Standing water on flat roof

razman · 11 May 2015

I recently had a 6m extension built with a flat roof which has a hidden drain (gully within the roof). Without going into details the builder totally messed it up and now I have standing water on the roof when it rains near the edge of the gully.

It does eventually drain off but takes a long time and I'm concerned that this will start causing leaks in the future.

What can I do to fix this without redoing the roof? Someone suggested spreading stones (6mm limestone chippings) across the whole surface which should help disperse the water. Any ideas?

ree · 11 May 2015

Why not post pics of the roof?

Dont spread chippings.

How many square metres in the roof?

How many gullies in total?

Any guttering?

Does the ponding actually drain off eventually, or does it evaporate off?

^woody^ · 11 May 2015

Yes avoid chippings.

You could build up the area with layers of felt, or you could just leave it

cotswoldbuilders · 11 May 2015

You state, the water lays at the edge of outlet.

Common problem, the outlet hasn't been recessed into the decking board.

razman · 12 May 2015

Extension Roof

razman
12 May 2015
1

Untitled

razman
12 May 2015
1

OK attached some pics I had on my phone. Looks like the gully edge was refelted at some point which has caused a lip preventing the water from flowing down.

Roof size is approx 50m2 but we have 2 big skylights so felted area is approx 45m2.

One gully which flows to a down pipe on the front of the extension.

I think the ponding evaporates and does not drain off.

ree · 12 May 2015

You "recently had a 6m extension built with a flat roof" but you now say that: "Looks like the gulley edge was refelted at some point"? Bit of confusion here?

I assume that the gulley is a square gulley roughly centred in pic 2?

Given all the above details then i think that you are going to have to live with it.
Maybe others, more experienced than me, can come up with a solution?

No experienced flat roofer would have built that roof that way - but you probably already know this.

FWIW: in modern residential flat roofing design, you never if possible design in chutes or internal drainage gullies - you most always fall the whole roof to guttering.

razman · 12 May 2015

Ok to give some more details. The extension was designed with a canopy and end to end 9m glass sliding doors. I did not want to have plastic guttering on the face of the canopy ruining the look and asked for a hidden drain.

However, AFTER the builder had done the gully he told me they had originally built the roof to have standard guttering and then changed it. He blamed the architect.

Soon after water started leaking into the canopy and dripping through the lights. At which point they refelted part of the roof.

No leak now just ponding. Builder claimed they used very high quality felt and shouldn't be a problem.....

I've seen on some websites, where the chippings are bonded with bitumen so are not loose. Is there any benefit in doing this or should I just leave it and stop worrying about it.

ree · 12 May 2015

Live with it.

Alastairreid · 17 May 2015

A little standing water at the verge is common and will do no harm.

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Standing water on flat roof

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