Best way to get a shallow slope on a flat roof

[David WE Roberts]

Looking at the roof design (I have looked back through previous useful threads).

I am considering 6" (150mm) * 3" (75mm) at 600mm spacing. The span is about 3.5m front to back and about 7.8m side to side. The proposed roofing is metal sheet. I am assuming the joists will span the short (front to back) span.

The issue now is how to engineer the slope (I have read that 1/2" per foot is adequate so a difference of 6" front to back should be O.K.).

I can see some obvious ways:

(1) Front and back wall level. Hang the lower end of the joist flush with the wall plate and rest the base of the high end of the joist on the wall plate. This will require the low end cutting at an angle to sit properly in the joist hanger and the high end notching to sit onto the wall plate. Sloping ceiling.

(2) Front wall one block higher than rear. Hang both ends from joist hangers. Both ends will need cutting at an angle to fit the joist hangers. Sloping ceiling.

(3) Front and back wall level. Hand the main joist parallel from front and back wall. Fit a profiled piece of timber above the joist. Simple hanging, no angles cut, less lateral force on the walls (not that there should be much anyway). Flat ceiling. Downside (if any) seems to be the odd shaped roof void which will give a lot of air space above the insulation at the higher end.

What dos the team think?

TIA

Dave R

[Steve Walker]

One thing to watch ... I used galvanised steel sheet for covering my shallow angle garage roof (existing wooden roof) and got water coming in - it turned out that at shallow angles, the water can run back under the lower edge of the sheet and far enough uphill to get into the structure. I solved the problem by screwing a 3/8" piece of angle 3/4" back from the edge as drip-edge. Luckily there was no problem with the high part of the profile (despite it being flat on top) and so I could just run the angle straight across the bottom.

SteveW

[Bob Minchin]

I'd use method 3 with half the pitch 1 in 48 should be plenty especially with steel which should have less tendency for ponding. ISTR regs for flat roofs suggesting 1 in 80 - I tend to use 1 in 40/50 and have no problems. The pieces of taper timber are called firring strips/battens specified by the length and the timber they are cut from

e.g. N number firring strips ex 3.6m x 50mm x 100mm will give you about 1 :40 fall N number firring strips ex 3.6m x 50mm x 75mm will give you about 1 :50 fall.

The short sides of your block work will need to be angled to match. Cut the blocks at an angle and finish off with a course of bricks. These will be hidden behind the roof fascia.

Bob

[Olav M]

What will it be? Shed or room? If you intend to fit a ceiling, then (3) is correct way. Fall should be 1 in 60 or greater. The "pofiled pieces of timber" are called firring, nailed to top of joists. If fall is in same direction as joists, their shape is a narrow triangle/taper, no less than 13mm thick at thin end. If fall is at right angles to joists, their shape is parallell (each piece) but decreasing in thickness from top of slope to bottom. Takes some thought and calculation. Remember measure twice cut once!! You will need to consider direction of joists and covering together, it has to be fixed into joists.

[David WE Roberts]

Spookily (especially the dimensions)

[John Rumm]

(3) is the traditional way to do a flat roof in a house or extension etc. The slanty bits to add on top being called "firings". The only thing to check is if it will give enough slope for your roofing sheets.

Regarding insulation, you need to decide if you are going for a warm or a cold deck roof. With a felted roof, warm deck[1] is quite easy these days. Lay rigid PIR foam boards on the firings, and then ply screwed down through the insulation, then felt etc on that. With corrugated sheet that may or may not work as well.

[1] Warm deck places the main roof structure inside the warm envelope of the room - the insulation and vapour control later being adjacent to the roof deck on top of the joists. Cold deck is the more traditional roof style with insulation stuffed between or under the roof joists, placing the roof structure on the cold side. This normally means you need some ventilation into the roof void above the insulation.

[Dave Starling]

John, I am going to install a warm deck on my garage. I was advised to lay a sheet of 9mm ply below the celotex. This prevents them squishing on the timber when you screw down the top decking. Also I was trying to work out how to apply foil tape on the bottom of the boards - in order to maintain the vapour barrier on the bottom side of the insulation. Obviously you can't hinge the joints down and at a width of 7 metres, trying to lift up an entire row could be tricky.

Im laying a deck with 18mm 8ft x 2ft T&G spruce ply. I am hoping that will give a stronger deck than butt jointed ply.

Dave.

[John Rumm]

I am not convinced its really necessary with PIR foam - its relatively dense and quite hard to deform much along the top of a joist. I laid mine directly onto the firings, and put 19mm shuttering ply on top. Screwed down tight, you get a couple of mm of compression of the foam on the joist, but not much.

You could simply lay a layer of visqueen sheet over the joists before the PIR boards - then there is no need to tape. Only thing to watch is that you are making it water tight as soon as the boards go on - otherwise you end up with puddles of water trapped on the sheet!

It probably will. As long as its well screwed down at all the edges so that they can't move relative to each other then you are usually ok (the differential movement is what will fracture the felt when the roof is walked on.

Note you can get insulation boards with the ply pre bonded to them. Its pretty thin ply mind you - about 6mm. Like you I wanted something stronger (and the plain foil faced insulation I could buy for a fraction of the price of the "warm deck" versions)

[David WE Roberts]

I intend to put a wooden wall plate all round the top of the wall. Any reason why I can't use a flat wall with firring strips on the top of the wall plate to geve a slope and a seal? It looks a lot easier than trying to cut a very shallow angle in dense concrete blocks.

[Dave Starling]

The approach I took to get me to the stage of putting the roof timbers on my garage are similar. At the top of the walls, screw some shuttering either side. One side lower than the other to give the desired shallow angle across the span. Use a pretty strong mix, level to the shuttering. When its set, lay dpc strip on and put your timber on that. I've got 150x50 regularised for a span of around 3 metres side to side and i'm going with 550mm spacing. I'm also going to use metal straps, from the walls bolted to the roof timbers- I think they are called hold down ties? Supposed to prevent the entire roof lifting off in strong winds.

Dave