Repairs: roofs and rainwater goods

Problem 13: Ponding on a flat roof

Initial checks should verify:

  • Has the deck beneath sunk?
  • Was it even laid to a meaningful drainage fall in the first place?
  • Are the outlets or secret gutter/downpipes well detailed?

Occasionally roofs have been re-covered when the felt was sound but the outlet pipes were blocked and water consequently backed up under the felt. The author has even seen schemes where the outlets and downpipes do not align, causing water to shed into the building.

Why check the roofing deck? Well, a variety of interesting materials have been used over the last 40 years, and some perform very indifferently.

Cement/strawboard or chipboard has all the qualities of soggy paper once water has penetrated it. Establish if you are only recovering with a new layer of felt or if one deck has been applied over a perished old one, when a complete re-deck will be required.

Sometimes it is worth taking a core sample through the felt to establish:

  • the type of roofing felt; and
  • whether the roof covering (felt or felt and decking) was applied directly over the top of an old failed covering (or failed deck).

Solution 1 (temporary)

A temporary solution could be to repair a felt flat roof (or asphalt) using a glass-reinforced plastic (GRP) 'pour on' finish to seal the old roof, or by patching the original material over any cracks or leaks.

GRP is reasonably effective – but such treatments are weak at sharp corners, arrises, and the like, and will still be at risk if there is much pedestrian traffic, for example, where a flat roof deck serves as access to flat conversions, or where there are balconies or perhaps unauthorised roof gardens to flats. (Cast iron chair legs soon leave tell-tale marks punching through the deck, despite tenants denying that they barbecue on the roof!)

Solution 2

When re-covering, it makes sense to look at and coordinate options to improve roof insulation.

The traditional fibreglass quilt laid in the roof void formed by the flat roof joists has limited efficiency. An air gap is still required to ventilate the roof, and insulation becomes a soggy mess once water gets in, which it inevitably does.

Before you write the specification, ensure that there is sufficient upstand for the roof covering, and look at improving this within the specification if appropriate. For example, if the roof has a secret gutter and it becomes blocked, then ideally the ponding rainwater should flow through some form of warning gully or similar outlet before it backs up and around the cover flashings and into the building fabric.

  • Ideally any upstands should be a minimum of 150mm with a cover flashing of 100mm or more turned over.

Flat roof under a foot of water

The author recalls lifting the trap onto a 5-storey office block flat roof to discover that it was under about a foot of water (about 300mm). Any higher and the water would have flowed down the access trap or into the plant room lift shaft.

Unbelievably the strawboard roof deck held up under the massive weight imposed on it and the roof covering was fairly sound - a couple of pin prick leaks had raised the alarm early.

The cost of disruption to the business of the roof had emptied itself through the building hardly bears thinking about.

Would the building designer have been liable for the poor original detail? Perhaps not. This design was common when the building was constructed. Or would the tenants have been liable for failure to maintain and inspect regularly?

Roofs are frequently repaired on a like-for-like basis, but it makes sense to improve them if possible.