Help with garden office roof design.

[Andy7]

I think I have my design for the warm-deck flat roof on my garden office worked out but would like to get some opinions from the various experts here (thanks, all for the great advice so far).
The roof is 6m x 6m, with a suitable slope for run-off.

So, here's what I have so far working upwards:

- Joists (2x3m lengths of 95x45mm C16 supported midway by internal stud wall) @610mm centers to suit...
- 18mm OSB3 (maybe T&G)
- Vapour barrier. Loose fitted, probably stapled at edges to keep it from blowing about while I work.
- 100mm celotex (or equiv if I can find cheaper than Wickes). Each screwed down at corners.
- Wickes Polyester capfelt stuck down with a goodly slap of...
- Wickes trowel-on roofing adhesive.

The instructions on the polyester capfelt calls for polyester roofing underlay, but does anyone think it's relevant for this design? Do you only need that if putting directly onto the decking?

Keen to get this as right as I can as it's not only probably the most expensive part, and the most important part if I want to be cosy and dry in there!

 

[^woody^]

If that is the order of construction, then the vapour barrier is not doing anything.

What about furring strips? Unless the joists are sloped.

I'd put the insulation below the deck, or between the joists, or between and under the joists, as this makes things easier when repairing or replacing the felt

 

[Andy7]

If that is the order of construction, then the vapour barrier is not doing anything.

What about furring strips? Unless the joists are sloped.

I'd put the insulation below the deck, or between the joists, or between and under the joists, as this makes things easier when repairing or replacing the felt

Isn't that a COLD deck you're describing? As I understand from many hours reading and re reading about this, I'd then have to worry about ventilating the space under the deck, then there's the risk of condensation from the underside of the deck getting into the insulation.

From what I understand the WARM deck option is supposed to be more efficient and simpler to construct.

 

[^woody^]

It may be a cold deck, but if the insulation is tight against the underside with no air void, then no ventilation is needed, and it is the simplest and more efficient to construct

 

[Andy7]

Hmmm... interesting. I see what you're saying.
Though I did see a video somewhere where someone did someting like that and the joists rotted because condensation got trapped.

I'd have to make sure that the insualtion was TIGHT to the top.
Also, I assume that I'd then need a good vapour barrier between my joists and the PB.

That also means the lighting, which is going to be downlights will need to be proplerly sealed agianst vapour transit into the ceiling. THAT might cause me to scratch my head for a while.

 

[noseall]

It may be a cold deck, but if the insulation is tight against the underside with no air void, then no ventilation is needed, and it is the simplest and more efficient to construct

Some bco's will not sanction this method (as a true 'warm' deck)no matter how robustly you tape or mastic the joints.

 

[^woody^]

They will presumably sanction a SIP roof panel though?

 

[Andy7]

They will presumably sanction a SIP roof panel though?

I've seen some interesting composite panels that have: PLY>VB>PIR>Underlay all in one. Trouble is, how on earth do you seal those between the joints? You'd really only have the capfelt as the waterproofing.

 

[noseall]

They will presumably sanction a SIP roof panel though?

I can't argue with your reasoning Woodbit but mebbe the SIP panel manufacturers know summat we don't and are in cahoots with B.C.

 

[^woody^]

It's normally BCO's with narrow minds, and who forget the basic principles that the regs are based on