Too flat a flat roof?

[MJNewton]

Addendum: Re-reading some of the suggestions I may have misunderstood on my first pass. Will give them some further thought - thanks. 

[MJNewton]

The builder visited today…

 

He was actually very good about the situation and quickly held his hands up once he’d checked and measured the firring strips himself. Whilst the finished roof does drain, and in his opinion wouldn’t prematurely leak, he acknowledged that its fall was below-spec it was my prerogative to be concerned and to expect a remedy. To this end we came up with a few options; the choice of which is entirely mine and he will pick up the bill:

 

1. Leave As-Is – He would give me a guarantee in writing that should the roof fail in the next x years (to be agreed) he would repair/replace and make good any consequential damage. To give me greater confidence he could also get the roofer to add another layer of the liquid membrane to the existing. He would also refund the initial £1000 roofer cost.
2. Remove and Rebuild – Rip up the entire roof deck (membrane and trims, top boards, insulation, bottom boards and firring strips) and replace (likely with bought firring strips this time – he had no idea they were available off-the-shelf hence why he always cut his own, normally without making a mistake with the measurements!)
3. Overboard and Recoat – Keep the old roof in place (bar the trims), add a new layer of OSB3 on top of suitable firring strips to give the correct fall and put a new waterproof coating on top.

 

I am not comfortable with option 1 as whilst I trust his word regarding the guarantee (we live in a small town, him and his son just down the road from me; there’s nowhere to hide!) my preference here is to avoid a leak in the first place rather than deal with one if/when it happens. I’d also like to draw a line under the situation rather than leave things hanging over me/us.

 

Option 2 was my initial preference as this gets me back to the position I was in before the mistake happened and therefore not be considered a ‘bodge’ in any way. However, his initial thoughts were that it could cause more problems than it solves. We didn’t get into too much detail about it but his concerns were about the destruction required as with all the fixings now hidden there’d likely be a lot of ‘coercion’ of materials and could lead to damaging other components. Might be worth pushing further though, particularly as I'd be more than happy to help with the removal side.

 

Option 3 sounds like a viable option as not only does it give me the opportunity for a new waterproof coating (I am leaning more towards a single-sheet EPDM membrane rather than another liquid coating - ever since it went on my confidence in it has not been the greatest as it is far from conventional and so seemingly not much information about it) but the extra board/membrane layer may also provide an additional noise and solar barrier? I certainly wouldn’t ever expect a leak through two surfaces either, at least not down into the living space! I am wondering about potential downsides though... It would add ~50mm to the roof height at the rear wall abutment which would eat into the 150mm lead flashing and the roof lantern upstand would require extra timber on top to increase its standoff height. Could there be a risk of interstitial condensation with this extra layer or would it be okay given it is on the cold side of the insulation? Should the gaps between the firring strips be filled with anything? (Note the strips will likely be 40mm to zero over a 3m span so not much volume there).

 

As before I would be grateful for your thoughts. The builder is being completely open and supportive of the situation and keen to make sure I am happy with the outcome – I just need to work out what the best outcome should be!

[newhome]

What does building control want to sign it off? Surely that’s one of the main considerations? 

[Mr Punter]

Firstly, well done to your builder!  Probably less than 1 in 10 would offer these options.

 

I would go with option 1 but perhaps negotiate on the money.

 

Option 3 seems a bit of a bodge.  They will need to penetrate the existing membrane to fix the firrings and new deck.

[Onoff]

Option 2 and his wife's recipe for lemon ice cream!

[MJNewton]

2 hours ago, newhome said:

What does building control want to sign it off? Surely that’s one of the main considerations? 

 

Good point. I've been so focused on finding the best practical solution (for me) I have forgotten to consider what BC might be happy with.

 

I did ask the builder what he thought and he reckoned BC would likely not be concerned as their focus is mainly driven by aspects of legislative interest e.g. safety, conservation of energy etc. I didn't feel he was wriggling out of anything, just expressing his opinion. I do know there's nothing in the regs about roof falls and all my informal chat with BC (not my BCO as he's on holiday) reveaaled was that there is a general requirement for good workmanship which in turn tends to require compliance with recognised standards. The thing is, BS6229 'Flat Roofs with Continuously Supported Coverings' seems to actually be a 'Code of Practice' which might mean it's not quite so black and white. Upstand and flashing details might however be.

 

I will pin my BCO down and see what he has to say on the matter, whether that is from a formal regs perspective or informal advice, so thanks for the prompt.

[MJNewton]

2 hours ago, Mr Punter said:

Firstly, well done to your builder!  Probably less than 1 in 10 would offer these options.

 

Yes, refreshing isn't it. It might be easier if I had something to push against though then I would insist on it just all being redone and be done with it!

 

Quote

I would go with option 1 but perhaps negotiate on the money.

 

Option 3 seems a bit of a bodge.  They will need to penetrate the existing membrane to fix the firrings and new deck.

 

Option 3 does have a whiff of bodge about it, and is the sort of thing that might be expected to be done when reroofing a previously-failed roof. Not really something one would expect of a two-week old example.

[Onoff]

I think with Option 3 I'd be worried about creating cavities allowing condensation and or insect / vermin ingress.

[Ed Davies]

Option 2.5 (as I suggested before)? I.e. remove the membrane and ply then start again from there? I too would think creating a cavity would be a bad idea. Thought you could get insulation with the appropriate slope already cut? Seconds and Co used to have lots of it because people had it cut to specific (wrong) sizes.

[MJNewton]

On 18/09/2018 at 18:46, Onoff said:

Option 2 and his wife's recipe for lemon ice cream!

 

I'm leaning that way (perhaps not the ice cream though, I don't want to be seen to be taking advantage of the situation ?).

 

A restrip and rebuild does at least put me back in the same situation as if nothing had happened, assuming of course that the builder's concerns about damage don't come true. I'm not quite sure what the issue is, perhaps it was a concern over the time it would take to do it carefully but I'm more than happy to get up there with a magnet and find the fixing screws and then remove the liquid membrane myself in those areas ready for unscrewing. Perhaps that's when I'll discover how good this Desmopol is when I fail to strip any of it off!!

 

Perhaps the BCO might say it is the most sensible option (for me) and everything else is a compromise even if they are content with them from a compliance perspective.

Edited September 21, 2018 by MJNewton

[Ian]

@MJNewton

It's very common on large commercial buildings that use single ply membrane roofs (membranes like yours) for the membrane to be laid completely flat for example along very long and wide gutter runs (most large new Tesco stores are done that way) so IMO there's absolutely zero chance of your roof leaking due to a slightly shallower angle of fall than the 1:80 recommendation.

 

It sounds like your builder is one in a million! If it were my roof I'd definitely go with option 1.

 

I'd agree with your builder that Option 2 carries a risk of creating unintended damage elsewhere which might mean that you end up with a worse job than the original.

Option 3 carries a significant risk of creating a problem with interstitial condensation in the zone between the 2 roof skins.

Edited September 19, 2018 by Ian

[Declan52]

How deep can the liquid membrane be laid?? 

Is there enough depth to get you nearer your target fall.

[MJNewton]

It is self-levelling and so I imagine it could be difficult to build up additional layers to create a fall. Besides which, we are upto ~20mm short and so it could cost several thousand to build it up that much... it certainly wouldn't ever leak though! ?

 

There was a bit of a development today; the builder phoned to say he'd spoken to the manufacturer and they said their product can be used even at zero falls and would provide a written guarantee (materials only) to that effect. I got the impression that he's now thinking that whilst he might not have got the normal fall right he's not actually ended up doing any wrong. He gave me their number and they confirmed the same thing, although they did concede that they don't have a BBA certificate that explicitly says this. The BCO is visiting tomorrow and so I will see what he thinks of that - surely BBA certs are important to demonstrate compliance with the regs and if that says a fall of 1:80 (recommended 1:40; it basically seems to be a copy and paste of BS6229) is assumed then surely it's not valid at anything less?

 

I also raised with the manufacturer that there were some runs and what looks like pinholes/bubbling in some areas... The roofer had previously said they were of no concern but the manufacturer didn't brush these off at all; indeed they have asked for photographs as they said this could be a sign of poor installation... When the weather gives me half a chance I'll be taking some and sending them over.

 

Next step BCO (tomorrow) and, somewhat unusual I imagine, I hope they are going to be strict about what is acceptable here!

Edited September 19, 2018 by MJNewton

[MJNewton]

Bit more of an update...

 

The BCO came out and listened to my concerns about the falls. He wasn't half as hard-lined about it as I thought he was going to be but did say he'd go back and speak with his manager about the situation, and he also said we should wait for the membrane manufacturer's opinion on the quality of the application as it could make the fall situation a moot point...

 

And it looks like it has... The manufacturer phoned me today and said the application of their product in this instance was 'shocking' with numerous issues including pinholing of the surface, the wrong joint tape being used, uneven thickness etc. They are going to put everything in writing on Monday.

 

I told the builder and he told the roofer. Both seem to be fuming and will be visiting site next week with the local supplier/trainer of the product used as the roofer seems adamant he hasn't done anything wrong. Not particularly looking forward to that one as I don't like criticising people's work, particularly in front of others, but I will have to at least point out all the bits that I sent photos of to the manufacturer so that they can all see what they've seen.

 

So, in my view, the fall issue has been overtaken by the membrane installation issue now. If the manufacturer isn't willing to provide a guarantee against failure in this instance then nobody else is in a position to do so either so that's my option 1 (leave as-is) gone. If the report says that water may have already got through then I think the line I'll be taking is that I want the whole roof structure stripping back to the joists (option 2) as if the roof structure was built well enough to keep moisture out then if any water does get in their it's going to stay in there and I'm not at all happy about that. With the roof stripped back we can of course then put the proper firring strips in before building back up.

 

I've been really enjoying the building of the extension so far but this whole roof business has really left a sour taste which is a real shame for all concerned. Hopefully one day I'll look back and laugh about it.

Edited September 21, 2018 by MJNewton

[Declan52]

Your not criticising the roofers work the manufacturer is. Just have it in black and  white what they reckon is wrong and show it to them all. Either the roofer done it wrong or he done it wrong because he was shown it that way by the local supplier/ trainer. Doesn't really matter either way. If it's not right it's not right. 

As you say the manufacturer won't give you a gaurantee so leaving the roof as it stands isn't an option. Only one left is strip it back and start again. And take lots and lots of pics this time.

Can the manufacturer give you drawings /pics of how it should be done so you can compare these to the work that will be done on the new roof so you can keep a beady eye on it as it is being done. 

Ask the local supplier when he is out is he able to call in and check the progress as well.

[Ferdinand]

7 hours ago, MJNewton said:

I've been really enjoying the building of the extension so far but this whole roof business has really left a sour taste which is a real shame for all concerned. Hopefully one day I'll look back and laugh about it.

 

I think you should look at it and think that you have correctly operated the "inspect, check,  correct" part of the process.

 

Catching it earlier than you could have done (eg instead of after the lantern was one) is a saving over worst case, even if it is a cost over best case (ie spotting the wrong angle on the firrings before they were installed).

 

It may not be an A+, but nor is it a D-.

 

Ferdinand 

[MJNewton]

Thanks both. I must keep reminding myself that any negativity on my part is merely the result of observations of what's in front of me and if its wrong it needs calling out. Whilst it is wrong to expect professionals not to make mistakes (we are all human), it is only reasonable to expect them to be rectified once identified. 

Edited September 22, 2018 by MJNewton

[Annker]

On 22/09/2018 at 08:19, MJNewton said:

Thanks both. I must keep reminding myself that any negativity on my part is merely the result of observations of what's in front of me and if its wrong it needs calling out. Whilst it is wrong to expect professionals not to make mistakes (we are all human), it is only reasonable to expect them to be rectified once identified. 

@MJNewton Interesting read this, can you tell me how it all ended up?

Reading through the thread I thought anything other than choosing Option 1 (leaving as is) would be madness; and then the plot twist with the suspect installation!

I imagine with the fullness of time the integrity of the roof has been well tested now.

  

Edited January 8, 2022 by Annker

[MJNewton]

3 hours ago, Annker said:

@MJNewton Interesting read this, can you tell me how it all ended up?

 

I can't believe this was three years ago now! Only feels like yesterday (I suppose the pandemic probably has something to do with that).

 

With the multi-faceted issues it all got rather complicated and so with my ever-sensible father (whose OCD, obsession with details, bad luck when it comes to trademen etc all come from!) I went on to explore what the best solution would be best. The technical director of the firm that supplied the Desmopol (they supply all sorts of roofing products, they're not the manufacturer) and he was the one person that I found supremely knowledgeable on all things roofing and, most importantly, completely trustworthy. A decent bloke and so one I decided to put my trust in. He suggested that ripping the whole roof and starting again might not be as perfect a solution as one might hope given the inevitable disruption to the rest of the construction, particularly if done by a fuming builder! Furthermore, he said he's spent his life up on roofs and so can spot a roof that will leak vs one that won't and he was absolutely confident that mine was of the latter. He assured me that the usual issues that plague too-flat flat roofs wouldn't apply here given how small it was and that the Desmopol product was approved for zero-falls application anyway (the BBE certificate I had said it wasn't but it turns out that was out of the date - there was a more uptodate one than confirmed its suitability).

 

So, I started to accept the flatness issue as more a case of less-than-perfect work that you get from many a tradesman and regarding the Desmopol application itself he and I end up recoating it together. His company paid for the products and we both spent the morning on the roof recoating it. He does actually run the training courses for this and other products and so was going to invite me along to one of them but that'd leave me doing it all myself which whilst I wasn't too concerned about I think he wanted to help a bit more than that whilst also ensuring that the issue was definitely concluded. In a way I'm glad I got the chance to reapply with his help as in 10/15 years time when I should be recoated I'll be confident knowing how easy it is.

 

So that's what we did and, three years on and - touch wood - we haven't had any leaks or anything and I think something would've happened by now. After a rain shower the roof drains in the same way every time with a little bit of pooling in one area, but that's stretching the term I think as it usually evaporates fairly quickly. It still haunts me a little whenever I see it and that happens often as my daughter's bedroom overlooks it! If it wasn't overlooked I'd probably have forgotten all about it by now I'm sure.

[Gus Potter]

@MJNewton

 

Thank you MJ for your initial post and the follow up. It is a great informative thread. A great demonstration of how a pragmatic approach, working together with suppliers / contractors / the technical advisors can lead to a good outcome without having to resort to stripping things out and all the grief that can bring.

 

Thanks again.

 

To add a bit of food for thought...

 

Often you hear bad reports about flat roofs, like all things in life bad news travels faster than good in general.The contractor /suppliers / installers / roofers are the first to get it in the neck when you start to see ponding / puddles on the roof. If you are thinking about a flat roof then you need to take quite a few steps back to prepare the structure in advance.. this needs forward planning!

 

To explain I'll use the following example. You build an extension out the back of your house. Much of the below also applies to new build. The roof timbers / joists.. could be metal and often span from the new front wall of the extension back to the line of your existing rear elevation.

 

You often want to open up the rear wall of the house to give a more of an open plan feel. You need say a steel beam / big concrete lintel to make a wider opening to access the new extension. This beam often rests on the old walls each side and adds point loads where before the load was spread more evenly. These point loads travel down the walls and often adds a bit more load locally to the founds that have sat there quite happily without moving. This extra load often causes some extra local settlement unless say the house is on solid rock.

 

Your new beam / lintel bends a bit in the middle. Near the ends of the extension the rafters are supported on a good length of wall, maybe the side /gable walls of the extension.. but half way along the existing rear elevation you have a found that is settling and a have beam that is bending. Thus you already have a dip that causes water to pond.

 

Steel beams bend under their initial load say from brickwork / floors / roofs above.. then they bend a bit more if you have say snow on the roof. But often they go back up a bit once the snow melts. But timber is a bit different.

 

Timber (concrete to a lesser extent) suffers from what is called creep, it's a natural material. You can see good example of this if you go into an old pub with exposed beams.. observe the sag / creep.  BS 5268 for example recommends a deflection limit of 0.003 x the roof joist span for some applications. So a 4000mm span can deflect 0.003 x 4000 = 12mm! But with modern flat roof membranes you can already see trouble ahead.

 

What happens is that the poor builder puts everthing in flat and level and a few weeks / months later it is no longer flat, the steel has deflected, the timber is creeping and the founds are settling locally. You then see ponding on the roof and start asking questions. If you look at each element in isolation it is often not much in terms of settlement  / deflection.. but when you add all the movement together you can soon see how the flat roof now has low spots in it.

 

In summary make sure your SE or structural designer knows that you are going for a modern " true flat roof" early on and this can avoid later issues.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

[MJNewton]

The only issue in this case was that he made a mistake cutting the firrings and didn't cut them steep enough. If he had (to the recommended 1:40) or had just bought them pre-cut then it would easily have accommodated any issues that might be expected to potential arise elsewhere.

 

I suspect it was just a moment of absent-mindedness as every other aspect of the build - foundations, brickwork, beams, slab etc was absolutely superb - mm accuracy on everything else that could be measured. Absolutely faultless. Indeed the BCO and several tradesmen I had in doing other work (e.g. sliding door fitter, screeder, plasterer) all commented what a superb job he'd done on the shell. They were quite taken aback when I mentioned the issue about the too-flat roof!

 

I think it's this aspect that made the whole thing that bit harder to come to terms with as it wasn't that he was rubbish, he just made what turned out to be a fundamental mistake. Of course, we all make them (I *certainly* do!) but I was paying someone here in part to avoid such rookie errors. That's partly why I mostly do everything myself then if (when!) it goes wrong I've only got myself to beat up about it.

Edited January 8, 2022 by MJNewton

[Onoff]

On 08/01/2022 at 19:59, Gus Potter said:

You can see good example of this if you go into an old pub with exposed beams.

 

Landlady was saying to me all the upstairs floors slope. Murder tiling the bathroom floor recently she said. Don't suppose Part A was a thing in the 15th century... ?

 

It doesn't look too bad from this angle. 

 

 

6.2% Audit Ale is lovely btw. 

[Adsibob]

I discovered a very similar issue on my new first floor extension flat roof. The GRP was probably applied 2 or 3 months ago. It never occurred to me to check this ad the builder is really experienced and in the whole had been very thorough and careful (to the point that I get frustrated that things take longer than I would like). But we have pooling! The photos show either side of the rooflight: on the left of the roof light there is quite a large puddle that is about 1m in length and 20cm or 25cm in width.

 

 

On the right of the rooflight, there is a much smaller puddle, maybe 15cm by 20cm. 
 

What concerned me was that these photos were taken yesterday afternoon at about 3:15pm, after a full day of sunshine (albeit the puddles would only have been in the sun for maybe 3hrs as this roof is on the west of the house and shaded by the second floor loft conversion before midday.

 

 So I sent them to a RICS surveyor who specified some other parts of the build (though not the roof, so he is independent ) and his view was that there was nothing to worry about in terms of leaks. He mentioned that this puddling can cause moss to grow which can then dislodge and block gutters, but I am trying to view it positively as just another trainee to get gutters cleaned once a year. 
 

Our build still isn’t finished, so I will raise it with my builder, but I suspect he won’t accept there are any issues. It’s a brand new GRP roof, so it should be pretty impermeable… I hope!

 

[MJNewton]

I always remind myself that green roofs must surely remain pretty soaked for days after any decent amount of rain?

[Annker]

@MJNewton Thanks for updated the thread, good to hear that the roof is performing as it should!