Debate persists about best practice when refurbishing metal roofs, touching on whether the overlap should be sealed, says Steve Cookson of HD Sharman Ltd.
06 August 2018 | Steve Cookson
The overlap is the gap at the end lap of roof sheets when providing cut-edge corrosion or full-roof coating treatments.
End laps in metal roofing systems have been sealed for many years, typically with a butyl strip seal, which historically was 9mm wide by 3mm thick.
This increased to 6mm wide by 5mm thick following testing carried out by the Metal Cladding and Roofing Manufacturers Association (MCRMA) and published in its Technical Paper TP16 in February 2004 and updated in April 2015 as Guidance Document GD 19.
The guidance was clear that the end lap should be sealed.
We chose to consult with independent industry expert Barry Jackson, who is widely known and respected in the roofing industry in which he has worked since 1968.
For the past 25 years, Jackson has operated as a consultant, offering roofing and cladding advice to developers, architects, building owners, surveyors, property managers and contractors.
He explains that it is important to ensure that sealants are adequately compressed to exclude water from the lap. The end lap sealing is designed to prevent water from being retained within the lap by capillary action and, therefore, reduces the risk of reverse side corrosion, noting that rainwater can contain pollutants that accelerate this process.
Improvements to pre-painted roofing and cladding products have come about because of advancements in steel substrate protection and multilayer coating systems.
However, if coating edge peel has occurred on older projects, it looks unsightly, causes concerns to building occupiers and should be treated fast.
If the building is out of warranty and treatment is to be carried out, Jackson recommends preventing water from being retained within the end lap by capillary action as it can cause deterioration of the underside of the roof sheet, where the coating is less robust than on the external surface.
The system used to seal the gap must be able to accommodate thermal movement and be applied by trained operatives from an experienced company that will issue an appropriate warranty.
Despite the above best practice to seal the gap, four main arguments persist for not sealing the gap. While they contain elements of truth, they do not represent the best reasoning. This is why we do not think they are valid.
Five reasons to seal the end laps
- It brings the roof back to "as built".
- It complies with British Standards and industry guidance.
- It reduces the potential for reverse side corrosion due to capillary action.
- The underside of the sheet is far more susceptible to corrosion than the upper surface.
- Ensures a full system refurbishment as opposed to just an aesthetic finish.
1. Any residual moisture already within the lap cannot escape
This is true, but even if you didn't seal the lap, the moisture would still be held through capillary action and cause further reverse side corrosion.
This is probably the most important reason to seal the gap and stop further corrosion, but it is heavily reliant on the correct preparation to ensure that the lap is thoroughly dry before sealing.
2. Any internal condensation could gather on the underside of the sheet and run into the lap where it would be held
This is also true. However, the upper end lap seal should have been installed close to the upper end of the lap to exclude water from this source.
3. If the lap is not leaking, why seal it?
Again, this is a logical argument but it's shortsighted, as it doesn't address the long-term greater risk
of reverse-side corrosion.
4. As less material is used, it is more cost-effective not to seal the lap
It is true that not sealing the lap will use less material and will also save on labour giving an initial cost benefit, but applying a full coating system to seal the gap will remove the need for continuing repairs and offers significant cost benefit across the building's life cycle performance.
Steve Cookson is technical director at HD Sharman Ltd