‘A Stitch in Time – Guidance Notes in Maintaining your Place of Worship’
Guidance Note 2 – Flashings
Flashings are used to seal junctions between different components in buildings.
On slate and tiled roofs lead was traditionally used. It is long-lasting (over 150 years if detailed correctly) but expensive to repair or replace. Lead has now been largely superseded by galvanised sheet steel for renewing flashings but it is susceptible to rust and requires to be replaced within 50 years of installation.
Lead flashing detail to abutment wall.
Flashings are often fitted under roof coverings which need to be lifted to allow new flashings to be installed. Where a good quality slate has been used with a poor-quality flashing this means stripping and refixing slates, an expensive exercise when flashings wear out first.
Slate roofing finishing against a raking abutment wall. When the roof was re-slated no abutment flashing was installed leaving this SA church roof open to water ingress.
Lead or copper are still the best materials for use as flashings with slate and tile roofs as they will last at least as long as the roof finishes. Flashings are essential to sealing roofs and failure to properly specify and/or fix flashings can greatly accelerate the deterioration of the fabric of a church.
Rusting galvanised valley flashing in slate roof
Because flashings are largely concealed under the roof coverings, and are often at high level, it is very difficult to properly inspect them; damage and deterioration can remain undetected for years. In extreme cases poor, past workmanship when repairing and/ or roof finishes results in failure to fit flashings leaving the fabric vulnerable to water ingress.
Where a lead abutment flashing is installed it is worn out and torn, once again leaving the fabric beneath vulnerable to water ingress. Damage like this is almost impossible to spot from ground level.
Care needs to be taken in the choice of materials used for flashings, especially where roofs are finished with sheet metal. Metals need to be compatible otherwise rapid corrosion can result. If, for example, a zincalume ridge flashing was used with a galvanised (zinc coated) iron roof the roof sheeting will rust more quickly, where a pure zinc flashing would be compatible/ non-corrosive. Where water from a galvanised iron roof runs into zincalume gutters and downpipes no corrosive action results.
It is essential that flashings be in good condition. Damaged or missing flashings leave the fabric beneath vulnerable to the ingress of water. It is as important to inspect the flashings as it is to inspect the roof finish.
When inspecting the condition of historic church roofs it is easy to overlook the flashings, even though they are often installed at the places on a roof subject to the heaviest flow of water.
It takes experience and knowledge to understand what to look for when assessing whether flashings are fit for purpose. This guidance note provides advice on some points to look out for.
If you are in any doubt about the condition of your flashings, or of your church roof in general, contact a well-respected heritage architect who has experience in dealing with church buildings to advise on a suitable course of action.