Maximum Overhang for a Flat Roof?

KC Cantiaci · 29 Oct 2018

Hi

Does anyone know what the maximum overhang can be for a flat roof construction depending on joist size?

We are building the roof and want to create an overhang that acts as a bit of a canopy above a bi fold door to shield it a bit from rain. We are using 9x2 joists for the span but these will be notched down to 5" for the overhang.

Any advice would be appreciated.

^woody^ · 29 Oct 2018

geraldthehamster · 30 Oct 2018

Is there such a thing as a span table for cantilevered joists?

KC Cantiaci · 30 Oct 2018

^woody^ said:
2m

2m.....with 5" joists?

KC Cantiaci · 30 Oct 2018

geraldthehamster said:
Is there such a thing as a span table for cantilevered joists?

I dunno. I couldn't find anything online hence the question. To be honest, I was only planning on extending out maybe 600mm or so and if I was running the 9x2's all the way, I wouldn't even doubt it. But as they will be taken down to 5" or thereabouts, it's made me question it. Not sure if there is anything like a discreet bracket of some kind and/or whether I need it?

geraldthehamster · 30 Oct 2018

I would have thought that 5x2 over 600mm would support the roof OK. Especially if it's a warm roof with OSB-Celotex-OSB bridging the overhang and the main roof. I'm not sure I'd want to jump around on it though.

Disclaimer: I'm not a builder, though I have DIYed a few flat roofs.

KC Cantiaci · 30 Oct 2018

geraldthehamster said:
I would have thought that 5x2 over 600mm would support the roof OK. Especially if it's a warm roof with OSB-Celotex-OSB bridging the overhang and the main roof. I'm not sure I'd want to jump around on it though.

Disclaimer: I'm not a builder, though I have DIYed a few flat roofs.

Thanks for the reply. Yes, it will be a warm roof. Another thing I'm debating is the OSB for the top? I'm going to use T&G for the deck and I was going to use T&G for the top but not sure if I need to. My main reason was more for keeping the boards level without lippage to create a smoother surface for the epdm. But I could use 11mm osb??

I've also yet to select a vapour barrier for the deck. The one I keep looking at is the Alutrix 600 but it's expensive. My thought is that if I'm driving screws through the insulation and barrier, I want as best a seal on it as possible. Didn't know what other alternatives there are??

geraldthehamster · 30 Oct 2018

To be honest I was a tightwad when I did mine, and used 11mm non-T&G OSB for the top and bottom deck. I was careful to stay over the joists while fixing the bottom deck, but once the Celotex was on and the top deck over that, it was quite solid to walk on.

EPDM is easy to do. if you haven't already, watch the videos on the Rubbaseal website.

Not an expert on vapour barriers, sorry. Except don't get too hung up about them. In theory EPDM is slightly breathable.

KC Cantiaci · 30 Oct 2018

geraldthehamster said:
To be honest I was a tightwad when I did mine, and used 11mm non-T&G OSB for the top and bottom deck. I was careful to stay over the joists while fixing the bottom deck, but once the Celotex was on and the top deck over that, it was quite solid to walk on.

EPDM is easy to do. if you haven't already, watch the videos on the Rubbaseal website.

Not an expert on vapour barriers, sorry. Except don't get too hung up about them. In theory EPDM is slightly breathable.

I've laid epdm over garage roofs but they weren't insulated. My understanding is that epdm is breathable for the top to allow moisture to evaporate but the vapour barrier is to go on top of the deck and below the celotex to prevent condensation from underneath?

^woody^ · 30 Oct 2018

KC Cantiaci said:
2m.....with 5" joists?

Yeah.

C24, close joist spacings and light loads will span about 4m, so half of that.

KC Cantiaci · 30 Oct 2018

^woody^ said:
Yeah.

C24, close joist spacings and light loads will span about 4m, so half of that.

Joists are going to be spaced at 400mm centres so to go out 600mm should be fine. Is there a standard overhang size for a flat roof? Some kind of usual practise for this sort of thing??

^woody^ · 30 Oct 2018

The minimum is 50mm to allow for a drip. Anything else is down to design, but the more the better.

Ensure to detail the fascia so that rain is cast off and does not track back under the soffit.

KC Cantiaci · 30 Oct 2018

^woody^ said:
The minimum is 50mm to allow for a drip. Anything else is down to design, but the more the better.

Ensure to detail the fascia so that rain is cast off and does not track back under the soffit.

Thanks Woody

geraldthehamster · 30 Oct 2018

My understanding is that epdm is breathable for the top to allow moisture to evaporate but the vapour barrier is to go on top of the deck and below the celotex to prevent condensation from underneath?

Yeah, that's the theory. Though if you think about it, unless your roof is leaking, the only moisture that might need to evaporate would be condensation from underneath.

Don't obsess about tiny fixing holes in the VB is all I'm saying.

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Maximum Overhang for a Flat Roof?

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