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jim jim is offline
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Default How much harder is it to make a flat roof with a small slope?

On Jul 20, 9:15*pm, John Rumm wrote:
On 20/07/2011 19:49, GB wrote:

I see them all the time - flat roofs that are completely level, and water
pools on the roof just waiting to get into the building. Why don't they get
built with a slight slope, maybe 1-2cms per metre? If that were done, the
worst of the water would run off rather than pooling and the roof would dry
out quickly once the rain stops.


There's 2 sorts of 'flat roof':

1. Completely flat

The roof sides, and the deck itself, are tanked ie completely
waterproofed. The resulting ponding is deliberate part of the design
used to control temperature of the roofing. There may even be an
arrangement to flood the roof in drought conditions.

There some like this in SE London which you can look down on from the
west side of the line into Charing Cross from Tonbridge.


Of course, that's harder to build, but how much harder? Is the main issue
packing out the joists so the roof slopes, or is there more to it than that?


The term "flat roof" really should me a misnomer in the sense that while
they should be flat, they should not be level!

The minimum fall on a roof should be about 1 in 80. Typically achieved
by placing firings (i.e. tapered timbers) on top of the roof joists.



2. Standard 'flat'

These have a slope of at least 1 in 80, but IIRC Ruberoid recommend
(or certainly did 20 years back!) at least 1 in 40.

Apart from having to resist snow, rain & tempest, a flat roof can
become extremely hot in sunny weather with consequent risk of
warping. Another factor is that a really heavy fall of snow really is
*heavy*! My guess is that 1 in 80 is the minimum required when the
roof is new to allow for resulting distortions and avoid ponding after
a few years of the worst our weather can throw at it.

A 1 in 40 (about 2.5deg) deck doesn't sound much, but shows a
pronounced tilt when viewed sideways on. That can be difficult to
make look 'level' if required for aesthetic reasons, for instance on a
double garage roof sloping to the side rather than front to back, when
viewed from the road.

HTH