Too flat a flat roof?

[Mathew Newton]

We have just had the shell of an extension built which includes a flat roof . Firring strips have been used to produce a fall to one side however this fall is around 20mm over the 3000mm span i.e. 1:150.

Everything I am reading states that, as per BS6229, flat roofs should have a minimum fall of 1:80 (i.e. 37.5mm in my case) and really ought to be desi gned for 1:40 (75mm) to allow for inaccuracies in construction, settlement of materials etc.

Unfortunately I have only just spotted this and the roof covering (consisti ng of OSB sheets, vapour barrier, 125mm solid board insulation, ply and a p olyurethane liquid membrane) along with cavity trays and flashing all now i n place.

Now I'm fretting about possible issues in the future. Am I right to be worr ied? Should I be seeking this being altered? I was going to speak to Buildi ng Control tomorrow to say what they say but whilst they might confirm the fall is low I can't see anything in the regs that mandates a fall and I don 't know whether BS6229 is mandatory either (it seems more of a code of prac tice).

Grateful for your thoughts.

P.S. For what it's worth I must admit to having a tendency to worry - to th e point of anxiety - about detail like this.

[tabbypurr]

of. Firring strips have been used to produce a fall to one side however thi s fall is around 20mm over the 3000mm span i.e. 1:150.

e a minimum fall of 1:80 (i.e. 37.5mm in my case) and really ought to be de signed for 1:40 (75mm) to allow for inaccuracies in construction, settlemen t of materials etc.

ting of OSB sheets, vapour barrier, 125mm solid board insulation, ply and a polyurethane liquid membrane) along with cavity trays and flashing all now in place.

rried? Should I be seeking this being altered? I was going to speak to Buil ding Control tomorrow to say what they say but whilst they might confirm th e fall is low I can't see anything in the regs that mandates a fall and I d on't know whether BS6229 is mandatory either (it seems more of a code of pr actice).

the point of anxiety - about detail like this.

IIRC some of the first flat roofs were fully flat with a lip all round. The intention was they held a thin layer of water which somehow protected the felt. Needless to say that method was replaced, the first slight hole and i t all went in there.

I don't see BR helping you.

NT

[Mathew Newton]

Even if they were to say 'theres nothing explicit in the regs but if I were you I wouldnt lose sleep over it' that in itself would actually be of considerable help!

[Brian Gaff]

Yes my neighbour many years ago did their build using family members and of course now the new owners have always had puddling and leaks despite several re applications of the outer surface etc. Obviously it took many years and you cannot prove a negative in that had it had a greater slope, would it have fared better in the same time scale, but it is a little worrying, and I do wonder if anyone actually checks this sort of thing properly. Brian

[Mannequin of mirth ...]

Everything I am reading states that, as per BS6229, flat roofs should have a minimum fall of 1:80 (i.e. 37.5mm in my case) and really ought to be designed for 1:40 (75mm) to allow for inaccuracies in construction, settlement of materials etc.

Unfortunately I have only just spotted this and the roof covering (consisting of OSB sheets, vapour barrier, 125mm solid board insulation, ply and a polyurethane liquid membrane) along with cavity trays and flashing all now in place.

Now I'm fretting about possible issues in the future. Am I right to be worried? Should I be seeking this being altered? I was going to speak to Building Control tomorrow to say what they say but whilst they might confirm the fall is low I can't see anything in the regs that mandates a fall and I don't know whether BS6229 is mandatory either (it seems more of a code of practice).

Grateful for your thoughts.

P.S. For what it's worth I must admit to having a tendency to worry - to the point of anxiety - about detail like this.

well Scottish building regs ...when regs used to make sense....used to say a flat roof should be over 1:10 .... if it is any help....so 1:8 sound OK .......don't worry...be happy.....

[Mathew Newton]

re you I wouldnt lose sleep over it' that in itself would actually be of co nsiderable help!

A bit of an update:

I spoke to Building Control this morning and whilst my inspector is away in holiday one of his colleagues listened to what I had to say. Based on that , and that alone, his view was that the fall was indeed unsatisfactory and could well be considered a violation of the regs. He confirmed there was no thing explicitly stated about falls in them but there is something of a cat chall 'quality and workmanship' requirement which generally requires, among st other things, compliance with relevant standards where appropriate to de monstrate compliance. Hence, the British Standard would be relevant here, n ot least given how accepted its fall figures are in the industry.

He did also say they'd need to see it really as there may well be a good re ason why it is the way it is, and if there is then they would always try an d take a pragmatic approach towards consideration of accepting it as they d on't like to insist on re-work if not absolutely essential. He did echo my concerns though about the fact that the roof lantern is yet to be installed and so any test of falls at the moment could be premature.

I then spoke to the builder who was, thankfully, very considerate of my con cerns. He immediately said he wondered if he might've made a mistake in the calculations but obviously couldn't really comment further without first c oming to see what's what. He's away on holiday so next week we are going to meet up.

The builder confirmed that the furring strips weren't off-the-shelf and so this in my view might explain how the mistake might've occurred as I'm sure we have all performed a calculation, been happy with the result but then r ealised once you'd built the damn thing you'd made an error (or indeed not realised until someone else points it out!.

So we are on pause until next week but so far so good. As things stand I se e no alternative but to strip and re-build as I can't see myself accepting the current construction and it sounds like BC might not either. It is unfo rtunate but mistakes do happen and I am currently content to consider it as just that.

[Mathew Newton]

1:10? That's not a flat roof.

Not sure what you mean with the 1:8 reference - ours is 1:150.