timber frame at stud locations. Wall ties should not be fixed to the sheathing material only. The appropriate wall ties and fixings are usually supplied by the timber frame manufacturer.
Wall ties are generally spaced at 450mm vertical centres, horizontally at stud centers (usually 400 or 600mm), and at 225mm vertical centres around openings and at expansion joints, unless otherwise specified.
Detail B 6.5.3 Plan view of typical wall junction/Detail B 6.5.4 General arrangement (60Kb)
Damp proof course (DPC)
Timber frame panels usually come with a damp proof course fixed to their underside. The laps in the DPC at the panel ends should be released and used to provide a continuous barrier to moisture. The DPM should be lapped with the DPC on the bottom of the sole plate across the full width of the internal wall leaf.
Damp-proof courses should be provided at all external openings and to all timber cavity barriers.
Provide damp-proof course to the bottom, back and sides of all window sills.
Damp-proof courses must be provided to all steel lintels and lintel angles. The breather membrane must be dressed over the damp proof courses provided to the lintel.
The DPC should always be fixed to shed water away from the frame.
Detail B 6.5.6 Jamb detail/Detail B 6.5.7 Cill detail/Detail B 6.5.8 Head detail (48Kb)
Detail B 6.5.9 Damp proof course (61Kb)
B 6.6 TIMBER FRAME - EXTERNAL WALLS
At eaves, the soffit board should not be carried over the top of the masonry leaf. To accommodate natural shrinkage of the timber frame, a gap should be left between the roof timbers and the top of the masonry outer leaf. Typical dimensional requirements for this gap are outlined in Table B6.6.1.
Typical verge detail formed with gable ladder is shown in Detail B6.6.2. To accommodate natural shrinkage of the timber frame, a gap should be left between the roof timbers and the top of the masonry outer leaf and the underside of the gable ladder. Typical dimensional requirements for this gap are outlined in Table B6.6.1.
To accommodate natural shrinkage of the timber frame, provide a gap at the locations indicated, dimensions of which can be taken from Table B6.6.1 below.
|Types of floor construction|
|Suspended timber ground floor when panels are supported on ground floor joists or perimeter joists
||Other ground floor construction
|Allowances for ground floor openings||5 mm||3 mm|
|Allowances for first floor openings||12 mm||9 mm|
|Eaves and verge for a single house||8 mm||6 mm|
|Eaves and verge for a two storey house||15 mm||12 mm|
Detail B 6.6.1 Eaves detail/Detail B 6.6.2 Gable verge/Detail B 6.6.3 Shrinkage locations (45Kb)
The cavity in timber frame construction must be ventilated in order to dissipate any moisture that may enter the cavity. Ventilation is generally provided by proprietary perpend ventilators fitted at 1500mm horizontal centres in the masonry outer leaf in the locations indicated in Detail B6.6.4. Consideration must also be given to the location of cavity barriers as set out in Section B6.12.
Fireplaces and chimneys in external walls
Generally there are two primary methods of constructing chimneys in external walls.
The fireplace and chimney stack can be located on the outside of the timber frame external wall panel. (See Detail B6.6.5) The fireplace is in a preformed aperture in the timber frame wall panel. The structural integrity of the timber frame is maintained and specially trimmed openings in the roof are avoided.
Detail B 6.6.4 Cavity ventilation/Detail B 6.6.5 Fire places & chimneys (37Kb)
The fire place and chimney stack can also be built inside the room usually after the internal linings have been fixed. This type of construction requires that the floor and roof members be trimmed around the chimney stack (see Detail B6.6.6).Care should be taken at roof level when detailing flashings around the chimney to allow for movement of the timber frame. Where metal ties are required between the chimney stack and the floor and the roof timbers, these should be fixed in accordance with the chimney manufacturer's or design engineers' recommendations.
If the wall, floor or roof timber members are less than 200mm from the chimney flue there should be at least 40mm between these members and the chimney. More in formation is given in Technical Guidance Document J.
Detail B 6.6.6 Fire places & chimneys (43Kb)
Detail B 6.6.7 (18Kb)
B 6.7 TIMBER FRAME - SUPPORT TO OPENING LINTELS AND TRIMMER BEAMS
Lintels within timber frame panels
Openings in load-bearing wall panels will include a lintel at the head of the opening to transmit loads to cripple studs on either side of the opening. The number of cripple studs required depends on the size of the opening and the load being carried by the lintel.
- Detail B6.7.1 shows typical load-bearing lintels in an external wall panel
- Detail B6.7.2 shows how heavy point loads from trimmer joists, multiple trusses, beams etc., are transmitted to the foundations by means of additional studs. The number of studs required is determined by calculation
- Detail B6.7.3 shows a typical load-bearing lintel in an external wall panel.
Occasionally steel columns may be required to transmit loads to foundations.
The timber frame panels will arrive on site with the stud supports already in place. Where studs are added on site (through error in manufacture or through mis-place-ment of the panel on site) they must be added in accordance with the timber fame manufacturer's instructions. Generally the sheathing should be fixed to all loose studs. If the sheathing cannot be accessed for nailing then all loose studs
should be fixed together and timber bridging should be inserted between the additional studs and those held in place by the sheathing.
It is always a good idea to walk through a timber frame building and make sure that there is timber in place to transfer loads through floors and into lower wall panels. Multiple studs in an upper wall panel should normally have multiple studs directly below them in the lower wall panel and timber within the floor to transfer loads. A check should also be carried out at the same time to ensure that there is enough timber for plasterboard support and fixing.
B 6.8 TIMBER FRAME - PARTY WALL CONSTRUCTION
Party walls are normally formed by two independent wall frames. Detail B6.8.1 illustrates a typical vertical section through a party wall in a two-storey semi-detached or terraced house. The function of the party wall is to provide an effective barrier against the spread of fire and sound transmission.
* The party wall should be completely imperforate. Gaps, irrespective of size, should be fire stopped.
* The two leaves of the party wall must be unconnected for their full height. Light weight thin metal ties may be used at 1.2m centres (usually as an aid to erection) subject to agreement with the timber frame manufacturer.
* Electrical sockets, switches, services, etc., should not penetrate or be fixed to the party wall linings. Where service have to be on a party wall, the wall should be battened out to form a services' cavity leaving the main fire and sound protection intact.
* Services, including cables, ducting etc., must not be located in the party wall cavity.
* Provide adequate fire stopping at roof level (see Details B6.8.1 and B6.8.3).
* HomeBond do not permit conventional masonry chimneys in timber frame party walls.
* Sound insulation quilt should be fixed to at least one of the party wall frames.
The combined width of the two frames should not be less than 220 mm (i.e. face of stud to face of stud; two 90mm studs plus a 40mm cavity) but a width of 250 mm is recommended for improved sound insulation.
For fire stopping, use wire reinforced rock fibre which must be thicker than the cavity to ensure that it completely seals the space (e.g. 65mm fire stop in a 50mm cavity).
The cavity at the top of the party wall must be closed and the wall/roof junction fire stopped. This is usually achieved by covering the top rail of the party wall spandrel with a 9mm non-combustible building board, which should extend over the width of the party wall.
The good sound performance of timber frame party walls depends on a number of factors:
* The structural isolation the two leafs.
* The use of dense wall linings usually plasterboard in two layers, with a combined minimum thickness of 30mm. In the roof spaces (i.e. between 2 uninhabitable areas) the plasterboard thickness can be reduced to 25mm.
* The use, in at least one leaf, of sound insulation usually glass fibre with a minimum density of 12kg/m3. Thicker and denser insulating material (such as rock fibre) will provide better sound insulation. The insulation must be supported and must be held in place without any gaps.
* The provision of a minimum distance between the inside face of studs of 220mm: However the preferred recommended distance is 250mm.
It is not recommended that ordinary block chimneys be constructed within the party wall. The junction of the chimney with the timber frame is difficult to seal properly for sound and fire requirements. However there are a number of proprietary systems that have Agrément certification and which may be suitable for use in the party wall. There are also prefabricated systems (again a number have Agrément certification) that form a chimney stack on either side of the party wall leaving the party wall intact and some join the chimney stacks together at roof level to form a single stack. The fixing of plasterboard behind these chimneys usually has to be carried out before the chimney is built; at least at ground floor level as the chimney width at ground floor is often greater than the maximum centres for plasterboard fixing (~600mm).
Party walls are required to be braced. This is usually carried out by fixing sheathing (normally plywood or OSB) to the ends of the panels; some manufacturers sheath the panels along the full length of the party wall.
Detail 6.8.1 Party wall construction (62Kb)
Fire stopping at eaves
The void formed by the slope of the rafters and the horizontal soffit to the eaves must be adequately sealed against fire spread at each party wall position.
This is usually achieved by nailing 9mm non-combustible building board to a framing around the eaves projection and filling the void with reinforced rock fibre mineral fibre. This is often referred to as the eaves box. (Detail B6.8.2 and B6.8.3).
Detail B 6.8.2 Fire stopping at party wall/Detail B 6.8.3 Fire stopping at party wall (180Kb)
Services to party wall
Detail B6.8.4 shows an acceptable method for providing services on the party wall. Services must not be built into the party wall.
Detail B 6.8.4 Services (572Kb)
The cavity, between the timber-frame party wall and the external masonry leaf, must be closed with vertical barriers (Detail B6.8.5). In addition to the vertical cavity barriers, a vertical fire stop seals the junction between the two party wall frames. This is usually achieved by using 65mm thick wire-reinforced mineral fibre quilt, stapled or nailed to each of the frames. This vertical fire stop should be carried up to the top rail of the party wall spandrel panel.
Where additional structural bracing is required, this will usually be in the form of sheathing materials. Generally partial sheathing of the party wall frame is sufficient. See Details B6.8.5/B6.8.6.
Sound insulation of party walls uses the structural separation of the two separate timber frame leaves, mass (provided by the plasterboard) and sound-absorbent quilt to achieve sound reduction. (Detail B 6.8.7A). This type of construction provides reasonable resistance to airborne sound.
Where sound resisting floors are required, these may be achieved in a number of ways. Detail B6.8.7B illustrates one such type of construction, a sandwich floor. The mass of the floor and the sound-absorbent blanket reduces airborne sound while the floating layer of the floor serves to reduce the transmission of impact sound. The ceiling should be battened out to form a services cavity; this will protect the integrity of the two layers of plasterboard forming the main barrier to the passage of fire and sound. Detail B6.8.7 should be read in conjunction with Part E of the Technical Guidance Document of the Building Regulations.
Detail B 6.8.5 Fire stopping/Detail B 6.8.6 Structural stability/Detail B 6.8.7 Sound insulation (169Kb)
B 6.9 TIMBER FRAME - INTERNAL WALLS
Internal wall construction is similar to external wall construction, with studs at either 400mm or 600mm centres. Often internal walls will be made with noggings (Detail B6.9.1) at mid-height; these can stiffen the panel for handling, can provide support to plasterboard edges and may be required to resist lateral buckling of the studs. Internal load-bearing partitions are generally prefabricated by the timber-frame manufacturer and should be installed prior to the fixing of the ceiling plasterboard. A damp-proof course should be provided to the underside of all ground floor partitions when sitting on concrete or blockwork.
Internal partitions are usually lined with 12.5mm plasterboard, fixed in accordance with the plasterboard and/or timber frame manufac-turer's recommendations. The following is recommended:
* All plasterboard edges be timber backed.
* Internal and external corner junctions must be arranged to provide support to both lining boards and may require an additional stud for this purpose. Usually about 20mm of timber is needed to ensure adequate timber and plasterboard edge and end fixing distances.
* Where internal wall junctions occur at stud centre-lines, additional studs or battens may be required to support adjoining board edges.
* Where no fire resistance is required, the wall and its' lining can be supported by horizontal noggings set between studs. The spacing of these noggins should be such that the recommended spacing of the plasterboard fixings (depending on the thickness of plasterboard and whether nails or screws are used) is not exceeded.
Detail B 6.9.1 Internal walls/Detail B 6.9.2 Internal wall junctions - plan views (39Kb)
B 6.10 TIMBER FRAME - INTERMEDIATE FLOOR CONSTRUCTION
Timber-frame dwellings can be built with all the common types of ground floor. The selection of appropriate floor type will depend on a number of factors including:
- End user requirements.
- Site conditions.
- Insulation standards.
- Contractor’s preference.
The most common type of intermediate floor in domestic timber-frame construction is the platform floor, so called because it acts as a working platform from which the first floor or upper floor wall panels can be erected. The platform floor is generally factory made as a floor cassette.
Detail B 6.10.2 shows the typical method of supporting a floor beam in a timber frame wall panel, using multiple studs or a post.
At party walls any gaps between the floor panels, irrespective of their size, should be fire stopped. In external walls any gaps between panels could be sealed with a waist band (usually OSB forming a continuous band between the first floor wall panel and the ground floor wall panel). Small gaps in the floor sheathing may require to be sealed. (Detail B6.10.3). The exposed header joist should be provided with preservative treatment.
Detail B 6.10.1 Floor support on wall panel/Detail B 6.10.2 Floor beam support (43Kb)
Detail B 6.10.3 (8Kb)
Support to internal partitions
Where load-bearing partitions occur above the floor, an additional joists (if the partition is parallel to the floor joists) or full depth bridging (if partition is at right angles to floor joists) is are normally required to transfer loads to the partition below (Detail B6.10.4).
Additional joists may also be required to carry non load-bearing partitions which run parallel to the floor joists. Short lengths of non load-bearing partition can usually also be supported on noggings fixed between joists.
Internal non load-bearing partitions at right angles to joist span, can normally be carried by the joists, but the additional load of the partition must be allowed for when calculating joist sizes.
Detail B 6.10.4 Load bearing partitions/Detail B 6.10.5 Internal non-load bearing partitions (54Kb)
B 6.11 TIMBER FRAME - FIXING SEQUENCE
The timber-frame manufacturer/design engineer should ensure that the site-fixing details specific to the project are supplied to the site. Nails used in the external walls should be corrosion resistant, e.g. galvanised, copper or stainless steel. A critical feature of timber-frame construction is the on-site nailing of the various timber components. When site fixing, it is important that the Site Fixing Schedule supplied by the timber frame manufacturer is followed. The recommended minimum site fixing is outlined below. (Subject to the supplier size fixing schedule).
* Sole plate to substructure Sole plate located on rising wall by means of 4mm dia stainless steel masonry nails at 300mm centres, long enough to provide a minimum penetration of 50mm into the rising wall. Alternatively stainless steel fixing clips located at 1200mm centres may be used. The clips should be long enough to provide adequate fixing into the face of the rising wall without damaging the blockwork (Detail B 6.4.3).
* Bottom rail to the sole plate 4mm x 85mm long nails at 300mm centres.
* Stainless steel holding down straps Fixed to the timber frame full length studs through the panel sheathing with the supplied strap manufacturer's stainless steel nails and fixed in accordance with their recommendations: 4 no. nails minimum.
* External panel to external panel 4mm x 85mm nails at 300mm centres, skew nailed and staggered.
* Head binder to wall panel 4mm x 85mm nails at 300mm centres long enough to provide a minimum of 38mm penetration into the panel top rail.
* Header joist to head binder/top rail 4mm x 85mm nails at 300mm centres, skew-nailed.
* Floor joist to header joist 4mm x 85mm nails 2 no. skew-nailed on each face.
* Blocking pieces between floor joists 4mm x 85mm long nails 2 no. each face, skew-nailed.
* Bottom rail to header joist 4mm x 85mm long nails, skew-nailed at 300mm centres.
* Solid bridging or blocking to joists 4mm x 85mm nails 2 no. each end of blocking skew-nailed.
* Herring bone strutting to joists 3.75mm or 4mm x 85mm nails 1 no. each side, skew-nailed.
* Joists lapped over internal partitions 4mm x 85mm long face nailed.
* Trimmers (i.e. two joists nailed together) 4mm at 300mm centres face nailed, long enough to provide 40mm point side penetration into the second joist.
* Flooring material to floor joists 4 no. 4mm x 85mm nails at 300mm centres face nailed and staggered.
* Floor panel to floor panel 3.25mm x 85mm nails at 300mm centres face nailed and staggered.
* Trussed rafter to head binder or top rail over studs Proprietary truss clips fixed in accordance with manufacturer's instructions or 2 no. (1 no. each side) 3.75mm x 85mm long nails, skew-nailed so as not to damage the nail plate.
* Bottom rail of spandrel panel to top rail/head binder of panel below 4mm x 85mm long nails face nailed at 300mm centres.
* Gable ladder to spandrel panel 4mm x 85mm long nails 2 no. each side of gable ladder bridging pieces, skew-nailed.
* Plasterboard fixings to walls Single layer 12.5mm plasterboard laid vertically, 2.65mm x 40mm nails at 150mm centres. All plasterboard edges must be backed by timber. See also B6.12
* Plasterboard fixings to walls All plasterboard edges should be backed by timber. Between habitable rooms:19mm flank type plasterboard laid horizontally, 2.65mm x 50mm at 150mm centres; 12mm plasterboard laid vertically 2.65mm x 65mm long at 150mm centres Between roof spaces: 12.5mm board. 2.65m x 40mm at 150mm centres; 12.5mm board on 12.5mm board 2.65mm x 50mm at 150mm centres. Stagger the vertical plasterboard joints.
Timber splitting can be reduced when hand nailing if the nails are blunted. Care should be taken to ensure that fixings actually go into timber and that they have adequate edge and end distances. Generally skew nailing should be used rather than face nailing (except when fixing plasterboard) especially when fixing panels together; skew nailing will help pull the panels together, however skew nailing will require longer nails. What fixings are available on site should be checked with what's specified in the Site Fixing Schedule and either the schedule should be altered (by the timber frame manufacturer) or the correct nails obtained.
B 6.12 TIMBER FRAME - FIRE SAFETY
Fire safety requirements for timber frame houses are the same as those for all other forms of house construction.
Fire resistance is a major means of specifying the performance of a building element (typically walls, floors and beams) in fire. Fire resistance is usually specified in minutes or hours; elements of a domestic dwelling are usually required to have a fire resistance of 30 minutes except for party walls (and compartment walls) which are required to have a fire resistance of 60 minutes.
Compartment floors (e.g. in an apartment) are required to have a fire resistance of 60 minutes as are any elements supporting the floor (e.g. internal load bearing walls or external walls). Requirements for cavity barriers and fire resistance in Technical Guidance Document B vary with the building purpose group and the building height.
Fire resistance has 3 components: Stability, Integrity and insulation. Stability means that the element will still be in place and able to carry load at the end of the specified period of fire resistance. Integrity refers to the passage of fire and smoke while insulation refers to the temperature rise on the unexposed face of the element.
The fire safety requirements for buildings are largely set out in Technical Guidance Document B. In obtaining a satisfactory degree of fire resistance and safety the following guidelines should be followed.
Detail B6.12.1 summarises the location at which cavity barriers occur in timber frame housing. To meet the recommendations for complying with Technical Guidance Document B, cavity barriers should be provided in timber frame walls as follows:
* around all openings, such as doors, windows, vents, openings for extract fans, meter cupboards etc
* in semi-detached and terraced units, at the junction of party walls and external walls. In the external wall on either side of a party wall.
* at eaves level.
Apart from the cavity barrier practice set out above, the timber frame houses should, as a matter of course, incorporate all other relevant fire safety provisions indicated in Technical Guidance Documents B and J to the Building Regulations particularly in respect of the following:
* means of escape
* Provision of fire alarms
* wall and ceiling linings
* Surface spread of flame
* Fire resistance
* integral garages
* roof covering
* radiation onto boundaries
* roof lights
* heating appliances, hearths, chimneys and flue pipes
* at the top of the external walls
Probably the most important aspect of fire safety is the provision of alarms and adequate escape routes.
Detail B 6.12.1 Cavity barriers and fire stops (23Kb)
Plasterboard (or similar fire resisting lining) provides the main protection to the timber framing in relation to fire and makes the largest contribution to fire resistance. For this reason it is important that the plasterboard is fixed properly particularly at party walls. Screws are generally recognised as a better fixing than nails; the recommendations for fixing screws are outlined below
The following is recommended (subject to the board manufacturer's recommendations or those of the timber frame manufacturer):
* Screw lengths
Single layer 12.5mm and 15mm plasterboard - 36mm
Single layer 19.0mm plank - 42mm
12.5mm plasterboard on 12.5mm board - 50mm
12.5mm plasterboard on 19.0mm plank - 60mm
Each layer must be independently fixed
230mm for ceiling and 300mm for walls, 150mm in racking walls and around floor edges
* Site control
Unless control on site is very good it is recommended that the screw fixing schedule is simplified. No more than two screw length should be used (42m and 60mm) and that all screws are put in at 200mm centres
* Plasterboard joints
Generally all joints and plasterboard edges should be timber backed (19mm plank edges don't need to be timber backed providing both edges are bound edges). This applies especially to floor edges at the external wall, party wall and load bearing walls. In the party walls the 19mm plank should be laid horizontally with its vertical joints staggered if possible but the vertical joints must occur over timber. The 12.5mm plasterboard should be laid vertically; again the vertical edges should be staggered if possible with the vertical joints of the board underneath. The vertical joints of the second 12.5mm board should be staggered with the vertical joints of an underlying 12.5mm board. It is good practice to have bound edge against bound edge; where boards are cut, the cut should always occur over timber.
* 3 Storey dwellings
These are required to have a full 30 minutes fire resistance and smoke doors. The stairwell walls are usually lined on both sides with a high performance lining such as Fireline or similar. In the ceiling to the roof space a similar high performance board is normally used.
* Apartment floors
Apartment floors should be battened out to provide a services cavity. This means that lights and wiring can be placed within the services cavity without compromising the fire and sound performance of the floor. All joints of the plasterboard should be staggered and this requires careful planning of the plasterboard layout and the position of noggins to pick up the second layer of plasterboard.