Powys Built Heritage
Powys Built Heritage

Reinforcement

Consolidation or reinforcement of the existing structure, should be achieved using the most conservative approach that is practicable, although limited reconstruction as existing may be unavoidable but should be limited. The survival of vernacular building techniques such as lath-and-plaster and haired plaster etc. is very important, and their restoration should be in compatible materials and finishes.

To top

Timber Restoration

Appropriate restoration to timber frames, roof structure, beams, floor joists and other structural timbers, should be based on a careful and comprehensive survey of the existing structure. In situ reinforced resin repairs to structural timbers are not appropriate unless justified on the grounds of avoiding major disturbance of the historic fabric. In general, timber should be made good by splicing a new section of matching timber to replace a rotten portion.

To top

Dry Rot

Wherever possible Dry rot eradication and timber preservative treatments based on an analysis and specification by an independent consultant, and using non-destructive techniques and non-toxic applications should be used. It should be noted, that experience has shown that the provision of damp-proof courses and membranes in historic buildings has often only diverted damp problems elsewhere. Dry rot is best eradicated by restoring the building envelope to prevent water ingress and the rot then left to die naturally.

To top

Damp-Proofing

Damp-proofing by traditional methods but only where damp is causing structural damage to the building should be considered; damp-eradication measures, such as improved drainage, the introduction of French Ditch, French Drains, Limecrete Floor or the lowering of ground levels, are preferable where practicable. Post-application damp proof coursing or a damp proof membrane often causes further damp problems in historic buildings.

To top

Roofing

Stripping of roof materials should be carried out carefully to ensure that all sound existing slates or tiles remain undamaged so that they can be re-used. All sarking must be vapour permeable. Replacement battens or laths should be pre-treated against fungal attack. They should be fixed to rafters with stainless steel nails; galvanised steel has a shorter life.

Re-slating or re-tiling should be carried out using sound slates or tiles salvaged from the roof, with any deficiencies made up with new or sound second-hand materials. The use of second-hand materials, matching the existing ones in type, size, thickness, colour and texture are eligible. The selection of existing slates or tiles for re-use should be carried out with care to ensure a significant life in relation to any new material to avoid repetition of the same problem in 20 years.

Substitute materials such as artificial slates made of fibre resin, artificial stone, concrete tiles etc. are not appropriate. If the existing slates or tiles are themselves an inappropriate earlier replacement, it may be appropriate to reinstate the original material provided that accurate evidence of the original material is available.

Fixing nails for slates and tiles should preferably be stout copper. Stainless steel nails are a possible alternative but it should be noted that they might be difficult to remove when repairs are necessary. Only nails with large diameter shanks should be considered.

Lead soakers and flashings should be provided at abutments of roofs with gables, chimneystacks, etc., although in some cases other locally traditional details should be retained where they exist.

Where very old lead survives, this should be regarded as a valuable part of the fabric of the building. Making good rather than complete replacement should be considered. New lead work should be specified correctly in respect of sizes of sheet and thickness, falls and details of joints and fixing. The work must be carried out in accordance with the guidelines and recommendations from the Lead Sheet Association. The specification of works should include details of any proposed lead work.

Chimneystacks

These are vulnerable to decay. Where decay is confined to individual stones or bricks these should be cut out and replaced to match. Where a chimneystack, which is important to the design of a building, has earlier been reduced in height it should be reinstated in its original form. If fractured, valuable earlier ceramic pots or fine decorative ones should be restored by ‘stitching’ across the fracture. Demolition and rebuilding stacks, even if using reclaimed materials, is not encouraged. It can only be considered where there is structural failure and should not be done merely to insert a lead tray.

To top

Rainwater goods

Cracked or broken cast iron gutters and down pipes should be replaced in matching material and section, e.g. half round, box or ogee gutters and round or rectangular section down pipes. Substitute materials such as PVC or extruded aluminium are not appropriate. Where the gutter forms part of the architecture, such as part of a cornice detail, any replacement sections must be exact.

Sound existing lengths of cast iron should be re-used after de-rusting. Down pipes should be fixed on spacers far enough from the wall that if a leak occurred water would run down the back of the pipe and not the wall.

To top

Stonework

Stones should only be replaced where they have lost their structural integrity due to deep erosion, or serious fracture or spalling, or where weatherings are no longer performing the function of throwing water off surfaces below. If erosion or spalling is only superficial, it should be accepted and loose water-holding material lightly and carefully brushed off. The redressing of surfaces is not appropriate in most cases.

Where moulded or carved stone needs to be replaced, the details of the original should be matched as exactly as possible. Replacement stones should match the original size, shape, colour, texture, qualities of durability and surface finish e.g.; rubble, tooled for ashlar. It is important to ensure that bed joints are correctly finished.

Ideally, stone should come from the same quarry as the original, provided the durability of the stone currently available is considered to be adequate. If this cannot be matched, geologically compatible stone should be used. Cast stone will seldom be acceptable. Replacement stones should normally be set to the original face line.

Plastic in situ resin-based mortar repairs to brickwork and stonework are not normally acceptable.

To top

Brickwork

As with stonework, damage is most often the result of water penetration, so exposed features such as cornices, string courses, copings and plinth offsets are the most likely to be affected. The number of bricks that are sufficiently decayed to be in need of replacement should be accurately identified. Only bricks that are beyond retention on structural grounds should be renewed. This is particularly the case with early brickwork.

The method of cutting out should cause the minimum of disturbance to the surrounding brick, brick slips should only be used for individual bricks and it should be remembered that there would be an inherent structural weakness. Replacement bricks should replace the existing ones in dimensions, strength and durability, texture of finish and colour. They should be laid in the same bond and width of joint. The appearance of the wall can be seriously impaired by different joint widths for areas of replacement brickwork. In the case of an example of early brickwork that may be kept to the absolute minimum, it may be justifiable to carry out limited special minor repairs (plastic repairs).

Re-pointing of stonework and brickwork

Re-pointing should only be undertaken where mortar has weathered out, leaving open or deeply recessed joints vulnerable to water penetration or where the mortar is very soft or loose. Loose pointing should be carefully raked out manually, using hand tools, NOT mechanically. Comprehensive (total) repointing is not recommended. Repointing should be confined to open joints only.

Examples of Good Pointing

The general principle is that the mortar should be slightly weaker than the stone or brick, therefore lime mortar is preferable. Mortar which is harder, for example cement, will prevent moisture from evaporating out through joints. The strength of the mix also needs to be related to the degree of exposure. Existing mortar can be sent for analysis so as to produce an exact match.

Example of Poor Pointing
Example of Poor Pointing

The mortar should be packed firmly into the joint using a pointing iron after all loose material has been flushed out. The joint should be finished in accordance with the original form, where there is evidence of it. This is particularly applicable where joints are finished with special treatments, e.g.; an incised line in the centre of each joint, ‘beak’ pointing, tuck pointed brickwork, finely finished masonry, ashlar stone, or joint filled with screened lime. For joints, which are not specially treated, a flushed finish, fractionally recessed, is usually appropriate.

Pointing at it worst and poor mortar match

Relatively recent forms of joint finish, such as ‘strap’ or ‘ribbon’ pointing raised above the surface, ‘bucket handle’, ‘weather-struck’, etc, are very unsuitable for historic building work.

To top

Restoration of Terracotta

Terracotta usually performs well if water can be prevented from penetrating. One of the most serious results of water penetration is the rusting of iron or steel members used to fix the terracotta to the underlying structure. Rusted fittings, which are causing structural failure, should be dealt with by carefully removing the terracotta units and de-rusting and treating the fixings. Where replacement is necessary, fixings should be stainless steel or non-ferrous metal. Broken units of terracotta, should be restored and fixed back in place by dowelling and bonding with epoxy resin. Small-scale restoration to individual blocks may be carried out with a special mortar to match the terracotta, using techniques similar to those for plastic repairs to stonework and brickwork and taking care to avoid feather edging.

Colour matching of such mortars should be achieved by choice of aggregate, not the use of pigments, which usually leach out in time.

To top

Ironwork

Both wrought and cast iron possess generally good anti-corrosion properties but where they are likely to be exposed to water and air it is essential that they are properly protected by a traditional 4 coat oil paint system. On no account should rust be over-painted. Rust should be removed by simple scrapers, wire brushes and sand papers, or by a roto-stripper or abrasive wheel, however this is not suitable for detailed areas.

Sandblasting is an effective method of cleaning cast iron, wet blasting, i.e.; grit applied with a high pressured water spray, or the use of a needle gun are preferable methods for in situ cleaning. These abrasive methods are inappropriate for soft wrought irons; flame cleaning followed by the use of a wire brush is the most suitable method.

To top

Joinery

Joinery forming an integral part of the building can include such external features as cupolas, balustrades of roof platforms/balconies, ornamental bargeboards, eaves, cornices, window and surrounds, doors and door cases, porches.

The fundamental principle to adopt in restoring the historic joinery is to replace only what is necessary, using timber of matching species and type of grain. Where new pieces are required they should be carefully jointed, using the same technique as was used for the original. Timber should be fully seasoned.

An important principle is that the moisture content of the timber used in joinery works should match the old. This is necessary to avoid differential movement between old and new and consequently distortion.

It is particularly important when restoring carved or moulded sections of joinery to ensure that the maximum amount of original material is retained. Where replacement is unavoidable, the new work should follow the existing work precisely. A section of the original timber must be kept within the structure for future reference.

Where restoration is carried out to windows, only those sections that are decayed sufficiently seriously should be replaced; wholesale renewal for the sake of convenience or economy should always be avoided. The profiles of decayed sections of glazing bars, etc; which need splicing should be copied exactly and be precisely married into the existing work. When the complete replacement of a badly decayed window is necessary, the existing design should be reproduced exactly.

examples of good scarfing

examples of poor scarfing and replacement

To top

Restoration of Glass

Historically important glazing, whether painted, stained, or plain, is a job for a specialist conservator. Particular attention should be given to the following points of detail:

  • The strength, texture, type and tone of putty used in re-glazing
  • The method of fixing the glass
  • The retention and careful conservation of all historically important windows fittings.
  • The retention and reuse of any original hand made glass.
  • Consideration to the colour of new glass
  • Liaison between glazier and the mason, blacksmith or carpenter, (as appropriate) is extremely important.

To top

Render

Restoration of external render or stucco and limited areas of renewal. There should be a presumption against total or substantial renewal, unless unavoidable. If render has been removed to the detriment of the performance and appearance of the building, its reinstatement may be the most appropriate form. Other works include the restoration of applied details and features, such as cornices, string courses, window architraves, columns, pilasters, ‘rusticated’ rendering etc. These should be restored with care and accuracy to the historic form or profile, and as nearly as possible to the historic composition. Glass Reinforced plastic (GRP) or similar replacement mouldings are not acceptable but certain proprietary in situ resin-based techniques may be appropriate.

To top

Cleaning of stonework / brickwork

The external cleaning of stonework and brickwork is only neccessary where there is such a build-up of dirt, paint or resin coatings on the surface that it must be removed in order to assess the extent of necessary restoration, or where the surface build-up is damaging the fabric of the building through chemical action. Cleaning for cosmetic reasons is not appropriate. Any cleaning must be undertaken to a suitable specification and carried out by specialist conservation contractors.

 

To top

Paint

A lime wash or microsilicate paint finish (e.g. Keim) is suitable for all new render. Brilliant white is a modern colour and is seldom appropriate for historic buildings. Opaque microporous paint finishes are recommended for joinery. Lead paint may be used on grade II buildings, subject to an appropriate licence being obtained. Transparent stains are only appropriate where there is evidence of a previous varnish finish.

To top

Copyright© Powys County Council. Powys County Council is not responsible for the content of external websites.

Click here for terms and conditions.

Archive images are courtesy of the Radnorshire Museum and Mr Pete Jenkins

Case Study ...



Cavendish House, Middleton Street

Cavendish House before and after restoration
Read more