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.
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?
Hmmm.
Is this a wind up?
'flat' rooves are not flat, they already have this slope built into them, and no, it's not hard to do, it's a taper - timber merchants will cut any sized taper you want, up to 6m in length - these are placed one upon each joist to achieve the fall.
I suspect because like my carpenters, the principles of trigonometry escape them. They insisted on laying out the rafters for the 57 degree pitched roof on the ground, and not trusting to sins coses and tans...
Bill: ere, how come this sheet of ply don't fit? Alf: I told you that it doesn't work if it ain't flat. Bill: so why dont we make it a bit bigger? Alf: oh YES, but how MUCH bigger eh? That's rocket science, that is. Trigernomettry. Not carpentry.
Err they do ...
Search for "firring strips"
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.
They usually are sloped. "Firring pieces" are attached to the roof joists to achieve this. The roof drain may be blocked. But flat roofs are bad news, you're right.
IMO, a physical check is so much easier than trying to get the maths right. If I had a pound for every calculation where I try and be clever with the maths, and fail, I could probably afford to emply an emmeritus professor of trig!
There's 2 sorts of 'flat roof':
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.
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
They are, of course, sometimes unavoidable. Properly done, they are not such bad news as most say.
They work OK when maintained. They just don't last forever like tiles/slates do. When they start to leak its often too late. And they always start to leak when the weather makes repair difficult.
Thanks all. The two I am thinking about are an extension roof done around 60 years ago and a garage roof done around 50 years ago. Maybe things were different then, or maybe they were just done badly, but they both pond.
Nothing lasts forever. Pretty well every house round here has had a new roof. Some were slate, some tiles.
While they would have had five new flat roofs.
That's just a poor job. It's not hard to keep the check kerb level and hide the slope of the roof.
MBQ
My flat roof is over 30 years old. The house isn't 150. And had been re-roofed before I bought it - and had it subsequently re-done.
Plenty examples of traditional roofs being badly replaced. So in some ways the same as a flat one - good workmanship and materials matter.
What my builder did last year. But the flat roof built on my dad's extension c1963 was truly flat; with roofing felt hot tarred on and solar chippings it was not very good but probably typical of the time.
+1 same with our garage/car port roof.
MBQ
It would be most informative to see your design whch hides the slope from all sides and keeps the roof line looking slim all round.
Yes you can hide it somewhat, but personally, I consider a step or kerb at the front of a garage looks a ghastly fudge when seen from the side. So often it just shows up flat roofing for what it is, a cheapo job. OTOH you have cut your suit to suit the cloth...
There are plenty of examples where a flat roof is the only option, planning wise.
Of course if it is done as a 'cheap' job the results will be poor. Which is true of everything. Including a conventional roof.
See fig. 3 in
completely.
Where does the step come from?
MBQ
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