Garden room roof support

[Tommy-Lee Staples]

Hi all,

I am planning to build a garden room which will be 6.6 meters wide and 5 meters long/deep.

I have managed to find joists that are 6 meters long which will give me the overhang on the front and back.

The roof will then have firring strips on top, 18mm OSB board and then EPDM rubber on top. There will be nothing else on the roof apart from the odd rain/snow.

Will the proposed plans with struts in-between make a strong enough roof? I would rather not have to install a steel if I can help it.

 

[RonnyRaygun]

Snow load depends on where in the country you are.

However, you should really allow for some access load too. Either way I think it would be a bit tight to rely on 50x200 joists at 400crs.

I'd want to go for 50x225mm C24 at 400mm crs or double up the 50x200s.

 

[Tommy-Lee Staples]

I am in South East London.

Thanks for your reply. What about it if I made a couple of the joists 'flitch beams' like below

 

[RonnyRaygun]

Right, so the snow load will only be around 0.3kN/m2.

But even allowing no loading for access you will still need the joists at maximum 300mm spacings (with 50x200 C16). You'd also need to allow enough slope that ponding doesn't become an issue.

Bear in mind that there will probably be occasions where you or someone else will be walking around up there. It's not, I assume, subject to building regs so it's up to you, but I'd still be doubling them up.

Flitch beams don't really help as the joists between will still have to span the full 5m

 

[George_engineer]

Yeah you'll need to go to 220mm depth to span 5m.

As it's a lot of timber it might be worth looking at the engineered timber option - I-joists or metal web joists.

 

[cdbe]

Use the shortest span (4.4m) fix some ladders for your overhangs, in the small room use 4x2. If you don't need a flat ceiling inside, skip the firings and just set the joists to fall.

 

[Tommy-Lee Staples]

Use the shortest span (4.4m) fix some ladders for your overhangs, in the small room use 4x2. If you don't need a flat ceiling inside, skip the firings and just set the joists to fall.

Thanks for your reply. I do need a flat ceiling inside.

 

[Dereekoo]

Running the calcs 200 x 50 C24 timbers spanning 5m with a standard snow loading of 0.75kn/m2 and a roof dead load of 0.25kn/m2 then the deflection would be 10mm as against standard max of 14mm, the max bending moment in the timber would be 50% of what it can actually take. Using C16 deflection would be 13mm and bending moment 75% of allowable

FYI https://www.timberbeamcalculator.co.uk/en-gb/span-table/flat-roof-joists?load=0.75&class=C16

 

[Tommy-Lee Staples]

Right, so the snow load will only be around 0.3kN/m2.

But even allowing no loading for access you will still need the joists at 300mm minimum (with 50x200 C16). You'd also need to allow enough slope that ponding doesn't become an issue.

Bear in mind that there will probably be occasions where you or someone else will be walking around up there. It's not, I assume subject to building regs so it's up to you, but I'd still be doubling them up.

Flitch beams don't really help as the joists between will still have to span the full 5m

Thank you for your reply, not subject to building regs. Okay your advise would be 300 crs or 400 crs with two joists bolted together?

 

[Tommy-Lee Staples]

Yeah you'll need to go to 220mm depth to span 5m.

As it's a lot of timber it might be worth looking at the engineered timber option - I-joists or metal web joists.

I was looking at I-Joists last night actually, I like the fact they are stronger, more energy efficient and can span longer distances. I assume would not need struts in-between?

 

[George_engineer]

I was looking at I-Joists last night actually, I like the fact they are stronger, more energy efficient and can span longer distances. I assume would not need struts in-between?

You would, but the supplier will be able to provide the information.

 

[RonnyRaygun]

Running the calcs 200 x 50 C24 timbers spanning 5m with a standard snow loading of 0.75kn/m2 and a roof dead load of 0.25kn/m2 then the deflection would be 10mm as against standard max of 14mm, the max bending moment in the timber would be 50% of what it can actually take. Using C16 deflection would be 13mm and bending moment 75% of allowable

FYI https://www.timberbeamcalculator.co.uk/en-gb/span-table/flat-roof-joists?load=0.75&class=C16

At what spacing? I think 0.25kN/m2 is a bit low once (presumably) plasterboard, OSB, insulation, firrings, and finishes have been added.

Plus it's most likely the joists won't be a genuine 50x200 but will be regularised down to 44x195 or thereabouts, at which point they fail, even if only 0.25kN/m2 is taken (C16 at 400mm crs). That's not allowing for the short term point loads that should be considered.

 

[Tommy-Lee Staples]

I have looked at the cost of I Joists and they are quite expensive. It would still be cheaper to bolt two joists together than doing it all in I Joists.

 

[Dereekoo]

At what spacing? I think 0.25kN/m2 is a bit low once (presumably) plasterboard, OSB, insulation, firrings, and finishes have been added.

Plus it's most likely the joists won't be a genuine 50x200 but will be regularised down to 44x195 or thereabouts, at which point they fail, even if only 0.25kN/m2 is taken (C16 at 400mm crs). That's not allowing for the short term point loads that should be considered.

440c/c. re 0.25Kn/m2 quite possible Ronnie but then the 0.75 snow load could be considered generous, its up to the OP to work actual loading out 22mm chipboard is about 0.15kn/m2, so 18mm OSB with rubber?? . Whether its regularised or not or to be plasterboarded who knows I just work on spec given by the OP.

 

[tony1851]

I was looking at I-Joists last night actually, I like the fact they are stronger, more energy efficient and can span longer distances. I assume would not need struts in-between?

I know you've now (sensibly) discounted I-joists, but how would they have been more 'energy efficient'?