600 c\c studs for garden room

[ricardo100671]

I am planning a small garden room build of 2.4x4.8m and was hoping to use 600mm stud centers with 11mm OSB sheathing and 90mm PIR to, not only, save on materials costs but recon this should also reduce thermal bridging. Since most ready-made garden shed typically have the same and given the extra rigidity of the OSB and insulation, I feel this should be good enough, but would like to hear from anyone who has experience having done similar or if there is some really obvious reason why this is a bad idea please.

Thanks in advance.

[SimonD]

My house whole timber framed first floor is built with 600 centres, but it does use 140 studs as opposed to 63 or 89. If you're using 90mm PIR then you'll want to be using the 140x38mm CLS or alternatively 145x45mm C16 anyway, so no problem at all. Just bear in mind that unless you specifically order 2400 x 1200 OSB, you'll be getting sheets that are 2440 x 1220 and will therefore need to shave them down when you sheath the frame.

[ricardo100671]

1 minute ago, SimonD said:

My house whole timber framed first floor is built with 600 centres, but it does use 140 studs as opposed to 63 or 89. If you're using 90mm PIR then you'll want to be using the 140x38mm CLS or alternatively 145x45mm C16 anyway, so no problem at all. Just bear in mind that unless you specifically order 2400 x 1200 OSB, you'll be getting sheets that are 2440 x 1220 and will therefore need to shave them down when you sheath the frame.

Thanks @SimonD I should have said, I am planning to use C16 47x95 stud, o fit the 90mm PIR and thanks for the tip on OSB size.

[LaChab]

That's exactly how I built our 2 "garden rooms", except ours are 4.8 x 3.0, and I used Rockwool (not as good, but cheaper and easier). Been 2 years now and no problems at all.

Have the osb on the inside and covered the outside with membrane and then featheredge, which seemed the cheapest solution.

[ricardo100671]

On 28/05/2026 at 11:03, LaChab said:

That's exactly how I built our 2 "garden rooms", except ours are 4.8 x 3.0, and I used Rockwool (not as good, but cheaper and easier). Been 2 years now and no problems at all.

Have the osb on the inside and covered the outside with membrane and then featheredge, which seemed the cheapest solution.

Thank you @LaChab Exactly what I neded to hear and the idea of OSB in the inside is genius, even more saving as no need to plasterboard and have good service for securing 

[Nickfromwales]

9 minutes ago, ricardo100671 said:

Thank you @LaChab Exactly what I neded to hear and the idea of OSB in the inside is genius, even more saving as no need to plasterboard and have good service for securing 

Have the rockwool held in place by zig-zagging bricklayers string-line, stapling it into position side to side, to keep it in place (from sagging and contacting the membrane). 

[Tony L]

1 hour ago, ricardo100671 said:

Thank you @LaChab Exactly what I neded to hear and the idea of OSB in the inside is genius, even more saving as no need to plasterboard and have good service for securing 

 

Is bare OSB, or even painted OSB a good idea inside a habitable room, especially a garden room, which could get very warm inside during the summer?  I'd be looking into the possible harmful effects of VOC off gassing if I was considering this.

[ricardo100671]

@Tony Thanks for your input. Yes I think that is a cencern that I may have overlooked, but, initially the room will be used as a workshop and if/when converted will be vapour barried before plasterboarding. Would this not address this ?

Also modern, I believe, OSB sold in the UK and EU is typically manufactured to E1 formaldehyde emission standards, and many products emit very low levels of VOCs. The highest emissions occur when the boards are new and diminish significantly over the first few months.

Edited 3 hours ago by ricardo100671

[ProDave]

I wired a house that was completely clad in OSB on the inside because the owner liked the look.  BC insisted on fireproof covering due to the risk of surface fire spread and they found a clear treatment that gave that protection while still looking like OSB.

[saveasteading]

14 hours ago, ricardo100671 said:

the idea of OSB in the inside is genius

 

25 minutes ago, ProDave said:

fireproof covering

If this is a shed that you will sit in occasionally then it probably isn't a bc issue. A fire is unlikely and you would walk out of the door.

If it is a bc issue then the osb is a risk. There have been full scale fire tests on exposed osb and the fire and smoke spread scarily fast. 

I once had a bco agree that masonry paint would suffice as protection as it doesn't burn. But that wouldn't suffice now as test results are required for everything.

The clear protection is intumescent varnish. It is expensive as it needs a thick layer then sealjng coat. : possibly more cost than plasterboard and finishing. It can look OK but can be a bit streaky. Plus, osb can flake and filler would look awful.

The same varnish would be required for ply or t and g.