B2: Trussed rafter roofs
The selection of truss configuration is dependant on a number of factors
* Timber size and strength class
* Roof profile
When measuring the span of a truss, measure between the intersection points of the lower edges of the rafters and ceiling ties.
For a trussed rafter to perform satisfactorily, adequate restraint (usually through the use of bracing, straps and truss clips) and bearing must be provided. Truss designers should check bearing, wall plates and specify its’ minimum strength class; typically C14 or C16.
To allow for tolerances, consider increasing the span by 25mm on either side to allow for adjustment on the wall plate.
Care should be taken not to introduce internal supports for roof trusses where the trusses have not been designed for such supports. In addition where trusses pass over nonload-bearing partitions, a gap (usually about 25mm) should be left between the top of the partition and the underside of
the truss to allow for the thickness of plasterboard and truss deflection.
Design of overall stability
To act as a single structural unit the trussed rafters should be braced in accordance with the recommendations of the truss, roof and building designer.
It is the responsibility of the trussed rafter designer to design the individual roof trusses and if requested the overall roof. The building designer may require additional bracing (for example to stabilise walls) in the roof structure.
Most roof trusses use c24 or tr26 timber. However, other strength classes may be used and the minimum strength class recommended for roof truss design and fabrication is C16.
It is recommended that roof trusses be manufactured under a quality system subject to monitoring by an appropriate third party. the national Standards Authority of ireland (NSAI) operates a roof truss Manufacturers Approval Scheme and maintains a national register of approved manufacturers; this register is available on the NSAI website. There are similar schemes in the UK operated for example by TRADA.
System owners is the term used to describe the companies that provide truss fabricators with the design system, engineering backup, software and punched metal plates used in truss joints.
B 2.2 TRUSSED RAFTER ROOFS - DESIGN INFORMATION
EN14250, IS193 & BS52683
Prior to the introduction of eurocode 5, roof trusses were designed using permissible stress standards; IS 193 (timber trussed rafters for roofs) in ireland or BS 5268: Part 3 (Structural use of timber - code of practice for trussed rafter roofs) in the u.K. in both cases the trusses should have been manufactured to en 14250: timber structures - product requirements for prefabricated structural members assembled with punched metal plate fasteners.
In Ireland IS 193 has been withdrawn and designs should now be carried out to Eurocode 5. In the U.K. designs may be undertaken to eurocode 52 or bS 5268-3 depending on the specification. For designs to Eurocode 5, reference should be made to not only the relevant national Annexes but also any non-contradictory complimentary information (NCCI) documents such as Swift 5 in Ireland and the proposed PD 6693 in the U.K.
For concrete roof tiles IS 193 gave a minimum dead loadings of 0.685kn/m2 for rafters 2(measured on slope) and 0.25kn/m2 for ceiling ties. The appropriate load from a standard 270 litre cistern should be taken as acting on the ceiling tie unless the cistern is larger. However, roof trusses should be designed for the actual weight of the roof covering and ceiling loads; heavier finishes will result in greater loads while lighter finishes may result in roof uplift and stress reversal.
This includes 0.11kn/m2 for the weight of supporting structure.
The building designer should specify the proposed dead load or roof covering, water cistern capacity and geographical location to the truss rafter manufacturer.
The trussed rafter designer should be made aware of the geographical location to enable correct the design to take account of the topography and correct local wind speed.
Wind speed maps are given in en 1991-1-4 for designs to Eurocode 5 and were located in technical guidance document A (Structure) to the 1997 building regulations for designs to permissible stress.
Detail B 2.2.1 Roof covering (33Kb)
Where possible the water cistern should be placed centrally within the roof void and be supported by over a minimum of four trusses. Cistern supports should be provided to transfer the cistern load to node points of the supporting rafters.
Locating tanks in hipped ends should generally be avoided unless special provision is made for support.
Chimney and roof lights
Where possible accommodate chimney, roof windows etc., in the standard spacing between trusses, and provide appropriate framing members.
Careful consideration at design stage should ensure that roof lights, dormer windows, chimneys, etc., should not coincide with the location of hipped or valley rafters and multiple trusses.
200mm. Any metal fixings should be at least 50mm from the flue. Similar requirements apply in the U.K.
It is recommended that cut rafters and framing members are supported by metal shoes rather than by skew nailing.
Refer to Section B6.6, drawings B6.6.5 to B6.6.7.
Detail B 2.2.2/3 Water cistern location/Chimney & roof lights (37Kb)
B 2.3 TRUSSED RAFTER ROOFS - BRACING
Adequate bracing should be used to ensure that the prefabricated trussed roof acts as a single unit. The design of the bracing members is usually the responsibility of the roof designer and/or the truss manufacturer.
The recommended minimum bracing details for the majority of prefabricated trusses spaced up to 600mm are outlined in detail B2.3.1.
Longitudinal bracing (binders):
Located at ceiling level and at internal node points, they add to the overall roof stability and help with the truss erection. They run at right angles to the trusses and should extend the whole length of the roof, finishing tight against the party or gable wall. Longitudinal bracing should be installed at the ceiling tie junctions with the struts and at the rafter apex.
Rafter diagonal bracing:
minimum of four diagonal braces being provided in the roof.
Detail B 2.3.1 Prefabricated roof trusses - minimum bracing (pdf 130Kb)
Detail B 2.3.2 Prefabricated roof trusses - additional bracing (35Kb)
Where it is necessary to brace the tie and strut members of the truss as illustrated by Detail B2.3.2, additional chevron bracing members should be fixed as illustrated by Details B2.3.3 and B2.3.4.
Detail B 2.3.3/4 Internal bracing (26Kb)
Bracing is usually specified by the truss fabricator/designer and relates to the individual trusses and their erection. Swift 5 and BS 5268-3 (as did i.S. 193) specify a minimum amount of bracing. However, the roof designer and building designer may specify additional bracing e.g. to cater for roof diaphragm action and for wall stabilisation.
Bracing timbers should be at least 97x22m, free from major strength reducing defects and fixed with two 75mm galvanised nails 2.65mm diameter at every rafter.
Any laps in bracing members should be carried over at least two trusses, Detail B2.3.5.
Detail B 2.3.5/6 Fixing bracing (45Kb)
B 2.4 TRUSSED RAFTER ROOFS - ANCHORING DOWN
Truss fixing Where prefabricated trusses may be subject to wind uplift they should be anchored to the supporting structure. The preferred method for fixing a truss to a wall plate is to use proprietary truss clips. All the nail holes should be utilised using appropriate nails in accordance with the manufacturer's recommendations (B2.4.1).
An alternative method is to use a twisted strap to provide a positive fixing between masonry and truss.
Skew-nailing should only be considered where the workmanship on site is of a standard high enough to ensure nail plates, joints and timber members will not be damaged by inaccurate positioning or overdriving of the nails. A minimum of two round wire nails of at least 4.5mm diameter and 100mm long should be used to secure each truss to the wall plate, one driven from each side.
Where nailing through the plate cannot be avoided the nails should be driven through the holes in the fasteners (B2.4.2).
Wall plate fixing
Wall plates should be fixed to the supporting structure usually by metal straps (or commonly bolts with
concrete ring beams). when using straps they should be a minimum of 1000mm long, 30x5mm in cross
section, galvanised or stainless steel and located between 1200mm and 2000mm centres. Straps
should be fixed to masonry by means of suitable shot-fired nails, screws or have tail built into the mortar bed joint. A minimum of three fixings should be made to the masonry, at least one of which should be located within 150mm of the bottom of the strap (b2.4.3). wall plates in contact with masonry should receive preservative treatment and it is good practice to place a dpc under the wallplate.
Care should be taken not to fix nails or screws into mortar.
Detail B 2.4.1/2/3 Truss fixing/Truss fixing/Wall plate fixing (44Kb)
B 2.5 Trussed rafter roofs - water cisterns
To reduce the likelihood of local deflection and ceiling cracking, the water tank should be supported on a system of spreader beams and bearers. These provisions are adequate for the majority of standard conditions.
The spreader beams should be located as near to the node points as possible (B2.5.1).
|3 Bed||270 litres||0.27|
|4 Bed||340 litres||0.34|
Water tanks with a nominal capacity of up to 270 litres should be spread over four trusses.
The water tank should be placed centrally within the bay, as illustrated in Detail B2.5.1, with the spreader beams located as close to the node points as possible. Skewnail the secondary bearer to the spreader beam and the primary bearer to the secondary bearer (B2.5.3).
Detail B 2.5.1/2/3 Arrangement/Plan/Location (40Kb)
Minimum size of water support members for 270L water tank
|Limit of span of truss
36 x 100
44 x 175
44 x 100
|11.0||36 x 100||75 x 150||44 x 100|
B 2.6 Trussed rafter roofs - hipped ends
Hipped ends in a prefabricated trussed rafter roof can be formed in two ways:
1. Using site infill timbers to form the hipped ends, see Detail B2.6.1. This method should only be used where the span does not exceed 6m.
2. Using specially designed mono-pitch trusses, see Detail B2.6.3.
For trussed rafters with spans greater than 6m a common form of construction for a hip end comprises of a number of identical flat top hipped trusses, spaced at the same centres as the main trusses, and a multiple girder of the same profile supporting mono-pitch trusses.
The flying rafters on the hip and mono-pitch trusses are usually supplied full length and are cut back on site to ensure that they meet the hip rafter. The hip rafter is notched over the hip girder to provide a support and is taken to the apex of the hip, where it is usually supported on a ledger fixed to the last full profile truss.
The corner areas of the hip are completed by using site cut rafters onto the hip rafter and infill ceiling joists spanning onto the hip girder.
The horizontal chord of the hip trusses requires lateral bracing to the hip girder. Detail 2.6.3 shows an alternative to the standard centres hipped end of detail 2.6.2; all in accordance with specialist manufacturer's instructions.
* two stage hip: for large hips usually in excess of 11m span
* standard setback hip: similar standard centres hip with girder position flexible
Detail B 2.6.3/4/5 Girder based hip/Two stage hip/Standard setback hip (38Kb)
To cater for outward thrust at the corner under the hip, the wall plates should be half lap jointed and securely nailed.
The corner should then be reinforced by means of an angle tie (Detail B2.6.6) securely nailed to the wall plates and/or a galvanised steel strap can be used to reinforce the corner. (Detail B2.6.7)
A galvanised steel dragon tie may also be used to reinforce the corner junction (Detail B2.6.8).
Detail B 2.6.6/7/8 Corner details (46Kb)
B 2.7 Trussed rafter roofs - spacing conditions
Condition 1: Truss spacings up to 660mm
At openings increasing truss centres by up to 10% (i.e. 660mm approximately for 600mm centres) causes no significant over-stressing of the tiling battens or truss. (See b 2.7.1)
S = standard truss spacing (normally 600mm)
C = increased truss spacing
B = reduced truss spacing
Condition 2: Truss spacing between 660 and 1200mm
This caters for spacing adjustments for openings greater than 10% standard spacing and up to twice standard spacing (i.e. between 660mm and 1200mm). the roof and ceiling should be given extra support provided by infill rafters and ceiling joists (see b2.7.2). Support of the infill timbers is provided by purlins, binders and ridge boards and by trimmers at the actual opening. See details b2.7.4/.5/.6.
The truss designer usually designs the above supports and supplies details for use on site.
Detail B 2.7.1/2 Truss spacing up to 660mm/Truss spacing between 660 & 1200mm (58Kb)
Condition 3:Truss spacing between 1200mm and 1800mm:
Spacing adjustments for openings greater than twice standard truss spacing and up to three times standard truss spacing (i.e. between 1200mm and 1800mm) usually require girder trusses on used either side of the opening. The girder trusses must be fixed together in accordance with the truss designers instructions. Tiling battens and ceiling material should be given extra support provided by infill rafters and ceiling joists.
Support of the infill timber is provided in line with each truss joint by a purlin, binder and ridge board and by trimmers around the opening.
The truss designer usually designs the above supports and supplies details for use on site.
The truss designer can supply special details for opening over 1800mm. Usually these will be similar to those shown in detail B.2.7.3.
Site cut infill support
Where truss centres are increased to accommodate openings, trimming may be unavoidable in some cases. Under these circumstances, full site instruction must be obtained from the truss and/or roof designer. All site work should be supervised by appropriate personnel.
1. For fire safety reasons all timber should be at least 40mm clear of the chimney where the distance to the flue <200mm.
2. Do not nail timbers directly to the chimney.
The trusses should be specially designed and fabricated for such locations and is not adapted from an ordinary truss by site cutting. (See Details B2.7.7 to B2.7.9). (In some circumstances the truss and/or roof designer may permit ordinary trusses to be cut. However, the designer should provide full site details for this work and the work must be properly supervised).
B 2.8 TRUSSED RAFTER ROOFS - LATERAL RESTRAINTS
Note that different details are applicable to timber frame construction where the timber frame manufacturer will provide details for the support of gable walls.
Location of straps
Straps should be provided at rafter and ceiling level, as illustrated by Detail B2.8.1.
Straps should be 30 x 5 mm in cross section,galvanised and carried over a minimum of two trusses. Solid packing and noggins are to be securely fixed with 50mm long wire nails,at least one of which should be in the second joist or rafter. The nail diameter should relate to the size of the nail hole in the strap.
Straps should be located at a maximum of 2m centres.
The requirements for strapping at rafter level depends on the height of the apex and wall thickness, as specified in Technical Guidance Document A of the Building Regulations. In general it is required in all standard pitched roofs in domestic masonry construction.
Detail B 2.8.1 Strap solutions (5Kb)
B 2.9 TRUSSED RAFTER ROOFS - DORMER TRUSSES
The prefabricated dormer truss combines a structural floor and roof in the same component. This type of roof construction generally offers no restriction on the location of ground floor walls, as the trusses span onto the external walls, which in turn offers greater freedom to plan first floor layout.
Technical Guidance Document F (Ventilation) requires that the minimum ceiling height for any habitable room is 2.4m. In an attic truss the minimum ceiling height of 2.4m should be equal to or not less than half of the area of the room measured on a plane 1.5m above finished floor level, i.e. area ABCD to be at least half the area of WXYZ (see Detail B2.9.1).
The application of a few basic principles at design stage can maximise the use of prefabricated components and minimise loose infill timber.
Dormer windows and stairwell openings should have multiple trusses either side with appropriate site cut infill timber between. Stairwells should be parallel to trusses and windows positioned opposite each other (Detail B2.9.2)
Where attic trusses are provided for future roof conversions, girder trusses must be provided the future framing of the stair opening. The future cutting of standard trusses must be avoided at all costs.
At T-junctions provide a corridor link between room areas as in Detail B2.9.3; this will reduce the number of site cut infill members. Use ground floor load-bearing walls to provide additional support to the attic trusses (see Detail B2.9.4).
Detail B 2.9.1 (8Kb)
Where this particular bracing member protrudes into the room area this may be overcome by packing
out the rafter by using a pack the same thickness as the bracing member (detail B 2.9.5).
Bracing details should be provided by the truss and roof designer. The building designer may have additional requirements.
Alternatively, plywood or OSB diaphragms fixed to battens and inserted between the rafters may
be used over the room area of the dormer truss. Sufficient diaphragms should be provided to
allow the line of action of the brace to be continuous (detail B 2.9.6).
Where possible, locate openings on the structural grid (usually 600mm) to match the truss spacing, this will reduce the amount of site cutting (detail B 2.9.7).
the slope of the rafter.
Detail B 2.9.8 illustrates alternative locations for the water cistern in a dormer truss. Where space is limited, two or more tanks in tandem may be required. The roof designer should inform the trussed rafter designer of the number of tanks, their location and capacity. tanks may be supported as illustrated by detail B 2.9.9. A minimum of 25mm should be provided between bearer b and the ceiling, to allow for long term deflection. Bearer C should be clear of the ceiling ties by a minimum of 25mm for the same reason. the cistern support should be designed by the truss or roof designer.
B 2.10 TRUSSED RAFTER ROOFS - VALLEYS
A valley intersection is formed by a series of specially fabricated diminishing trusses collectively called a valley set. The valley set transfers the rafter loads down to the underlying trusses in a uniform manner, by incorporating vertical webs normally at 1200mm centres. Each valley set truss in the set must be secured to each rafter it crosses
Special instructions will need to be given to the manufacturer on the use of intersecting valley infill rafters.
The truss designer should specify site details such as the restraint of the trusses under the valley set. Particular attention should be made to catering for wind forces especially uplift forces.
Over openings, support of the main roof trusses is provided by a proprietary girder truss shoe securely connected to a multiple girder truss (detail b 2.10.3).
Girder trusses must be fixed together in accordance with the truss designers requirements.
Bracing of the valley set is usually provided by horizontal binders fixed to the vertical webs and then fixed back to the main roof (Detail B2.10.2). If the horizontal binders do not coincide with the rafters of the main roof then noggings should be provided to enable adequate connections to be made.
In addition, tiling battens or bracing members can be fixed to the underside of the rafters of the main roof and extend 1200mm beyond the valley line as a means of additional stability to the main roof.
B 2.11 TRUSSED RAFTERS ROOFS - AUXILIARY DETAILS
Gable ladders should be designed for wind uplift forces and may need to be fixed to the external walls.
Separating wall fire protection
To prevent the spread of fire between dwellings, ensure the party wall is completed along the line of the slope of the roof and adequately fire-stopped with asuitable material.
Proprietary fire stopping systems should have appropriate third party certification such as that provided by the Agrément board.
Note: Detail B 2.11.2 is not appropriate for all buildings. Refer to Technical Guidance Document Part B (Fire Safety) of the Building Regulations.
B 2.12 Trussed rafter roofs - site practice
Site storage should be kept to the minimum. Store trusses clear of the ground, on bearers located at the support points, with suitable props provided where required (Detail B2.12.1).
Where trusses are laid flat, bearers at close centres should be provided to ensure level support (Detail B2.12.2). Where bearers are placed at different heights they should be vertically in line with those underneath. The timing of erection of the roof should be such that the trusses are exposed to the elements for the shortest possible time.
Where trusses are covered, the protection should be arranged so that there is adequate ventilation and any accumulated water can drain away.
The greatest stress at truss joints is generally caused by handling. Handling of trusses should be planned to take into account weight, size, access, lift height and whether manual or mechanical handling is required. Where possible trusses should be lifted at node points, with the apex upright (Detail B2.12.3).
10% (i.e. 660mm approximately for 600mm
centres) causes no significant over-stressing
of the tiling battens or truss.
(See b 2.7.1)