# Roof steel calculation



## Lazurus (20 Mar 2021)

Anyone advise how to calculate what size universal beams I require to span 5m roof. These will be sitting on block and brick walls to carry flat (slight pitch) roof. Trying to second guess the builder and get some prices worked out?


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## LJM (20 Mar 2021)

You first need to calculate the load ie the weight of the material sat on top. If there’ll be any additional load such as heavy snowfall, or foot traffic, this come in too.

but consider the roof material to be a uniformly distributed load, and go from there. You would normally multiply the load, to give a margin of safety; the margin is greater if it’s to be subject to dynamic loading or if it’s safety critical.


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## LJM (20 Mar 2021)

Once you have a load, you can calculate deflection of a proposed beam section to test it’s suitable


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## Spectric (20 Mar 2021)

Could you not use a shallow pitch apex roof, much better than a flat roof and will last longer.


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## RobinBHM (20 Mar 2021)

I'm not sure what you mean.

Do you have an opening of 5 metres in the wall that needs to support a roof

Or are you talking about the joists having to span 5m ...in which case 50 x 225 C16 will span allowing 0.75kn/m2 snow load.


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## LJM (20 Mar 2021)

I think he simply has a 5m span. Likewise, I would simply use c16 or an engineered wood joist (easily handled by one person at that length); I presume there’s good reason for specifying steel.


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## Lazurus (20 Mar 2021)

Yes the work shop is 8m x 5m so a 5m span, looked into timber but the calculation gave something like a 16" joist so steel would be preferred so not to lose internal roof height.
Thanks for all the advice @LJM


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## Lazurus (20 Mar 2021)

oh and @RobinBHM


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## RobinBHM (20 Mar 2021)

Lazurus said:


> Yes the work shop is 8m x 5m so a 5m span, looked into timber but the calculation gave something like a 16" joist so steel would be preferred so not to lose internal roof height.
> Thanks for all the advice @LJM



There's some good info here that gives span tables for flat roofs

For a workshop you could probably get away with 50 x 200 C16, there will be a bit of sag but that would be easily overcome by increasing the fall, say to 1 in 35.

A steel for that span is probably going to be 150 anyway.



Rafter span tables for surveyors - Roof construction | Right Survey


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## Spectric (20 Mar 2021)

Have you looked at engineered products like I beams, Gulam beams or wavy webed beams, have seen these at long lengths and you are not supporting a floor above. These all use design to deliver strength rather than just mass.


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## HOJ (20 Mar 2021)

I use the Trada span tables to Eurocode 5, from table 7.2 a C24 44mm X 220MM Joist will span 5.09m @ 400 Ctrs with a dead load (gk) of 0.75KN/m2.

Alternatively, contact Read Brothers in Norwich they would do a drawing and calcs for Easi Joists, greater spans and lightweight.

I would suggest steel is an expensive option, I have not long ago installed a steel beam, 3.4m long (178 x 102, 19KG) which cost £91.06 +VAT


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## Lazurus (21 Mar 2021)

The easi joists looks just the thing, I have emailed Reads with and enquiry, great advice thank you @HOJ


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