How much slope is needed on a flat roof?

[tintin99]

Hi,

Back in Jan you guys gave me some very helpful info on replacing a flat roof on a free-standing 6m x 3m garden room. Well, four months on I'm finally fixing the deck and hope to finish the whole thing this week...weather permitting.

Anyway, to provide the slope I used 50mm to 0mm firrings but now when I put a spirit level on the deck there are some areas where it appears to be dead flat. I'm naturally worried I'm going to get puddling in those areas. I'll be covering the roof in grit but I don't know whether that will help to reduce puddling (I suppose it will hide it if the water's not too deep!).

So any advice would be welcome...am I likely to get significant puddling and is there anything I can do about it at this stage...the only thing I can think of is to increase the slope by somehow jacking up the whole roof structure at the back edge.

Thanks in anticipation!

 

[breezer]

AFAIK the "grit" is only there to protect the felt itself from sunlight.

 

[masona]

Anyway, to provide the slope I used 50mm to 0mm firrings but now when I put a spirit level on the deck there are some areas where it appears to be dead flat.

Where about is the flat area?

 

[tintin99]

I've checked again over the whole area and the only place where it's dead flat is towards the back right corner. The gutter will be at the front. However the angle of the slope does vary over the rest of the roof and is pretty small in some places - the firrings weren't cut very well so that's probably why.

 

[masona]

Hmm, is it possible to raise the back about 50mm or fit another firring on top?

 

[tintin99]

I think adding extra firrings would be too much work now. It might be possible to jack up the whole structure and fit an extra length of 50mmx50mm beneath the back wall plate, but that wouldn't be ideal because it would create a triangular gap down the sides (which I could fill with a couple of firrings I suppose).

Hmmm...50mm to 0mm firrings are pretty much standard aren't they?...so I guess they must work...it just seems such a small slope.

 

[masona]

I think adding extra firrings would be too much work now. It might be possible to jack up the whole structure and fit an extra length of 50mmx50mm beneath the back wall plate, but that wouldn't be ideal because it would create a triangular gap down the sides (which I could fill with a couple of firrings I suppose).

Put a fasica board to hide the gap.

Hmmm...50mm to 0mm firrings are pretty much standard aren't they?...so I guess they must work...it just seems such a small slope.

They do work if the ceiling joists are level.
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