Flat Roof Span 5.4m Joist Sizing

[muh3]

Hello all,

I'm in the process of planning a large garden room about 6.4m x 5.6m, the clear span would be 5.4m. Looking at the span table I can't see any timber over 220mm but looking at it I should be fine with 47x250 timber?
This will be a warm roof with 18mm plasterboard, joist, 18mm OSB, 100mm insulation, 18mm OSB and EPDM. so a fair bit of weight.

I might also put 10 solar panels on it later date probably with a ballast system. Would that work with the 250mm timber or should i get 300.

Thank you.

 

[George_engineer]

The risk with flat roofs is ponding and with timber it can creep and deflect over time. If water then collects in the centre this adds to the weight causing more deflection and ponding and so on.

You should split the span with a steel beam (a 178UB should be sufficient).

If this requires building regulations then you may need to get an engineer to specify the steel beam.

 

[freddiemercurystwin]

A 178 over 5.6m? Seems a bit small to me.

 

[George_engineer]

It's only a timber roof. If it were a floor then I'd agree. But bumping up to a 203 wouldn't increase cost significantly.

A lot will depend on the solar panel ballast.

 

[muh3]

The risk with flat roofs is ponding and with timber it can creep and deflect over time. If water then collects in the centre this adds to the weight causing more deflection and ponding and so on.

You should split the span with a steel beam (a 178UB should be sufficient).

If this requires building regulations then you may need to get an engineer to specify the steel beam.

Hi George, thanks for that, I did not consider a steel beam. Building regs is not needed (under 30m2), I had a quick look, this would be 133kg for a 6.4m beam. That is not going to be easy to get in the back of the garden and up the roof. I assume i could then just us 6x2 from the beam as the span is only 2.7m.

How would I best support the beam. The walls are 4x2 timber, would i just double or tripple up the 4x2 under the beam, the garden room will also sit on concrete blocks, I assume it would be best to make sure there is a block under the beams timber to support the weight.

I have added a drawing.

 

[noseall]

Hello all,

I'm in the process of planning a large garden room about 6.4m x 5.6m, the clear span would be 5.4m. Looking at the span table I can't see any timber over 220mm but looking at it I should be fine with 47x250 timber?
This will be a warm roof with 18mm plasterboard, joist, 18mm OSB, 100mm insulation, 18mm OSB and EPDM. so a fair bit of weight.

I might also put 10 solar panels on it later date probably with a ballast system. Would that work with the 250mm timber or should i get 300.

Thank you.

Posi-Joists.

 

[^woody^]

Get that hipster dude to either calculate the required joist sections instead of relying on over-cautious tables, or use calculated engineered joists. Work out the PV panels weight, not guess, and don't forget snow loading.

Alternatively, to design out the risk of settlement of large single span roofs by using a much greater fall than 1:80, or form a very shallow double pitch but still within the tolerance of being a flat roof.

 

[jeds]

Exactly right. Get a quote for posi joists.

 

[George_engineer]

If it's a nice square building then posi-joists are a good option.

It may cost a little more than a steel beam and solid timber joists but you will just need to cost them up.

 

[noseall]

Exactly right. Get a quote for posi joists.

Wonderful additions to any property. Plumbers and Sparks love em.

 

[jeds]

I specify them a lot and builders usually call and say; is it ok if we use 8x4s instead? to which I say no. Next job with the same builder they usually say; can we have those metal joists again. Once you go posi, you'll never go back.

 

[Ivor Windybottom]

Are you staying within the 2.5m height limit for permitted development? This applies if any part of the building is within (either 1m or 2m?) of the boundary.

I haven't checked but if the truss joists are taller than timber for a given loading then this may be an issue. If they're equivalent then that's definitely of interest for me too so would be interested to hear from those who use them, I'm currently about to build something very similar.

 

[mrrusty]

So many garden rooms seem to go with flat roofs to get under the 2.5m. It opens up a lot of possibilities to put planning in (£200ish) - you can then have a pitched roof <2m from the boundary. This is what I did and my build is in progress. (I am also BR exempt as >1m from boundary even though timber frame). You don't need fancy plans - hand drawn to scale is good enough, and the site and location plans you can download from t'internet for £10 each. Unless there are strict planning rules in your area that mean it would be refused. In my case as part of the application I pointed out my preferred design with a ridge p[ut a 2.3m eaves 1m from boundary but under PD I could put a 2.5m wall at the boundary. Passed OK even though there is a neighbour house <10m from my build. https://community.screwfix.com/threads/garden-room.256842/

 

[muh3]

Hi all, I did go down the planning route to create a warm roof and have 2.4m headroom, waiting for a decision. I have emailed a couple of SE's and waiting for a reply with a price. I also had a look at the posi joists but none of the calculators allow for weight for solar panels. I assume this is done again through an SE but I will email one of the companies as well.

 

[noseall]

Are you staying within the 2.5m height limit for permitted development? This applies if any part of the building is within (either 1m or 2m?) of the boundary.

I haven't checked but if the truss joists are taller than timber for a given loading then this may be an issue. If they're equivalent then that's definitely of interest for me too so would be interested to hear from those who use them, I'm currently about to build something very similar.

Not sure what they'll spec', but we once used these monsters on a 7m+ span...

Bigjoist1

• noseall
• 19 Nov 2018

440mm deep with 125mm top and bottom chords!

We use Posis on all our floors.