Repair, reuse, re-roof

A couple of posts ago we mentioned the restoration of the roof space at The Crown in Helmsley. Our Building Conservation Team have a helpful Advice Note for re-roofing a Listed Building, like The Crown, the crux of which is explained below.

Looking over the roofs at Staithes. Copyright Fridge Productions Ltd.

When re-roofing listed and historic buildings it’s important that the work preserves the character, history and appearance of the roof and therefore the building. The roof structure should generally be repaired rather than replaced, and historic roofing materials should be re-used wherever condition allows, or otherwise replaced on a genuinely “like for like” basis.

In the North York Moors, vernacular building roofs may be constructed from local timber of chestnut, elm or ash, unprocessed, round or waney-edged and sometimes even with the bark still on. On more grander buildings, timbers may be of sawn oak, whereas on “polite” buildings dating from the 18th and 19th centuries roof timbers may be of high quality, planed Baltic pine. On buildings dating from the mid-19th century or later roof timbers are likely to be constructed from regular, machine sawn lengths. Roofs may incorporate timbers that were originally part of an earlier building, or which survive from an earlier period in the building’s history, such as upper crucks cut down from full (medieval) crucks when a cruck building was raised and reconfigured in the 18th century.

This historic truss incorporates a tie beam which has been re-used from an earlier roof structure. Copyright NYMNPA.

This barn retains its original chestnut roof structure which has a character that could not be replicated with new timbers. Copyright NYMNPA.

Whatever the timber in a historic roof, it tells the story of the building, is characteristic of its age and type and it should be preserved during re-roofing works IF the building as a whole is to retain its character.

Many old roof timbers will be curved and distorted; they may have been inserted as green timbers and distorted as they seasoned, or have bowed under the weight of the roof. However there is rarely a need to replace them for that reason. They may hold valuable evidence about the building such as peg holes, graffiti or jointing techniques, and they are irreplaceable because unsawn timbers are no longer readily available.

Severely infested timbers which have lost their structural integrity may require replacement, but this is rare especially in oak or chestnut, and usually it is possible to strengthen the roof without the removal of historic timbers. Where roof timbers have been subject to insect attack they should be treated with a suitable insecticide, but most old timbers will show signs of woodworm holes which may not be active and are unlikely to have penetrated beyond the sapwood and so should not mean the timbers need to be replaced.

An exception in regards to replacement are machine sawn square timbers in regular dimensions which can be replaced on a like for like basis because the style of roof can be replicated, with no intrinsic loss of character.

 In order to repair and strengthen historic roof structures several methods may be used to avoid replacement. For instance, traditional timber repairs which involve splicing new timber into the old where it has decayed can be achieved by means of scarf joints, in which the decayed timber is cut away and formed into a lap joint to connect with a new section of timber. The two timber sections are then pegged or bolted through, thereby restoring the integrity of the timber. Supplementary timbers can be added side by side with the existing timbers where these are undersize or in a weakened condition. Purlins or rafters can be supplemented with new timbers, whilst leaving the original in situ.

New timber spliced into the old timber beam. Copyright NYMNPA.

Metal plating can be used to reinforce joints that have become loose or have failed due to movement of the timber or decay over the years, or to bridge thin, weak, split or cracked lengths of timber. Such plates can be fabricated in mild steel (painted in a red oxide paint to inhibit rust) or stainless steel to suit the dimensions of the timber, and then fitted to the timber using nuts and bolts.

Mild steel plate to strengthen existing rafter. Copyright NYMNPA.

Metal brackets can be fabricated to strengthen supporting features like purlins where tenons (joints) may have failed, and steel “shoes” may be used to extend the base of rafters onto the wall plate where rafter ends have rotted away, or steel angles used to strengthen and stiffen the connection between rafter and wall plate. Where they will be visible in the roof space, these steel features can be attractively designed and painted to ensure a restrained appearance.

Most veteran roofs within the National Park are covered with pantiles or Welsh slates, although there are a small minority of buildings which retain stone slates, Westmorland slates or thatch. Whatever the historic material of the property, care should be taken to achieve a good match in sourcing replacement materials IF the original has perished. Imported slates can rarely match the purple and blue-black colours of Welsh slates, and also age differently and may have a shorter lifespan. Handmade pantiles are significantly different from modern machine made tiles, particularly those varieties which are “interlocking” or given an artificial patination. By contrast, handmade pantiles have a rougher surface which will weather faster and will acquire the patina of the original tiles in time, as well as exhibiting variation in tone and slight inconsistencies in shape and finish which give them a handmade appearance. New handmade tiles are still produced, but reclaimed tiles and slates are also available as a like for like replacement.

Pre-coloured machine made pantiles (left) have an artificial manufactured appearance. Copyright NYMNPA.

Handmade tiles, in contrast to machine made tiles, with colour variation, surface texture and more irregular appearance. Copyright NYMNPA.

Any necessary modern ventilation requirements (Building Regulations) can be achieved discreetly, avoiding vents in the roof slope. It may be possible to used concealed ventilation on the eaves or ridge instead.

If you need further information, sources of materials or any clarification regarding the need for listed building consent or planning permission before re-roofing your listed building, get in touch with our Building Conservation Team.

DON’T FORGET the bats. Bats are legally protected. Some bat species roost in roofs and so any building work which is likely to disturb bats needs to be planned carefully starting with a survey. The Bat Conservation Trust has useful advice available on their website.

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