Too flat a flat roof?

[MJNewton]

We have just had the shell of an extension built which includes a flat roof. Firring strips have been used to produce a fall for rainwater runoff however this fall is around 20mm over the 3000mm span i.e. 1:150.  Everything I am reading (including BS 6229) states that 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 that our roof is only 1:150 and yet the roof (a warm roof construction consisting of joists topped with firring strips, OSB sheets, vapour barrier, 125mm solid board insulation, ply and a polyurethane liquid membrane top coating) and all associated cavity trays and flashing etc is all now in place. Bar the roof lantern the roof is complete.

Now I'm fretting about possible issues in the future. Am I right to be worried? Should I be seeking this being altered (i.e. deconstructing to joist level to allow steeper firring strips and then rebuilding from there)? I was going to speak to Building Control tomorrow to say what they say but whilst they might agree that the fall is low I can't see anything in the regs that mandates a particular fall and I don't know whether BS6229 is mandatory either (it seems more of a code of practice). 

 

For what it's worth I must admit to having a tendency to worry - to the point of anxiety - about detail like this. I also feel really quite sick about the situation as I don't like confrontation and do like the builders; see them most days locally, am friends with a mutual friend of the builder's son etc and so really want to avoid a falling out if at all possible. I also really respect their workmanship. I have previously already make them put a fair bit of effort in to move an airbrick and so they know I'm a one for detail and worry over what someone else might consider to be nothing; they may well see this to be just another case of that but I can't help but feel I should be addressing this now rather than regret doing so in a few year's time once the roof fails due to a fall-related issue.

Grateful for your thoughts/advice. 

 

P.S. I hope it doesn't come to this but the contract states that should we end up in dispute we agree to jointly fund an independent surveyor and be bound by his view/recommendation. I have yet to pay for completion (the final invoice only arrived today).

 

Edited September 13, 2018 by MJNewton

[Ian]

I presume that you’ve had some wet weather since the roof was finished? If so, does rainwater pond anywhere on the roof or is it ok?

The British Standard is not a mandatory requirement. The reason for stating that there should be a minimum design fall of 1:80 is because with falls that are shallower than that it’s easy to end with flat spots in the roof which hold water.

 

 

Edited September 14, 2018 by Ian

[Onoff]

Is there anything in the tech spec for the liquid membrane that says about minimum falls?

 

You might do well to have a look at Approved Document Part H based on this I found:

 

http://www.roofconsult.co.uk/articles/bauder.htm

 

The building regs are statutory so there's really no dispute that would require an independent surveyor imo. 

 

You can download from here:

 

https://www.planningportal.co.uk/info/200135/approved_documents/71/part_h_-_drainage_and_waste_disposal

 

As said if it's been done well, allows smooth run off etc maybe it'll be ok. I'd try and get BC on your side asap to add weight if you decide to delay payment. Maybe even try and work a hefty discount to offset future issues if it fails or some written g'tee from the builder for the expected life of the roof. 

 

(I feel your pain btw. Our flat roofs on the dormers have exactly this issue). 

 

[Onoff]

Might give you some comfort for the future even if it's "wrong":

 

"It shows excellent adhesion to the substrate, high flexibility, high resistance to weathering, as well as UV resistance. It shows high resistance to standing water, thus it can be used on flat roofs without particularly good slopes".

 

https://isomat-pu-systems.com/solutions/waterproofing-conventional-flat-roof-polyurethane-liquid-membrane/

 

Post up the membrane system make and the peeps on here will be lining up to read the tech spec and comment!

[Nickfromwales]

20mm over 3000mm will be more than enough to stave off standing water. Get a length of 28mm copper pipe, stick a 20mm chock under one end and pop a marble in.  

This isn't something you should be worrying about tbh, and most 'flat' roofs I've seen have been in for decades with pooling water, leaf litter, grot and more and don't leak one drop.

As long as the boarding has been done well, and the firring pieces in at 400mm centres max, then I'd say this'll work just fine. As asked, have you done a run-off test with a hose / had a bit of rain on it yet? Proof is in the pudding     

[Onoff]

2 minutes ago, Nickfromwales said:

20mm over 3000mm will be more than enough to stave off standing water. Get a length of 28mm copper pipe, stick a 20mm chock under one end and pop a marble in.  

This isn't something you should be worrying about tbh, and most 'flat' roofs I've seen have been in for decades with pooling water, leaf litter, grot and more and don't leak one drop.

As long as the boarding has been done well, and the firring pieces in at 400mm centres max, then I'd say this'll work just fine. As asked, have you done a run-off test with a hose / had a bit of rain on it yet? Proof is in the pudding     

 

My experience is a bit different!

 

 

As much to do with "new" felt being laid over old and it rouching up forming horizontal barriers to the run off. Joins are bad too, where they have butted pieces together rather than overlapping. FA slope mind.

 

[ToughButterCup]

7 hours ago, MJNewton said:

[...]

For what it's worth I must admit to having a tendency to worry - to the point of anxiety - about detail like this. I also feel really quite sick about the situation as I don't like ....

[...]

 

Sweating detail and worrying  about it is completely normal.  There is not one self builder here who has not lost many nights sleep over detai. Not one. (Shit I spellt detail wrong. Arggghhh)

 

Except @Onoff

[Nickfromwales]

5 minutes ago, Onoff said:

My experience is a bit different!

Yes, but felt over felt isn't a good example and the OP expressed anxiety over their brand new, sarked, membraned roof so I'm going with apples not rotten tomatoes  

[MJNewton]

Thank you everyone for the input; I knew you'd sympathise with the anxiety side of the issue but also offer impartial advice!

 

In answer to the questions raised:

 

The membrane that's been used is Desmopol PU (an alternative source of similar information also available here). One thing I have noted is that the BBA certificate assumes (in paras 4.5-4.6) a minimum fall of 1:80 (almost twice what I've got).

 

Regarding actual run-off it definitely does drain away from the house but some water is left behind. I don't know whether that's just surface tension that would affect all 'flat' roofs though. Here are some photos from this morning, but I must add the caveat that it is difficult to say how faithful a reproduction they are as some aspects look worse in real life and some better:

 

 

 

 

 

The sheet is covering a 2.5m x 1.5m opening for the roof lantern. This is expected to weight upto 250kg and so whilst the joists are 170mm x 50mm C24, with headers and trimmers tripled up and bolted together, I may well be expecting some deflection - perhaps over the long(er) term. With a 20mm overall end-to-end fall I am assuming the midspan-to-end is therefore roughly 10mm and I can't help but feel that this doesn't give me much headroom before my flat roof really does become flat.

 

I am feel awful not only about the risk aspect of the situation but also the consequences on the builder having to redo to the point where I am considering offering to contribute to offset at least some of their losses (e.g towards the materials that cannot be reused). Just to add to my concerns the membrane installer was a sub-contractor (to the builder) and so they are something of an innocent victim in all of this.

Edited September 14, 2018 by MJNewton

[Triassic]

One approach would be to discuss your concerns with the builder and possibility of leaks in the future. Follow up your discussions with an email covering the points raised and discussed. Then forget about it. Using modern materials, the roof is unlikely to leak and if it does you then go back to the builder to rectify.

[ToughButterCup]

18 minutes ago, MJNewton said:

[...]

Regarding actual run-off it definitely does drain away from the house but some water is left behind. I don't know whether that's just surface tension that would affect all 'flat' roofs though.

[...]

 

A tiny amount of water spreads itself thinly. 

If there is a pool of water that consistently forms in the same place ( a low spot) , the normal precipitation / evaporation cycle will , over time leave a ring caused by small bits of dust trapped in the surface film.

Our flat roof has three small 'pools' and has had for the last five or more years. I have no concerns whatever about them. None. And it has been done to a much much lower standard than yours.

 

Its Friday.

Not long until a pint of foaming liquid a good chat and a family meal, and time to watch a trashy film - take the mickey.... eh?

[Mr Punter]

It is a bit annoying they didn't use 1:60 firrings but it won't make any difference to you.  The roof will leak if the membrane fails, whether the fall is correct or not.  Correct membrane install and upstand detailing is what is important here, not fall.

[MJNewton]

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 nothing explicitly stated about falls in them but there is something of a catchall 'quality and workmanship' requirement which generally requires, amongst other things, compliance with relevant standards where appropriate to demonstrate compliance. Hence, the British Standard would be relevant here, not 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 reason why it is the way it is, and if there is then they would always try and take a pragmatic approach towards consideration of accepting it as they don'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/acceptance of falls at the moment could be premature if they are borderline.

 

I then spoke to the builder who was, thankfully, very considerate of my concerns. He immediately said he wondered if he might've made a mistake in the calculations but obviously couldn't really comment further without first coming 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 for where/what to cut but then realised 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 see 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 unfortunate but mistakes do happen and I am currently content to consider it as just that.

Edited September 14, 2018 by MJNewton

[newhome]

Just as a small digression, back in the day flat roofs used to be anathema to most people. They invariably leaked, and it wasn't a question of if, but when. I've noticed that many people seem to incorporate flat roofs into their new build or extension with open arms these days. Is the construction different or more superior now thus making them so much better than their older, leaky relatives? 

 

 

[ProDave]

11 minutes ago, newhome said:

Just as a small digression, back in the day flat roofs used to be anathema to most people. They invariably leaked, and it wasn't a question of if, but when. I've noticed that many people seem to incorporate flat roofs into their new build or extension with open arms these days. Is the construction different or more superior now thus making them so much better than their older, leaky relatives? 

 

 

Things have moved on since the days of a bit of plywood, a layer of roofing felt and a lot of hope.

I would not even have felt of a shed roof today. Useless stuff.

[newhome]

I had a house with a flat roof (fibreglass) years ago. Bloody thing leaked. The one and only time I’ve ever gone up a ladder to roof level (carrying stuff too!) and that was only because I was too poor to get someone to fix it (mortgage interest rate hit 15%!). I did manage to stop it leaking fairly miraculously but didn’t come off that roof until I had finished as I knew I would never be brave enough to go back up again as it was terrifying lol. 

[Simplysimon]

1 hour ago, newhome said:

Just as a small digression, back in the day flat roofs used to be anathema to most people. They invariably leaked, and it wasn't a question of if, but when. I've noticed that many people seem to incorporate flat roofs into their new build or extension with open arms these days. Is the construction different or more superior now thus making them so much better than their older, leaky relatives? 

 

 

EDPM roofing material is far superior to the felt roofs of old, however, i have fitted many torch on felt roofs and found it to be a good product. i did burn the felt to the osb as opposed to sticking basic felt to the roof by pouring hot tar over the osb and then torching felt to that. 'but that's the way we've done it for years'.............

[Stones]

On ‎14‎/‎09‎/‎2018 at 07:46, MJNewton said:

 

Regarding actual run-off it definitely does drain away from the house but some water is left behind. I don't know whether that's just surface tension that would affect all 'flat' roofs though. 

 

 

If you think how your car looks after it's been freshly waxed and it's rained...surface tension does have an effect. 

[MJNewton]

Yes, and even fairly steep car panels can hold onto water somewhat. 

 

I have decided that whilst the risk of my roof failing might be low the consequences of it doing so are potentially significant. I think it therefore reasonable to want to minimise the risk of this happening and so accepting something that doesn't meet the recognised standard is not an option. Hopefully the builder will reach the same conclusion... 

[Onoff]

Such a shame as looks a nice job from the pics. 

[MJNewton]

It is a shame, and I feel bad both for the situation and the potential losses for the builder (which I may offer to offset slightly). My assumption at present is that it was a simple mistake; I'd be much more comfortable if it was a result of cutting corners but that's just not his style. 

[Ed Davies]

Assuming a fix is decided on, is there a need to replace the whole roof? Any way to replace just the liquid membrane and top layer of ply? I'm thinking something like taking them off, add a layer of sloped insulation then replace them.

[PeterW]

Like @Ed Davies suggests, can’t you just add another ply layer on the top ..? Add firrings, insulate between them using PIR or even EPS then top with ply and membrane again ..??

[Declan52]

How are the joists attached below. Are they nailed onto a wall plate or sitting on hangers.

Could you use acro props underneath and lift the joists up using a plank running down both sides and then have different size packers put under the joists to give you your fall. 

[MJNewton]

There might be alternative fixes but I think they'll all be trading labour for materials and vice versa and so simplest (ie. a straightforward rework) might just be the best. 

 

An additional covering of ply and a new membrane might work but it might look a bit of a bodge as a conventional re-roof probably wouldn't even take that approach preferring instead to remove the existing covering.

 

I did wonder about raising the joists and packing underneath but they are notched to fit in a steel beam so likely aren't going anywhere, and I wouldn't trust a membraned roof that had been lifted even as one so it'd need to be replaced either way. It'd also need the upstands removing and so would need a new covering anyway, and the lantern upstand may also need modifying to remain remain true to its original orientation. 

 

All food for thought though and I imagine the builder and I will kick the ball around a bit before deciding what best to do. 

Edited September 15, 2018 by MJNewton