Creating a fall on a flat roof: and attaching the wall plate for it

[ToughButterCup]

Bit of help needed here please.

Part of our roof is flat. I am determined to avoid at least some of the problems associated with flat roofs if I can. One strategy is to create a significant fall on the roof.

 

The question is how to create that fall  easily and  hopefully cheaply.

The plan:

The elevation:

 

The architect has noted that we should use soft wood (SW = soft wood I assume ?) tilting fillets. I note from this post that we  should make the fall 1 in 40 or steeper. (Thanks @tonyshouse)

• The Fall: Is there any reason why we should not build that fall into the POSI joists in the first place? (instead of using tilting fillets?)
• Attaching the wall plate (rim joist) As planned, the wall plate (rim joist) attachment system (threaded bar resined into the concrete behind, I should imagine) will butt up to the Durisol block face: and that is the  face which is backed by  185mm of graphite polystyrene - and then 120mm of concrete. On the other side of the Utility and boots room, the wall plate abuts the 'internal' face - the concrete 'side' of the Durisol block, so there is no potential problem with that face. In short, does the 185mm of insulation matter when attaching the wall plate (rim joist)?

 

To help you visualize my concern, here's a schematic image of a Durisol block.

 

[PeterW]

If you put the fall in the joists then you will need to cut the end of every joist to 1:40 which will be a real challenge. 

 

The other option is to create a flat deck on top of the posijoists and use tapered insulation on the top of the deck. Means that everything is flat when constructed and then the slope is built on top. 

 

On the wallplate issue I would ask Durisol as you're relying on the compressive strength of the Durisol plus the tensile strength of the bolts. 

[najem-icf]

Why not use tilt firrings?  Most timber merchants do them - they will give you the fall you are looking for without the need to install your posi's at an angle.  Water will run no problem at a gradient shallower than 1:40.  There doesn't seem to be a large span there so you could achieve this without great difficulty.  Alternatively, ask the posijoist manufacturer what angle you can go to (that's if you have already had them made) - they are engineered joists and the webs are calculated according to the stresses based on their proposed installation.  As such, if they are installed at an angle, they could cause stresses not factored and the webs could potentially fail.  Not saying that *will* happen - but the potential worst case scenario.

 

For the roof covering, I would recommend using Firestone Rubbercover - I have used it several times now and never had any issues with it.  It is a single ply membrane and is flexible enough to accommodate expansion and contraction without compromising the integrity of the membrane.  I have attached a photograph of one I did some years ago for my brother.

 

Hope this helps.

[nod]

42 minutes ago, recoveringacademic said:

Bit of help needed here please.

Part of our roof is flat. I am determined to avoid at least some of the problems associated with flat roofs if I can. One strategy is to create a significant fall on the roof.

 

The question is how to create that fall  easily and  hopefully cheaply.

The plan:

The elevation:

 

The architect has noted that we should use soft wood (SW = soft wood I assume ?) tilting fillets. I note from this post that we  should make the fall 1 in 40 or steeper. (Thanks @tonyshouse)

• The Fall: Is there any reason why we should not build that fall into the POSI joists in the first place? (instead of using tilting fillets?)
• Attaching the wall plate (rim joist) As planned, the wall plate (rim joist) attachment system (threaded bar resined into the concrete behind, I should imagine) will butt up to the Durisol block face: and that is the  face which is backed by  185mm of graphite polystyrene - and then 120mm of concrete. On the other side of the Utility and boots room, the wall plate abuts the 'internal' face - the concrete 'side' of the Durisol block, so there is no potential problem with that face. In short, does the 185mm of insulation matter when attaching the wall plate (rim joist)?

 

To help you visualize my concern, here's a schematic image of a Durisol block.

 

Hi Ian

 

i would go with the tilt fillets on the top

Youd be making a lot of work for yourself creating a fall with the joists

Berrys will make your taper fillets to any fall you require and deliver free of charge

i used 70mil  down to 10 mil over 5 mtrs  £4 each

 

i wouldn't worry about the wall plate If you use 15 mill threaded bar in a staggered pattern All fixed with resin It will go nowhere 

[PeterW]

@recoveringacademic I've re-read that post. 

 

The architect has drawn the joists flat but the insulation above is sloping. It would infer sloping insulation. 

[ToughButterCup]

3 minutes ago, PeterW said:

@recoveringacademic I've re-read that post. 

[...]

It would infer sloping insulation. 

 

Yes, I think you are right. He did mention that at one stage......

[PeterW]

So the fillets aren't required.... you need one or the other ..!

[Mr Punter]

The tapered insulation costs a fortune.  Use firings (tilt fillets) will be much cheaper.  Is there some sort of deck above the insulation?  What insulation type?  How is it to be fixed down?

[ToughButterCup]

13 minutes ago, Mr Punter said:

[...]  Is there some sort of deck above the insulation?  What insulation type?  How is it to be fixed down?

 

Thanks @Mr Punter

Here's what I know:

 

and here is the relevant bit of the BR submission:

 

I hope that makes sense to someone.....

Ian

[Construction Channel]

Your architect is very good at drawing pretty pictures. But I don't think he was overly worried about your wallet. 

 

- it is drawn with firing pieces (tapered bits of wood fixed to the top of level joists)

 

- the tapering of the insulation was just to create the gutters. E.g you would cut out a 6" wide channel at the low end starting at 0" deep in the Center of the roof and ending up 2.5" deep at either end where your hoppers are. 

 

- he mentions useing marine ply. Our bco has always been happy with 18mm osb 3 which is possible 1/3 of the price. 

 

- he also mentions no mechanical fixings or second layer of ply above the insulation. Just suggests "sticking" the VCL to the marine ply then "sticking" the insulation to that and then "sticking" the roofing material to that. The last part I can  understand and should be fine with the correct insulation board but the the other bits I would be a little sceptical about. 

 

- also for your wall plates fix them down about every 500mm with concrete screws and then every 1500 with plate straps. ( the plate straps at least should be in your spec.) also yes to the half laps. 

[ToughButterCup]

4 minutes ago, Construction Channel said:

[...]

- he mentions useing marine ply. Our bco has always been happy with 18mm osb 3 which is possible 1/3 of the price. 

 

- he also mentions no mechanical fixings or second layer of ply above the insulation. Just suggests "sticking" the VCL to the marine ply then "sticking" the insulation to that and then "sticking" the roofing material to that. The last part I can  understand and should be fine with the correct insulation board but the the other bits I would be a little sceptical about. 

[...]

 

Thanks for the nudge about marine ply. Now I know what I want for Christmas.

 

''...Other bits....' the sticking bits? 

[Construction Channel]

It all depends what VCL is being used normally we would just use thin gauge polythene but as we all know not a lot sticks to that. So I assume he has specked you a suitable product that he can guarantee will adhere to both the foil faced PIR and to the ply. 

[PeterW]

I reckon he's counting on the weight of the sedum holding this lot down ....

 

270mm of PIR is more than enough for that roof - you could get away with 200mm and still be well inside BRegs without even considering the sedum. 

 

How much of a flat roof is there ..?? 

[Construction Channel]

5 hours ago, Construction Channel said:

- also for your wall plates fix them down about every 500mm with concrete screws and then every 1500 with plate straps. ( the plate straps at least should be in your spec.) also yes to the half laps. 

it appears i may have put this in the wrong thread, this is for plates, not rim joists

[Simplysimon]

there is no need for tapered firrings to run in the same direction as the joists, you can work out the fall over the distance and every 400/600 centres cut a rectangular section to run at 90deg to the joists. the sections will all be a different height to create the fall and if you have a saw you can make them yourself these are graded firrings

[jamiehamy]

8 hours ago, Construction Channel said:

 

 

- he mentions useing marine ply. Our bco has always been happy with 18mm osb 3 which is possible 1/3 of the price. 

 

- he also mentions no mechanical fixings or second layer of ply above the insulation. Just suggests "sticking" the VCL to the marine ply then "sticking" the insulation to that and then "sticking" the roofing material to that. The last part I can  understand and should be fine with the correct insulation board but the the other bits I would be a little sceptical about. 

Our roof has a 5 degree pitch. Joists, 18mm t&g osb, Elotene foil faced self adhesive VCL,  150mm Kingspan (can't remember which) stuck down and EPDM contact adhered to that. It's all there 18months later as good as the day it was laid :-) 

[Calvinmiddle]

We have a small pitch as well, just did it with the joists and didn't worry about have a flat ceiling on the inside. But our element of flat roof is in the living room which is open to the steeper pitched roof kitchen.

Will link to the post with the house pics in a bit

[Construction Channel]

12 hours ago, jamiehamy said:

Our roof has a 5 degree pitch. Joists, 18mm t&g osb, Elotene foil faced self adhesive VCL,  150mm Kingspan (can't remember which) stuck down and EPDM contact adhered to that. It's all there 18months later as good as the day it was laid :-) 

 

Fair enough. How did they fix down the trims around the edges? 

[jamiehamy]

1 hour ago, Construction Channel said:

 

Fair enough. How did they fix down the trims around the edges? 

I'll get a pic when I can but the rubber laps down to the fascia - and there is a trim nailed in which catches the rubber and guides the runoff into the gutter. It's a nice mechanical fix right round the perimeter. 

[Tennentslager]

Check out this site for EPDM supplies and pictures

https://www.rubberroofs.co.uk

I bought from them as they supply the exact size you need...most others sell by the full metre, annoying if you need it 3.1x 8.1 for instance.

[jamiehamy]

1 hour ago, Tennentslager said:

Check out this site for EPDM supplies and pictures

https://www.rubberroofs.co.uk

I bought from them as they supply the exact size you need...most others sell by the full metre, annoying if you need it 3.1x 8.1 for instance.

I used these guys and Permaroof Shop. Both very helpful and know their stuff. 

[Silage]

Ca I ask does anyone have detailing of drainage for  a parapet roof. It was mentioned above that a drainage 'gutter' be formed in the insulationto fal to the outlet position? Is this necessary. Would not a depression at the outlet be sufficient? My architechts drawings show neither. I am going with posi-joist, firings, 18mm OSB, VCL, 220mm insulation (120mm on green roof), mechanical fixings, EDPM. Firestone mention topping the insulation with 12.7mm ISOGARD HD which is fibreglass covered PIR - is this necessary, why not 11mm OSB?

 

[jack]

2 hours ago, Silage said:

Can I ask does anyone have detailing of drainage for  a parapet roof. 

 

This is what was designed for ours:

 

 

The onsite detail was slightly different, in the the channel walls aren't tapered in plan, as shown on this.

 

I'd absolutely include these channels, ensuring falls the full length of each channel and out through each parapet. You don't want any pooling water to sit there, and potentially go through freeze/thaw cycles (something I'm sure was a factor in the leaks we had).

 

One thing we don't have, but will get around to one day, is an emergency overflow pipe set above the outlet level, draining outwards to a highly visible place. If somehow all the gutters get blocked, and water builds up, I'd like visible evidence rather than waiting for problems.

[Silage]

@jack great reply thanks.