A15: Restoration and conservation

The restoration of a timber building or element involves returning a heritage object to a known earlier state, without the unnecessary introduction of new materials.

Conservation involves the prevention of decay and the prolonging of the life of the particular timber element. This work should be done without damaging the timber element or the building it is a part of. Historical evidence should not be falsified.

Reconstruction generally involves altering a heritage object by the introduction of new or old materials to produce an end result which respects the original.

Restoration and reconstruction often, by necessity, occur in the conservation of old buildings. Pastiche should be avoided but properly executed replicas, in particular cases, are acceptable.

Where completely new works are required in an otherwise conservation setting, the use of architecturally well designed forms and carefully chosen materials of a contemporary nature is far more appropriate and respectful of an historic setting than bland pastiche.

TIMBER WINDOWS

The most common window up to the early 18th century was the side hung casement. The sliding sash window superseded the casement window and dominated window design for over 200 years. It is important not to confuse Georgian and Victorian sliding sash design.

The typical Georgian timber window was not equally split between top and bottom sash. The convention was to have nine panes of glass over six below, with a matching split pattern for the shutter panels of three over two. The Victorians often replaced the Georgian windows with equally split top and bottom sashes. The Victorians also introduced plate glass in lieu of the original Georgian crown or cylinder sheet glass. In addition, the Victorians `improved' the sash window construction by the addition of window horns (extensions to the bottom of the top sash).

Glazing bars varied in their width and thickness with each architectural period. Early 18th century Georgian window glazing bars were wider and usually heavier than mid to late 18th century glazing bars.

Sashes and window weights

If a new sash is required it should be a faithful historical copy. It is important to match the weight of such replacement windows with the original so that the original window sash cords, pulleys and weights can be reused where possible. The rule of thumb is that the weights for the upper sash are a little heavier than the sash itself so as to maintain it at the top of the frame. The opposite is true of the lower sash where the weights are usually a kilo lighter than the sash so as to ensure that it sits firmly on the sill.

Preventing draughts

A common problem with old sliding sash windows is their rattling under wind pressure and subsequent draughts. Proprietary systems are available to draught strip sliding sashes such that they perform as well as any standard replacement window on the market.

Repairing joints

Wear and tear and the inconsistencies of some old animal based glues can lead to the loosening of timber joints over time. If the timber is sound the joints can be reglued and cramped. Where additional strengthening of the joint is required insert a stainless steel or brass screw from the inside (to avoid external rot problems) through the tenon or dovetail joint, ensuring that the structural integrity is not compromised.

Thermal insulation and avoiding condensation

The use of original shutter panels will give added heat insulation and security to sliding sash windows. All buildings need both heat and ventilation in order to avoid mould growth, decay and condensation. If there are no shutter panels, and after draught stripping it is still necessary to improve the thermal performance of sash windows, then secondary glazing is preferable to altering the glazing bars of the original window design. The divisions of the new internal secondary glazing should be aligned with the meeting rail and/or the glazing bars of the original window. To reduce the effect of a double image reflection, the glazing bar members of the secondary window should be painted a dark colour.

Maintenance

Windows need maintenance inspection and repainting every three to five years. When painting, the elements of the window should be painted in a sequence to avoid the sashes sticking.

A common problem with old windows is the cracking of the linseed oil putty used to glaze the panes of glass. The result is often water ingress at vulnerable corners. There are a number of modern glazing mastics which can be overpainted to match the original putty look and which out-perform putty in terms of lifecycle and protection of the timber glazing bars. Special mastics (to specialist order) can also be got for larger projects with colours to match traditional paint finishes. While some conservation purist might recoil at such a suggestion it is far more preferable to use such compatible special mastics than allow the proven poor track record of putty to undermine historical windows.

Replacement

If, as a last resort, the original windows cannot be conserved then historically accurate replacement windows should be made. Sliding sash windows are not any more difficult to make than other common window types. Replacement in PVC or aluminium can in no way accurately replace historical timber ones. PVC windows have a life span of 30 to 40 years with repolishing required after 15 - 20 years.

They have thermal expansion and window ironmongery replacement problems. Plastic windows are environmentally unsupportable due to the resources and energy required in their making and their inability to be recycled.

Ireland has many good joinery workshops which can accurately make historically accurate replacement windows under specialist architectural guidance. The choice of correct species and method of preservation would be similar to that of modern windows detailed elsewhere in this specifier's guide.

INTERIOR JOINERY AND FITTINGS

Like timber windows, interior joinery such as staircases, doors, (including ironmongery), architraves, panelling and mouldings are part and parcel of the architectural heritage of an old building. They should not be replaced by either inappropriate modern or pastiche joinery. Where heritage joinery is beyond repair or a significant new addition is required, then a contemporary design of a high standard could be the most appropriate approach. Remember that historical joinery and panelling was always painted except for special feature oak and mahogany handrails, doors and panelling.

STAIRCASES

The medieval spiral stairs developed into more open and feature type staircases in the 18th and 19th century. The urban Georgian houses used the dog leg staircase in a rectangular or elliptical manner with a series of half and quarter landings. Most of these staircases were made of pitch-pine and were always painted. In prestige houses the stair handrail would often be made of mahogany or oak and polished in beeswax. Balusters became more slender and refined during the 18th century. Today such balusters are visually elegant but may require careful strengthening in public use buildings to comply with current building regulations. Seek expert advice if such interventions are required.

DOORS AND ARCHITRAVES

The simple ledged door was used in vernacular buildings into the 20th century. The late 17th century saw the widespread introduction of panelled doors in wealthier houses throughout Ireland. Panels were either recessed or raised and fielded with a chamfered edge.

Flat panelling became more common as the 18th century progressed until the application of decorative head moulding became the norm from the middle of the 19th century onwards. Georgian architraves, panelling and mouldings are in general visually lighter than their Victorian counterparts.

Historical ironmongery such as locks, hinges, number plates, door knobs and bell pulls should be preserved. There are a number of specialist brass and metalworkers who can repair and refurbish such ironmongery.

PANELLING

The use of timber wall panelling in public buildings and larger houses became popular at the end of the 17th century. Unlike earlier medieval panelling which was not proportioned in any particular way, 17th century panelling was governed by the rules of classical proportion. The panelling itself was usually of softwood, painted in the popular colours of the time. On occasions oak or mahogany panels finished in beeswax, was used to denote rooms of special importance. On other occasions painted softwood skirtings, panelling and dado rails formed the lower part of the wall with decorative plaster panels above. From 1880 onwards the dado rail became less prominent while the skirting board grew taller and more prominent.

TIMBER FITTINGS

Timber mouldings in Georgian and Victorian buildings can be extensive and appear intricate. However, all mouldings are based on two historic and simple forms - the convex quadrant called the ovolo (cavetto when concave), and the right angled, flat faced fillet. Most 18th century mouldings were based on Roman examples while most 19th century mouldings were based on Grecian examples.

Do not use standard hardware shop mouldings in restoration work. Remember that mouldings and panelling were directly proportioned to the proportions of the particular room or space in which they were situated. A good joiner can repair or match damaged mouldings or panels under architectural guidance.

PAINTING

When stripping old timber do not use blowtorches or hot-air guns because of the damage they may cause to the original wood and the risk of fire they pose in historic buildings. The original colour of historical timber work can be determined by chemical and/or spectrometer analysis by a paint specialist. A number of matching historical paints are available from specialist suppliers in Ireland and the U.K.

FLOORS

Most timber floors in heritage buildings would have been made up of square edged floor-boards with a painted finish. Tongue and groove floor-boards are a relatively new floor type. In prestige buildings or particular rooms, oak, mahogany or special inlay floors were laid. Great care is required in repairing an inlay floor. Rare species can be identified by a wood anatomist from a small sample.

TIMBER DECAY

Timber is an organic material. As such it is subject to degrade through wet or dry rot and insect attack unless specially detailed or preserved by various methods. In historic buildings, the basic building technology used in their construction was often not conducive to the well-being of timber. In particular, bonding timbers were used in masonry wall construction where dampness was present due to the absence of a damp proof course and porous wall construction.

The first priority in any heritage building is to eliminate all sources of water penetration. When the basic fabric of the building is secure from water ingress the conservation work can begin on the other elements of the building including the timber components.

WET ROT AND DRY ROT

The fungi that cause wet and dry rot are not active below 20% moisture content. If the timber moisture content can be held below 20% there is no need for the saturation use of preservatives particularly in the case of dry rot. However, if there is a significant damp or insect hazard beyond control then an appropriate Hazard Class treatment should be used as detailed elsewhere in this guide.

Wet rot is a term for a group of fungi which is usually found where timbers are in ground contact or subject to constant wetting. Affected timbers will need to be removed and the source of damp eliminated.

Dry rot is the most serious of wood rots, but should not be a cause of panic preservative treatment using saturation irrigation systems unless it is very extensive and aggressively active. Dry rot needs moisture to survive. It is still widely believed by many specifiers that dry rot can obtain moisture from adjacent timber below 20% moisture content and thereby continue to spread throughout a building.

Recent scientific research shows that such a belief is incorrect. Dry rot is normally restricted to areas of timber and masonry which are subject to periodic but severe wetting. However, in areas of restricted ventilation the decay can spread to adjacent areas of drier timber. It is true that dry rot spores can lie dormant for a number of years. However, these spores will only become active above 20% moisture content. If the timber is kept dry the dry rot will die.

What is most important is the need to discover the location and cause of the dry rot outbreak and its present level of activity. Then eliminate all sources of moisture. Use dehumidifiers to reduce the relative humidity and thereby help dry the timber to 16% moisture content or below. Affected timbers should be removed to beyond 300mm past the last signs of decay. Replacement timbers should be pressure preservative treated to the required Hazard Class treatment. Non-decayed timber adjacent to affected wood should have a preservative paste or injected treatment carried out. Similarly, adjacent wall surfaces should be spray treated to kill spores.
For heritage buildings consideration should be given to a remote monitoring system which can detect moisture content levels in hidden areas and thereby indicate if there are conditions conducive to the growth of dry rot.

INSECTS

The most common type of wood damaging insect is the furniture beetle or woodworm. It is now less common inside centrally heated houses, preferring wood at a higher moisture content. The young grub (larva) remains in the affected wood for two to five years before it emerges from its flight hole during the period May to August. Severely damaged wood must be replaced with preservative treated timber. The preservative must contain an appropriate insecticide, and not just a fungicide, treated in accordance with its hazard class requirements.Where floor joists or boards are attacked they should be spray treated on all surfaces wherever possible.

Death watch beetle is seldom encountered in Ireland today but it has been found in old oak timber in some heritage buildings. The grub can stay in the wood for up to five years before taking flight in spring time. These insects require the wood to be damp and adequate moisture is necessary for infestation. Attack is frequently initiated in decaying timber, so the drying out of the timber will do much to eradicate death watch beetle attack.

Powderpost beetle is occasionally encountered in the sapwood of recently installed hardwood species with large pores, principally ash and oak. It cannot attack the sapwood of these species or of softwood species. Attacks normally peter out within a few years even if untreated and any occurrence in older buildings is likely to be extinct.

Ambrosia beetle or "Pinhole borer" attack, characterised by dark-stained holes of varying sizes, occurs only in freshly felled or "green" timber. This attack cannot continue once the timber is dried. It is normally of no structural significance.

FIRE SAFETY REGULATIONS

If the public have access to a heritage property then the owner or `person in control" is responsible for their safety in accordance with Section 18(2) of the Fire Services Act, 1981. In addition, the Building Control Regulations may apply and a Fire Safety Certificate be required. There can be a conflict of heritage interests and fire safety requirements. For example, the upgrading of the fire resistance of existing floors could threaten decorative ceilings below the floor. The sub-dividing of halls and staircases to restrict smoke movement can destroy the architectural integrity of these spaces. Similarly, original timber wall panelling or panel doors may not meet surface spread of flame or modern fire rating requirements, respectively. To maintain heritage features and at the same time comply with the Building Regulations fire requirements expert advice is needed. There is an element of flexibility in the Building Regulations' which allows the Fire Safety Certificate applicant to demonstrate that there are alternative solutions to the standard Regulations. The onus is on the applicant to clearly demonstrate from first principles that any proposed alternative to the Regulations will provide an equivalent level of fire safety.