Flat roof pitch

[KimB]

Hello all, I’m after some advice. I’m planning my garage build which will be starting soon. My garage will be 3.25m wide but the roof will overhang, maybe to the overall width of 3.5m or slightly wider? (Excluding gutter width). Anyway, what would the fall need to be on that span?

As of this current moment I can’t remember the name of the material I was going to use but it’s like a big rubber sheet that gets stuck down and is already pre made to the size you want. Hopefully people will know what I mean haha. Thanks

 

[dan treg]

epdm sheet i think you mean . did one about about 2 weeks ago think the fall was aroung 5 degrees

 

[eLuSiVeMiTe]

Aim for 1:40 to make sure you have at least 1:80 allowing for deviations.

Generally build the deck flat then use stepped or angled fittings to get the slope

 

[KimB]

5

epdm sheet i think you mean . did one about about 2 weeks ago think the fall was aroung 5 degrees

5deg? That’s like 1:80 drop isn’t it? Seen a lot?

 

[KimB]

Aim for 1:40 to make sure you have at least 1:80 allowing for deviations.

Generally build the deck flat then use stepped or angled fittings to get the slope

What do you mean by deviations?

 

[eLuSiVeMiTe]

Try as you might a wall/wall plate will never be perfectly level.
C16/24 timber varies in width.


Say the roof ends up +- 5mm or so out by giving a steeper pitch covers for this.

Best practice

 

[roofman]

50mm to 0mm fairings on or across the joists will be ample for a fall over 4m using EPDM

 

[KimB]

50mm to 0mm fairings on or across the joists will be ample for a fall over 4m using EPDM

So 200mm over a 4m span? Is that right?

 

[roofman]

So 200mm over a 4m span? Is that right?

No 200mm is far too much as your facia will need to be too tall to cover the joists as well as the fairings...honestly 50mm or iff your worried 60mm is ample for a 4m span...imho there is a fine line between ample fall and keeping your facia boards not looking like your roof has a top hat on.

 

[eLuSiVeMiTe]

50mm to 0mm fairings on or across the joists will be ample for a fall over 4m using EPDM

To add to this. If the firings are counter to the joists then 25mm to 75mm

 

[roofman]

this is the fairing https://www.roofingsuperstore.co.uk/product/tapered-firring-for-flat-roofs.html

 

[mike os]

What roofman said

 

[KimB]

No 200mm is far too much as your facia will need to be too tall to cover the joists as well as the fairings...honestly 50mm or iff your worried 60mm is ample for a 4m span...imho there is a fine line between ample fall and keeping your facia boards not looking like your roof has a top hat on.

Wow as little as that, I guess being one big roof sheet helps save the need to have such a steep slope?

To add to this. If the firings are counter to the joists then 25mm to 75mm

I’m not following

this is the fairing https://www.roofingsuperstore.co.uk/product/tapered-firring-for-flat-roofs.html

Blimey thanks for that that’s great to know about those.

 

[eLuSiVeMiTe]

I’m not following

If your nailing them directly over the joists they can taper to nothing.

If there bridging the gaps ,(counter battened) then they need to be thicker to add strength at the shallow end.

 

[eLuSiVeMiTe]

Some jewsons/Travis branches have sawmills and will cut firings to suit.



Another option is stepped firrings.
Cut straight (slight bevel if you like) getting taller every joist. Easy to do on site with a tablesaw or skilly.

 

[KimB]

If your nailing them directly over the joists they can taper to nothing.

If there bridging the gaps ,(counter battened) then they need to be thicker to add strength at the shallow end.

I see what you mean. Yes I’d have them on the joists so I have something to nail the roof sheets to. Great to know it can be as shallow as that. Will save me so much head room with the plan I have for my garage.

 

[KimB]

Some jewsons/Travis branches have sawmills and will cut firings to suit.



Another option is stepped firrings.
Cut straight (slight bevel if you like) getting taller every joist. Easy to do on site with a tablesaw or skilly.

Any pics? I think I know what you mean but I’m not sure on the name of things.

 

[eLuSiVeMiTe]

I looked earlier. Probably some on my pc but I'm on a newish phone.
Just straight cut decreasing in height.

If firings are spaced at 400c each
Stepped firring would be 5mm ish taller than the previous.


Counter battened firings are good for ventilation as they let air circulate between joists.

If your garage is long and narrow really should cut your joists across the shortest length not the long run.

 

[eLuSiVeMiTe]

I'm terrible for taking pictures

Did find this roof though. Was a fun one to do.

Won a local LBA award. Not that I got any of the credit.

Kept the facias parallel and had a leaded step up the solid insulation to the top deck.

Came out very smart.

 

[Arc Tourist]

Hello all, I’m after some advice. I’m planning my garage build which will be starting soon. My garage will be 3.25m wide but the roof will overhang, maybe to the overall width of 3.5m or slightly wider? (Excluding gutter width). Anyway, what would the fall need to be on that span?

As of this current moment I can’t remember the name of the material I was going to use but it’s like a big rubber sheet that gets stuck down and is already pre made to the size you want. Hopefully people will know what I mean haha. Thanks

Hi Kim, I'm building a garage at the moment with basically a flat roof. The length of the span doesn't matter, what matters is the drop per foot, which in my experience the "minimum" would be 1/2" per 12", or round it off to 3 cm per 30 cm.

In my case the span is 5 metres wide, so I am lowering the opposite wall by one block. Here is a rough sketch, hope it helps.