Avoiding Problems with Flat Roofs

Avoiding Problems with Flat Roofs

If you have a bitumen felted flat roof on any of your properties, it will be a constant drain on resources, typically needing replacement every 7 years, often less in a sunny location. Sunlight bakes the bitumen and drives off volatile oils, causing cracking as it becomes brittle. This is exacerbated by repeated thermal expansion and contraction with the daily cycle. If you have one and can’t afford to replace it immediately, it’s important to maintain the pale coloured gravel on the top surface. This reflects sunlight back and helps to shield the roof from the worst of it’s harmful effects.

In the long run it’s cost-effective to replace it with something more durable. A pitched roof is ideal, with proper timber rafters covered in a microporous sarking (not cheap bituminous felt which deteriorates just like a flat roof), and slate or tiles.

Unfortunately if there is a low window which would be covered, a pitched roof may not be possible. There are many alternatives to bituminous felt, try a google search for flat-roof-replacement. Light-coloured systems have an advantage in reflecting sunlight. Whatever system you choose, it should never, ever be applied over chipboard decking. It swells massively on contact with water and falls apart very quickly. Exterior grade softwood ply is fine. There’s no reason to use hardwood ply, it’s too expensive.

Glass Reinforced Plastic (GRP) offers a fairly durable solution, but suffers problems with osmosis similar to GRP-hulled boats, and can crack if walked upon. Again it should be finished in a pale colour to reflect harmful sunlight back. (There is a movement to paint urban roofs white as a counter to global warming. It reflects heat and light back into space, as well as protecting your roof.)

Wire mesh on my flat roof ready for roofkrete

My personal favourite is the Roofkrete system . I had this fitted 20 years ago to a kitchen extension I built on my own house, when the process was still quite new. Today there’s no visible deterioration, and the company claims it should last the lifetime of the building. They even say you can make a fire on it without serious damage, but I wouldn’t recommend it. The process is similar to that used for concrete boat hulls. A cementatious slurry is trowelled over three layers of galvanised steel mesh, and sets rock hard. It can cover a variety of bizarre shapes as you can see from the website. Like any cementatious product it must be protected from rain and frost when curing. It’s not cheap but as with so many things you get what you pay for.

When any roof is replaced the ceiling underneath should be insulated to modern standards, i.e. 270mm of fibreglass. If your tenants are warm and the bills are reasonable, they’ll probably stay longer. To illustrate, the owner of the house near mine recently replaced his flat roof, but didn’t insulate. When it snowed, the snow melted on it very quickly, but less so in lines over the joists, which provided partial insulation. Normally joists would be considered a cold bridge through insulation, so the roof was suffering from massive heat loss.

Next week I’ll discuss how to repair a leaking flat roof that you can’t afford to replace.

I’m also considering a couple of blogs on why you shouldn’t always trust a RICS survey to spot structural defects, or rely on a building inspector to spot poor workmanship.

Ps my bonus blog on retaining good tradesmen also comes out today.



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