Garden building roof joists

[JDW]

Hi, I'm after a bit of advice on this build if possible. It's the roof, can I go with 44 x 95 finished C24 instead for the roof joists (perhaps at 400 or 450 centres?

I've possibly dropped a spheroid and had the C18 70x44 stud delivered now I'm thinking I should have gone 95x44 to better support the planned 145 C24 joists..?

I'd appreciate any advice and guidance, thank you.

Cheers

Jim

 

[Jameshow]

I think you will be fine.

The benefit of going to 95mm studs is for insulation.

If your worried you could go to 400 centres under each joist.

63 X 36 cls would definitely be a bit light.

Cheers James

 

[topchippyles]

Hi, I'm after a bit of advice on this build if possible. It's the roof, can I go with 44 x 95 finished C24 instead for the roof joists (perhaps at 400 or 450 centres?

I've possibly dropped a spheroid and had the C18 70x44 stud delivered now I'm thinking I should have gone 95x44 to better support the planned 145 C24 joists..?

I'd appreciate any advice and guidance, thank you.

Cheers

Jim

That will be fine

 

[JDW]

That was easy, thank you.

 

[JDW]

I think you will be fine.

The benefit of going to 95mm studs is for insulation.

If your worried you could go to 400 centres under each joist.

63 X 36 cls would definitely be a bit light.

Cheers James

Thanks James, I am insulating. I'm putting 50mm pir in the floor (3x2 joists). I was also going to put 50mm in the walls - not enough room for circulation? perhaps I could go 40mm pir?

Cheers

Jim

 

[Spectric]

Remember a lot of strength in a roof truss is not in the material but due to the design. I used a ridge beam 200 x 40 and then constructed the roof trusses in situ rather than make them first and then install. I used 95 by 45 CLS for them and then overboarded (Sarkin) before getting the roofer in to fit slates.

 

[Jameshow]

I think 20mm air gap Will be fine.

Any water vapour can evaporate away easily enough. I would put the board up against the OSB /ply you are using on the inside.

Cheers James

 

[JDW]

I think 20mm air gap Will be fine.

Any water vapour can evaporate away easily enough. I would put the board up against the OSB /ply you are using on the inside.

Yes, I'm going to put the pir up against 9mm WBP internal lining; and use breathable roofing underlay under the external cladding and under the roof felt.

Cheers

Jim

Cheers James

 

[kwigly]

, can I go with 44 x 95 finished C24 instead for the roof joists (perhaps at 400 or 450 centres?
[/QUOTE]

No.
stick with your 145 deep roof joists.
(the 44x77 studs should be OK for walls, but have a double thickness header along the top to distribute the load from the roof joists)

 

[JDW]

, can I go with 44 x 95 finished C24 instead for the roof joists (perhaps at 400 or 450 centres?

No.
stick with your 145 deep roof joists.
(the 44x77 studs should be OK for walls, but have a double thickness header along the top to distribute the load from the roof joists)
[/QUOTE]

Thank you, I appreciate your input but I now have conflicting opinions. Why not the 95's?

Cheers

Jim

 

[Steve_in_Lincs]

This may help:

https://www.buildingregs4plans.co.uk/guidance_flat_roof_structure.php
You have an open span of nearly 3 metres, and as per the link you need to consider environmental loads as well as the basic roof covering weight etc.

I have used 145 x 47 C24 joists for what could loosely be called a floating floor in our bungalow refurbisment, and to say they bounced before the mid span supports were put in is an understatement. To use anything less in a flat roof most likely will not provide the structural integrity required for the span you have in your drawing, hence why the link above indicates a minimum 150 x 50 as a common rafter size. Personnally I would go for 175 or even 200. 200 being the optimum as it means you could infill with 200mm of rockwool or similar for insulation, as that also is now the minimum insulation depth now specified in building regs for homes, but probably a bit over the top for a garden building, but then it depends if you want to use it in the middle of winter I guess. I would also double up on a couple of the studs too, to go with the double thickness header.

There will be a lot of weight above your head, so kwigly's advice is sound.

 

[Jameshow]

Yeap 150mm deep.

Or as an old chippy told me 18" span per inch of depth!

Cheers James

 

[JDW]

Thanks Steve, my concern was whether the 44x70's could happily support the bigger roof joists.

I'll go with 145 roof joists, double up on every other stud and double the headers. I'd rather over engineer and keep the roof up nice and sound.

I am planning to use it all year round - roof insulation:

A. 50mm PIR + 95mm air gap
B. 100mm PIR + 45mm air gap
C. 50mm PIR + 95mm rockwool
D. 100mm PIR + 45mm rockwool
E. None of the above/ something else?

 

[Steve_in_Lincs]

Regarding the roof joists, don't forget the noggins, they will add some strength to the outer sheeting and stop the joists twisting. Roof insulation. Using all Rockwool is probably the cheapest option, and while not the easiest to install from underneath in the roof space - which is how I did mine, due to low roof pitch angle, you could always put yours in place before you cover the roof. Also, you may want to consider a breathable membrane on the inside of the joists and around the internal side of the studs before cladding. This should provide an almost air tight inner seal, which will stop moisture getting into the structure from inside, but still allow the building to breath. As for an air gap between the rockwool and outer skin, I am not sure if it would need one if you have the insulation packed in well enough, so there is no gap between internal and external skins, as the roof and walls shouldn't sweat if no moisture can get in. The effect would be much the same as if the structure were spray foamed, as is quite popular in warehouses and farm buildings. Others may have more knowledge/experience than me on insulation, but I have 200mm rockwool in the roof of the bungalow and 150mm between the floor joists, and I have been very impressed at its performance over this last very cold winter, with the heating running only on eco mode, the place has been very cheap to heat, and maintaining a constant temperature has seen the boiler barely ticking over. But if you have the budget for PIR all over, then 100mm would be the minimum I would have thought, as is similar for houses.

 

[Fergie 307]

To give you some idea I have used 4x2 framing in walls, and 8x2 joists hung between a central girder spine and the walls. Total span 24 feet. Just make sure your wall uprights are positioned below the joists, the wood is very strong in compression. For what you are doing I would have thought 150mm is adequate, but Personally I would still go for 4x2 for the walls, stronger and gives you more room for insulation. If you already have all the timber in the smaller size then I wouldn't double up the uprights, just put them at smaller centres, and double up the top plate.

 

[RobinBHM]

Thanks Steve, my concern was whether the 44x70's could happily support the bigger roof joists.

I'll go with 145 roof joists, double up on every other stud and double the headers. I'd rather over engineer and keep the roof up nice and sound.

I am planning to use it all year round - roof insulation:

A. 50mm PIR + 95mm air gap
B. 100mm PIR + 45mm air gap
C. 50mm PIR + 95mm rockwool
D. 100mm PIR + 45mm rockwool
E. None of the above/ something else?

On my garden cabin 4.5m x 4.2m I did 120mm celetex on roof, 50mm +50mm walls, 70mm floor.

I used 95x 45 for wall studwork, 50mm celetex in between, then 18mm Osb then 50mm outside, then 25mm battens, then cladding.

Inside I did 18mm moisture resistant mdf.

The 95x 45 with 50mm insulation allows electric cables to go in and if you put on 18mm board, it gives over 50mm from internal face to cables, so you don't have to worry about safe zones.

Also a 50mm space allows plenty of room for drilling the studs to run the cables.

I went for insulation levels that pretty much match building regs as I figured spending a few hundred quid up front would save heating costs. I have to say it's made a fantastic office, In the winter if I use it in the evening and heat it, when I go back in the morning, it's still retained some warmth. And it feels bone dry, never a hint of condensation, I have got A4 and A3 printers - the paper never gets damp.

I heat it with a fan heater for half an hour, then a oil filled rad, once it's reached say 18 deg, it needs little heat to maintain warmth.

I used tyvek house wrap externally and a 25mm air gap before cladding - that's vital for a dry structure

 

[JDW]

That's a great working example Robin, very helpful, thanks. Can you give me some more info on the roof design if poss?

145's? 400 centres? Slope?

An air gap makes sense to me to allow air flow in the roof, same as the walls but some people say push your PIR right up against your roof OSB3 so there's no room for condensation - maybe both techniques work, I don't know.

Cheers

Jim

 

[RobinBHM]

That's a great working example Robin, very helpful, thanks. Can you give me some more info on the roof design if poss?

145's? 400 centres? Slope?

An air gap makes sense to me to allow air flow in the roof, same as the walls but some people say push your PIR right up against your roof OSB3 so there's no room for condensation - maybe both techniques work, I don't know.

Cheers

Jim

The roof of my garden cabin has a 2m x 1.5m glazed lantern so the roof construction was quite different - it's made of 2 glulam beams 90 x 225 then the joists are just short ones about 1200 or so long each about 90mm one end own to about 60mm.

I did a warm roof with 11osb over the joists, then vapour barrier, then 120mm insulation, then 18mm Osb, then EPDM.

I would suggest you do a hybrid roof with insulation in between the joists, tight to the top surface. Tape all the insulation with foil tape around each joist to complete the vapour barrier.

Hybrid systems aren't tested and therefore not approved by building control or insulation manufacturers but as long as you do robust detailing to ensure you have a complete vapour barrier on the warm side, you should be fine.

Have a look at Oakwood garden buildings on YouTube - he has a video explaining flat roof construction.

 

[Jake]

Sounds like an interesting build Robin - any pictures or drawings anywhere? (I'm in the process of planning a similar home office thing).

 

[RobinBHM]

Sounds like an interesting build Robin - any pictures or drawings anywhere? (I'm in the process of planning a similar home office thing).

I did all the drawings using good old pencil and paper! -I will try and see if I can find them and post and will take some pics

When I built it, I was running a joinery company which manufactured the lantern roof and the windows / doors.

I have to say the lantern transforms the room -it makes it feel so much more spacious as although the flat ceiling perimeter is only 2150 to meet permitted development high the lantern glass is at 2600 (I dug out so its 2500mm above ground level.