Building Flat Roof

8 Mar 2018
Reaction score
United Kingdom
Hello All,

I am building a workshop, the plan was for 8m x 5m which planning & building regulation granted permission. But would like to have more space and build it bigger.

The new plans will be for 10m x 7m.
I am stuck on the flat roof design, any advice is greatly appreciated.

How should I design the flat roof? Use RSJ? Span distance? Suitable size joist? RSJ? Max Centres? etc.

I did search the web and came up with I beam, JJI, Easy Joist etc but quotes came in at £1999+vat which is expensive for a workshop.

Original Plan:
Warm Flatroof - GRP>18mm OSB3>90mm Insulation>VCL>12mm Plywood deck>firring>75 x 225 joist>12.5mm plasterboard>3mm skim

Thank you,
Sponsored Links
How about glulam beams?

90 x 405 has a loading of 2.4t UDL over 7m length.

2 glulams, 1 at 3.3m one at 6.6m. Then you could use 175 x 45 joists in between.

A 7m 90 x 405 costs about £250+vat

Im not a SE so above is just an idea, so its not professional advise.

The snow load on a 70m sq roof is a lot!
Sponsored Links
We are currently using Posi-Joists on a 7.1m clear span and they are big bu££ers! 120mm top and botton chords and 421mm deep. £2880 all in for 15 units.

The blocks look tiny compared to the joists

Wow that is a big joist, If there is no other alternatives I'll see what the best price I can get the joist for.

I did message a few SE but probably cheaper to use I beam jji etc.

Thanks for the picture it gives a good idea on the size.

Was considering low pitched roof but guess it will be as much.

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local