Joist size for flat roof

[muks11]

Hi all, first post so please go easy on me.

Recently submitted plans for planning permission for a single storey rear extension which is 5.5m deep and 8m wide. It will have a flat roof with Kingspan TR31 insulation above the timber joists. The TRADA doc has 3 loading tables for flat roofs (0.75, 1.00 & 1.5KN/m². Does anyone know which one I should be using for a flat roof of this size?

 

[DevilDamo]

The Trada tables I have list 0.5, 0.75 and 1.0. Where did 1.25 come from? Anyway, providing the flat roof will only be used for access/maintenance every now and then, you should be ok using the 0.75 section. Bear in mind you will have to take into account the "imposed loading" as there are also different tables for that. And another thing... you may find your LABC refer to the Eurocodes and not the Trada tables, which are a little more stringent.

 

[tony1851]

Usually 0.75 is ok for a flat roof, though if you live in an area of higher snowfall, go for 1.0.

 

[tony1851]

you may find your LABC refer to the Eurocodes and not the Trada tables, which are a little more stringent.

Agree, some do. But at the end of the day, OP can pick what cross-section size suits him best. On a flat roof, deflection is likely to be the governing factor and that's only a serviceability issue.

 

[jeds]

The Trada tables I have list 0.5, 0.75 and 1.0. Where did 1.25 come from? Anyway, providing the flat roof will only be used for access/maintenance every now and then, you should be ok using the 0.75 section. Bear in mind you will have to take into account the "imposed loading" as there are also different tables for that. And another thing... you may find your LABC refer to the Eurocodes and not the Trada tables, which are a little more stringent.

He's talking about imposed loads. The 0.5, 0.75 and 1.0 you are looking at are dead loads.

The answer to the question depends where you are in the country. Anything South of Cheltenham/Oxford is 0.75 - anything between there and a line from York/Carlisle is 1.0 - and anything above that is 1.5.

So Leicestershire would be 1.0

 

[DevilDamo]

He's talking about imposed loads. The 0.5, 0.75 and 1.0 you are looking at are dead loads.

I checked that before replying. The imposed loads in the Trada tables are 0.60 1.02 and 1.50, hence querying where 0.75, 1.00 and 1.50 came from.

 

[muks11]

I used the TRADA document which someone has kindly uploaded in this forum somewhere which is where the figures come from. Anyway, I'm just after rough estimate as to what size I would need to understand the costs involved. My SE will be doing the calcs and should confirm correct sizes.

Thanks for your help all.

 

[jeds]

The OP said;

The TRADA doc has 3 loading tables for flat roofs (0.75, 1.00 & 1.5KN/m².

You said;

The Trada tables I have list 0.5, 0.75 and 1.0.

The OP is referring to imposed loads. The loads you list are the dead loads. Different thing.

The answer for the Leicestershire area is still 1.0kN/m².