Garden room

nigelm · 13 Aug 2008

I am in the process of designing a garden room and would like some advice as to the correct construction method that I should use.

At the moment I am thinking of a wood frame with 70x50 studs on 500mm centres, this will be filled with rigid foam insulation boards. Outside will be sheeted with 9mm ply, a breathable membrane, tanalised battens then 18mm cedar cladding. Inside 9mm ply sheeting covered with t&g softwood.

The roof will be 200x 50 joists (4m span) on 500 centres, 18mm ply, rigid insulation topped with 3 layers of bitumen felt.

Any advice re the suitability of this construction method / material sizes would be most appreciated.

(size of the finished building will be approx 4metres square.)

Deluks · 13 Aug 2008

Sounds like you have it licked. Although you could easily do away with the ply inside and out. Just staple the breathable membrane over the studs, then vertical battens to attach the cladding to. Inside use a vapour barrier (polythene sheet) over the studs, then fix the t&g straight over this.

For the roof, if you intend to insulate between the rafters, then a simple 25mm layer of insulation over the top, covered with ply then felt, should do the trick.

What's the intended use?

^woody^ · 14 Aug 2008

Yes its OK, but change the joist and stud centres to suit standard board widths ie 400 or 600 centres.

Also if possible go for insulation between joists, and if needed below the joists, and this avoids an ugly thick roof when viewed from the outside - or just adapt the fascia detail to make it look thinner

And have you considered EPDM instead of built up felt? It may perform better

duvet77 · 14 Aug 2008

^woody^ said:
Also if possible go for insulation between joists, and if needed below the joists, and this avoids an ugly thick roof when viewed from the outside - or just adapt the fascia detail to make it look thinner

It is preferred to have insulation above the joists to create a warm roof otherwise you need to start ventilating between the roof joists.

You'll want a 500 gauge polythene vapour barier to inside of walls and ceiling as already mentioned. You could use duplex plasterboard if you weren't having a timber interior.

I'd personally keep the 9mm ply (or OSB) sheathing beneath the breather membrane, it'll add extra rigidity to the whole structure

Shytalkz · 14 Aug 2008

duvet77 said:
I'd personally keep the 9mm ply (or OSB) sheathing beneath the breather membrane, it'll add extra rigidity to the whole structure

Agreed, omit the internal one though.

^woody^ · 14 Aug 2008

duvet77 said:
It is preferred to have insulation above the joists to create a warm roof otherwise you need to start ventilating between the roof joists.

No you don't, it will be an inverted warm roof and no ventilation is required as long as the insualtion is tight against the underside of the deck

nigelm · 14 Aug 2008

Thanks for the input.

The building will be a small living space away from the house. The intention is to add heating and make it as comfortable as possible.

One idea I would like to investigate is to have the roof hidden behind the cedar cladding giving a nice clean finish. I am not sure how to acheive this but have seen some examples. Any ideas?

Nigel

duvet77 · 14 Aug 2008

^woody^ said:
duvet77 said:

It is preferred to have insulation above the joists to create a warm roof otherwise you need to start ventilating between the roof joists.

Click to expand...

No you don't, it will be an inverted warm roof and no ventilation is required as long as the insualtion is tight against the underside of the deck

Insulation between joists or under joists = cold roof

Insulation over joists = warm roof

Insulation on top of roof covering = inverted warm roof

Thats what my construction books say anyway.

duvet77 · 14 Aug 2008

nigelm said:
One idea I would like to investigate is to have the roof hidden behind the cedar cladding giving a nice clean finish. I am not sure how to acheive this but have seen some examples. Any ideas?

Nigel

You could use a parapet detail at the wall/roof junction so the external wall extends up past the roof level and hides the flat roof. All you'd see is a coping on top of the parapet. Drainage would be a bit trickier though as you'd need to form a hole through the parapet with a hopper rather than the bog standard fascia/gutter combo. It would be a trickier and probably more expensive way to do it.

On a side note, which way are you doing you boards? I think vertical boards can look quite nice.

nigelm · 14 Aug 2008

You could use a parapet detail at the wall/roof junction so the external wall extends up past the roof level and hides the flat roof. All you'd see is a coping on top of the parapet. Drainage would be a bit trickier though as you'd need to form a hole through the parapet with a hopper rather than the bog standard fascia/gutter combo. It would be a trickier and probably more expensive way to do it.

Could I use a parapet at the front / sides, but leave the rear side open and fit the guttering there.

On a side note, which way are you doing you boards? I think vertical boards can look quite nice.

I was thinking about using vertical cladding, maybe a 'splayed' profile, I thought this may help keep the area behind the cladding ventilated.

duvet77 · 14 Aug 2008

nigelm said:
Could I use a parapet at the front / sides, but leave the rear side open and fit the guttering there.

There's no reason why not. Are you doing the work yourself? The junction where you go from one to the other would need special attention and as you may have seen on another thread about a garden room on here, some builders will just fudge things.

I was thinking about using vertical cladding, maybe a 'splayed' profile, I thought this may help keep the area behind the cladding ventilated.

Not quite sure what you mean by splayed profile... do you mean feather edge board; thicker down on edged tapered to a thin edge?

If you're using vertical boards, you usually use vertical counter battens and horizontal battens to maintain continuous ventilation from top to bottom.

TRADA have some good guides on timber cladding but you have to pay for them.

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