# Workshop roof fall calculation



## Lazurus (4 Jun 2021)

Ok so the new build is now up to dpc, I am sourcing some easi joists for the roof but cannot get my head around the calculation for the firings to give ne a reasonable advised 1:40 fall over a 5 m span. Can anyone clear my befuddled aging brain on what size firings I need to cut to give me the correct fall? These are to go under the easi joists.


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## Jameshow (4 Jun 2021)

125mm or 5"

6" wouldn't hurt though for little more!

Cheers James


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## Adam W. (4 Jun 2021)

1 mm in every 40 mm

5000/40 = 125

1:40 over 5 m = 125 mm


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## Adam W. (4 Jun 2021)

1:40 is minimum for foul drains a roof can survive with less fall.


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## topchippyles (4 Jun 2021)

Anything between 100-150mm will do it but i would go at 150mm myself


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## Lazurus (4 Jun 2021)

Bl00dy hell that was quick. Lol many thanks


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## RobinBHM (4 Jun 2021)

Adam W. said:


> 1:40 is minimum for foul drains a roof can survive with less fall.



Just to elaborate on this:

According to BS 6229 & BS 8217, *flat roofs* should be designed with *minimum* falls of 1:40 to ensure a finished *fall* of 1:80 can be achieved, allowing for any inaccuracies in the construction. This applies to general *roof* areas along with any internal gutters.

1 in 80 is theoretical min fall for rain to run off.

However flat roofs may have joists that dip and felt often has bumps and ridges and a 1 in 80 fall may result in ponding.

When building orangeries with either fibreglass or EPDM membrane I found a fall of 1 in 50 to 1 in 60 worked fine


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## Adam W. (4 Jun 2021)

I doubt that easi joists will creep if they've been sized correctly.


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## HOJ (5 Jun 2021)

When you say "These are to go under the easi joists" are you raising one end of the joists to give the fall, firings usually go on the top.

Unless that's the intention, with the firings under neath to give you a flat ceiling.


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## OldWood (5 Jun 2021)

I suspect you are talking about 'furrings'- 'firing' is lighting a fire or shooting a gun!! Sorry to be pedantic, but then just to be doubly sure I was correct, I did find this on Google as a touch of 'questionable' humour






Urban Dictionary: furring


Having sexual intercourse in the wild dressed up as an animal.




www.urbandictionary.com





I did enjoy Adam W's rule of thumb, which does conveniently turn into 1 inch beam depth for every metre of span - isn't it fun to muddle up units.
Rob


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## Adam W. (5 Jun 2021)

Thank you.

I should have said 1:40 = 1" every 40"

which makes um er some" every 5000 mm


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## Lazurus (6 Jun 2021)

Yes Firrings to go on top of easi span beams with warm roof over these. Again many thanks to all


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