Traditional Roofs Repair and Conservation – Historic England Conference

FROM SOURCING SLATE and stone, to preventing lead corrosion, and growing reed for thatch, the Traditional Roofs Conference on 3 March 2020, presented a fascinating range of heritage roofing research and knowledge.

Historic England, the organisers of the event: Repair and conservation: facing the challenge in the 21st Century, updated the 150-strong audience on its research into historic roofs, as well as inviting a range of guest speakers to Church House in Westminster, London.

Metal theft

The Church of England is probably the biggest UK customer of roofing lead. Dr David Knight from the CofE started the conference, speaking about churches’ ongoing battles against metal theft from roofs. This has not just an economic cost, for what are often small congregations trying to raise the cost of replacing lead roofs, but a human and environmental one too.

Responding to the “plague of lead theft”, Dr Dale Dishon explained that Historic England changed its policy on roof coverings in 2017 in Metal Theft from Historic Buildings: “Where risk of lead theft is too high it’s not reasonable to expect like for like replacement.”

Keith Roberts, a chartered engineer and surveyor specialising in roofing, ably weighed up the benefits and disadvantages of replacing lead roofs with terne coated stainless steel.

In the second half of the morning, Terry Hughes made a powerful plea for historic vernacular roofs to be recorded in detail, particularly using photography and video. This should happen before re-roofing starts to ensure regional roofing designs and often skilled roofing solutions are preserved for posterity.

Chris Wood, recent Head of Building Conservation and Research at Historic England, described how historic roofs can often be seen to have performed over decades and centuries, in contradiction of modern thinking. Introducing new elements such as insulation to comply with BS5534 is not always appropriate. New guidance on heritage stone and slate roofing is now being formulated and it’s expected that a Code of Practice will be published in 2021.

Sourcing Slate and Stone

Examining historic roofs, it’s frequently the skill of the craftsmen roofer that is revealed, Richard Jordan said. He spoke of the many historic roofs he has worked on and the variety of materials and methods used. He also set out the challenges involved in sourcing authentic materials for reroofing, which he and daughter, Sara Mae, have faced as many of the quarries supplying regional stones have now disappeared.

Helping to solve the problem of authentic roof replacement, Clara Willett explained how Historic England has been creating a Building Stone Database http://mapapps.bgs.ac.uk/buildingStone/BuildingStone.html mapping buildings and the quarries that provided the vernacular materials to construct roofs across England. Now finishing the Greater London region, the database will be completed this month allowing identification of likely sites to excavate new stone.

Thatch materials and fire

Alison Henry, Historic England’s Head of Building Conservation, presented the findings of research into which varieties of thatching straw are the most productive and effective, trialling different husbandry regimes when growing the splendidly named traditional wheats: New Harvester, Yeoman, Squareheads Mater, Maris Widgeon and Maris Huntsman. Performance is now being monitored on outdoor thatched roofing rigs over 25 years.

Fires in thatched roofs is another area of Historic England research, Alison Henry reported. These are frequently associated with buildings where wood burning stoves have been installed. The study found that thatch roofs are not a fire risk per se, but become vulnerable to convection ignition when chimneys or flues are damaged or blocked.

Lead corrosion

In the afternoon, Chris Wood again took the stage explaining the research that has been done into preventing underside corrosion of roofing lead. Following long-term trials of a various solutions, the study concludes that a chalk emulsion is most effective. A recommendation to apply chalk coating is now to be included in the LCA Practical Guide to Lead (replacing the LSA Lead Manual), available from the Lead Contractors Association and FTMRC later this year.

Ventilation and condensation

Dr Bill Bordass spoke of the buffering effect many old building materials achieve by being slightly air permeable, allowing ventilation of roofspaces and preventing condensation build up. Studying humidity levels and air pressure inside and outside of the roofspace, Bill said that transfusive construction and the ‘sponge effect’ can actually improve building performance, but “there are no hard and fast rules when it comes to old roofs”.

Dr Brian Ridout’s presentation confirmed this view, in his talk on preventing roof timber decay. He spoke of the apparently decaying timber sometimes seen in old roofs that has nevertheless survived for centuries. However, he cautioned that the introduction of moisture, whether through roof leaks or condensation can be a catalyst in changing roof timber conditions to trigger problems.

The prospect from Church House looking across Dean’s Yard to Westminster Abbey.
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