Flat Roof with Parapet detail - correct fall

[Ay8452]

Hi Folks,

 

Just trying to get my head round drawings from my architect for a single storey rear extension 3.5m with a warm flat roof (GRP) and parapet wall detail all around. He has specified furring strips from the old house out to the rear (purple arrow) with 2 parapet wall rainwater outlets (marked in blue). Surely the area marked red will collect rainwater, what's encouraging it to move/drain out the water outlets? Ignore roof lantern that is not going to be built we've opted against it.

 

Thanks 

[Redbeard]

I think if I had been your architect (which I never would be as I am not one!) I might have played with the levels of the majority of the flat roof so that , without diminution of the insulation, I could get a channel in hard against the parapet with the outlets, with a slight fall each way from the middle. The fact that you have chosen GRP makes that easier than with any other covering I can think of. And Robert is your Mother's brother.

[nod]

From memory on our previous build I had fairings made 70 mil to zero over 5 meters 

Outlets same as yours and created a slits bump where you have marked red Keep the outlets low and you will be fine I did ours over five years ago and apart from when leaves block the outlets never collects water 

[jack]

1 hour ago, Redbeard said:

I think if I had been your architect (which I never would be as I am not one!) I might have played with the levels of the majority of the flat roof so that , without diminution of the insulation, I could get a channel in hard against the parapet with the outlets, with a slight fall each way from the middle. The fact that you have chosen GRP makes that easier than with any other covering I can think of. And Robert is your Mother's brother.

 

We had a lot of room to play with, and ours dealt with it like this:

 

 

As built, we ended up with a parallel-sided channel rather than the complex things the architect drew.

 

It's important to make sure that there are decent falls all the way to the exit of the outlet into the downpipe. In our case, some of the falls were a little marginal. As the underlying USB has sagged a little over time, there are now places in the gutters where water pools.

[Nickfromwales]

The biggest issue I see with these installs is the outlet is formed first, which creates a high point. 
 

Then the main roof material gets laid to them and then that has to climb up over that step, albeit a few mm’s, but that’s enough to cause water to pool in between / around the outlet(s). 

 

On the next one I’m getting in the drivers seat, and the 18mm OSB3 deck will get a load of attention to furring, levels and falls, and then the lot will be getting a second sacrificial layer of 9mm OSB3 which will be cut around 300mm square away from the start of the outlets to create a step down there. 

 

The outlets will be formed to stop short of the step where the 18 meets the upstand edge of the 9, and left to cure, with layers built to lose say 4mm of the 9mm OSB step.
 

Then the main roof covering will go to the cut out sections, get dressed in over the preformed and cured outlet material, and then be welded / bonded down to have the steps falling down not up.

 

I’m quite fed up of roofers pacifying me and saying it’s fine, they all just think that standing water and leaves needing to be washed out all the time is fine, as the roofs can handle standing water.

IMO that’s a load of crap, we can do better people!!

[saveasteading]

Get the water off the roof by the simplest method. To me that is a single slope and an oversail then a gutter and downpipe.

 

Architects should have a month of their course working with roof repairers. That should be the end of valleys, parapets and internal downpipes.

 

Philistine I know.

But the primary purpose of a building is to keep us warm and dry.

Second is durability and keeping the bears and burglars out.

Third is the need to impress the neighbours or amuse designers.

[Nickfromwales]

3 hours ago, saveasteading said:

That should be the end of valleys, parapets and internal downpipes.

Remind me to come and visit your simple cube sometime, lol. 🙄

[saveasteading]

1 hour ago, Nickfromwales said:

your simple cube sometime, lol. 🙄

It doesn’t leak. 

 

[jack]

14 hours ago, saveasteading said:

Architects should have a month of their course working with roof repairers. That should be the end of valleys, parapets and internal downpipes.

 

I've told the story before, but within weeks of moving in, we woke one morning to water coming through the ceiling in our bedroom. We found found another leak in the bathroom 6-8 metres away. The weld of the membrane on two of the scuppers through a parapet wall had failed (including the one on the left in the drawing above).

 

We'd had words with the roofing contractor over the welds when they were originally done. I thought they looked terrible but he assured us they were fine. He ended up getting out the Resitrix rep to assure us that all was good. The rep wrote a "everything done to the contractor's usual high standard" email, despite a lot of the rest of it looking like a dog's breakfast.

 

When they came back to fix the scuppers, we made them check and replace all the others. The welds on two of those had failed too, so 4 out of 7. He assured us (and I believe him) that they'd never had this issue before. I personally think they didn't clean the scuppers properly before installing them, and there was a film of dust or whatever from storage preventing decent adhesion. Touch wood the repairs have stood up for over 9 years so far.

 

I'd never have a flat roof again, although the real weakness is the parapet wall. Keeping the outflows clear is a massive annoyance given how many big trees we have nearby, and the difficulty of getting onto the roof. 

[saveasteading]

9 minutes ago, jack said:

never have a flat roof again,

I'm wondering what we call flat? 

I did hundreds of steel buildings at 10% slope ie 1 in 10. I would not have gone flatter.

Parapets or valleys only if a client's architect insisted, and then I designed  in lots of big outlets,  and weirs, and told them to clean twice a year.

 

[SteamyTea]

@Nickfromwales

 

When a waste pipe is fitted, it is not at too steep and angle, from what I understand, this is to allow the turds and Daily Mail to be washed down the pipe.

If that is the correct theory, what is the minimum angle used that works reliably?

 

With a flat roof, I would think that orientation is also important. Most of our rain comes in from the SW, that same SW wind is also of higher speed than others. That could cause water to be blown uphill and away from any drainage.

The above is mental musing as I have not looked at this in detail.

This is what I am looking at.

 

 

[BotusBuild]

That sir, is gorgeous.

[BotusBuild]

We have a GRP flat roof that pools in a couple of areas. GRP was done well.

When the green roof goes on I will have to do something about one of the areas. The other is minor and should be addressed by the green roof makeup shifting the excess water to the outlets anyway.

[Nickfromwales]

3 hours ago, BotusBuild said:

We have a GRP flat roof that pools in a couple of areas. GRP was done well.

When the green roof goes on I will have to do something about one of the areas. The other is minor and should be addressed by the green roof makeup shifting the excess water to the outlets anyway.

But really annoying when a bit of GAF could have reduced the pooling to zero?

 

4 hours ago, SteamyTea said:

With a flat roof, I would think that.....

.....gravity and resistance would be key to a successfully discharged deluge of rain, regardless of whichever direction it was delivered from.

 

4 hours ago, SteamyTea said:

When a waste pipe is fitted, it is not at too steep and angle, from what I understand, this is to allow the turds and Daily Mail to be washed down the pipe.

Different physics and dynamics methinks. A turd leaves the bowl with the provision of energy created from the flushed water, with those forces being concentrated within the confines of the bowl and downstream 4" pipework, further assisted by gravity of the requisite falls.

 

Flat roofs have a lesser amount of energy per m2 to call upon in the pursuit of getting 'random shite / other debris' from A-B, so when a roof is designed to almost get rain to B, it's got feck all chance of conveying anything else; this is even more of a problem when the journey has some of it going up steps / hills.

 

4 hours ago, SteamyTea said:

That could cause water to be blown uphill and away from any drainage.

Would need to be a hurricane, as the parapet walls deflect a lot of the wind in actuality. Nevertheless, when the wind subsides, the water will follow the path of least resistance; down the slopes, and into the outlets and downpipes, like a good little boy.

 

 

If the roofer wasn't also a turd......

[Nickfromwales]

15 hours ago, jack said:

I'd never have a flat roof again, although the real weakness is the parapet wall. Keeping the outflows clear is a massive annoyance given how many big trees we have nearby, and the difficulty of getting onto the roof. 

What if the roof had excellent falls towards the scuppers, and the scuppers offered zero resistance to the flow of water, and then the scuppers had a fall designed in which promoted 'stuff' leaving at a rate of knots in an orderly fashion? Copious rainfall should, in theory, then be your caretaker.

[jack]

8 hours ago, Nickfromwales said:

What if the roof had excellent falls towards the scuppers, and the scuppers offered zero resistance to the flow of water, and then the scuppers had a fall designed in which promoted 'stuff' leaving at a rate of knots in an orderly fashion? Copious rainfall should, in theory, then be your caretaker.

 

We actually have a reasonable fall on the side of the roof where we had the problems:

 

But yes, better falls to and through the scuppers would certainly help.

 

One thing that would still concern me is the amount of crap that would end up going down the downpipes and into the soakaway. I have some screens on the outlets to catch bigger stuff to reduce this, but over the years a lot of stuff still gets through. To an extent this is an issue for all roof types, but it feels like at least some of the leaves that fall onto a pitched roof get blown off eventually, or get shot over any anti-leaf measures in the gutters.

[Tetrarch]

I had a parapet wall specified on my 28sq m extension as I was trying to avoid downpipes on the rear elevation (side scuppers). Apart from the technical challenges you are encountering there is an aesthetic component as well.

 

My original drawings looked like this:

 

It turned out to be completely misleading as a warm roof is way thicker than the drawing would indicate and it would have left an enormous space above the patio doors. In the end I kept the side parapets and had a single deepflow gutter with a rain chain as a feature

 

You can see that if I'd kept the rear parapet the rear wall would have dominated the elevation

Regards

 

Tet

Ps Yes - I know, still waiting on my lead-man to return to finish off the parapets....

[Nickfromwales]

4 minutes ago, jack said:

 

We actually have a reasonable fall on the side of the roof where we had the problems:

 

But yes, better falls to and through the scuppers would certainly help.

 

One thing that would still concern me is the amount of crap that would end up going down the downpipes and into the soakaway. I have some screens on the outlets to catch bigger stuff to reduce this, but over the years a lot of stuff still gets through. To an extent this is an issue for all roof types, but it feels like at least some of the leaves that fall onto a pitched roof get blown off eventually, or get shot over any anti-leaf measures in the gutters.

You can discharge the down pipe into a grated back gulley, to trap at least some of it, but most soakaways can cope with leaf litter as it does break down over time. 

[Nickfromwales]

1 hour ago, Tetrarch said:

I had a parapet wall specified on my 28sq m extension as I was trying to avoid downpipes on the rear elevation (side scuppers). Apart from the technical challenges you are encountering there is an aesthetic component as well.

 

My original drawings looked like this:

 

It turned out to be completely misleading as a warm roof is way thicker than the drawing would indicate and it would have left an enormous space above the patio doors. In the end I kept the side parapets and had a single deepflow gutter with a rain chain as a feature

 

You can see that if I'd kept the rear parapet the rear wall would have dominated the elevation

Regards

 

Tet

Ps Yes - I know, still waiting on my lead-man to return to finish off the parapets....

The rain chain is cool. 👌. 

[saveasteading]

2 hours ago, jack said:

the amount of crap that would end up going down the ...

OK. My preference. My career was in big buildings and so eaves at 6m or more. 

I knew people don't clean gutters.

 

So I preferred a bigger than standard hopper so the leaves etc flow fast into the rwp 100 x100.  Nearly everything gets down that.

(At standard hoppers, the flow streamlines like a wier, and stuff gets left behind) 

At the bottom, fit a grated grill and poise the rwp about 100mm above that.

Thus crud is not on the roof or reaching the drains or soakaway and clearing is simple by hand.

This can be made more sophisticated by shaping the rwp bottom, or adding a plastic shoe.

 

I did this for our own office building. I had a catchpit before the rw harvester, and it was redundant:   Barely any stuff ever in it.

 

I think this principle works for the roof under discussion. 

 

Big sticks on the roof are another matter.

[Spinny]

You could use a large cricket between the roof outlets to push flow outwards towards them. (You can google flat roof cricket)

 

We have 3 rooflights and one is a long one against the wall so the wall below continues seamlessly up into it. The architect omitted to design any detail behind the roof light so we do get some ponding behind it when it is very wet. I realised their mistake only after the firing strips had been done and too late to organise any insulation crickets made from PIR to go under the membrane.

Tis another of the gotchas you only learn from experience. Like putting your lighting plan up against the structural engineers beam and joist plan before you find you cannot put lights where you wanted them.

[Nickfromwales]

What are crickets on a flat roof?

 

 

 

Crickets are triangular structures designed to divert water away from areas of a roof that may collect water. In residential roofing, we tend to think of chimneys as the prime example – chimney cricket. Here you would place a cricket behind the chimney in a triangular pattern so water doesn't collect behind it.1 Jul 2021

[Nickfromwales]

3 hours ago, Spinny said:

You could use a large cricket between the roof outlets to push flow outwards towards them. (You can google flat roof cricket)

 

We have 3 rooflights and one is a long one against the wall so the wall below continues seamlessly up into it. The architect omitted to design any detail behind the roof light so we do get some ponding behind it when it is very wet. I realised their mistake only after the firing strips had been done and too late to organise any insulation crickets made from PIR to go under the membrane.

Tis another of the gotchas you only learn from experience. Like putting your lighting plan up against the structural engineers beam and joist plan before you find you cannot put lights where you wanted them.

I'm not sure I'd want those under my membrane when they can't reliably take point pressure?

 

Good if it's an inaccessible roof which you don't want to go up and carry out maintenance on frequently though.

[saveasteading]

50 minutes ago, Nickfromwales said:

– chimney cricket. 

Memorable.

[Nickfromwales]

1 hour ago, saveasteading said:

Memorable.

My alter ego lol.