flat roof dilema - please help if you can

[wannabebuilder]

Hi all

I am building a garden room and don't really know much about flat roofs etc (IT guy).
I have built all the sides and floor/base and the total height so far is 2308mm.

That leaves me with 192mm to construct the roof to comply with building regs.

Now it's too late to build a warm roof because I don’t have enough space to fit everything and still be under 2500.

So my question is: is it ok if i have 100x47 roof joists filled with 100mm kingspan with deck board on top of that + fibreglass roof and then to use foil wrapped plaster board for the ceiling.

What worries me is the condensation, but if everything is filled to the max then there won't be any air gaps so there should not be any condensation right?

The building is going to be completely square and there won’t be any overhang so I can’t have soffit vents??

Please let me know what you think about that

 

[freddiemercurystwin]

Why does it need to comply with Building Regs? Is this being inspected and you are expecting a certificate? I think I'd be going for something like filling the full depth of the joists with your Kingspan and then a thin continuous layer over the top providing a layer between your joists and the roof finish. Not ideal as its neither one or the other and doesn't give you much for a fall either. Not a problem if BC aren't involved, maybe if they are ..........

As you say foil backed boards too.

If you are having a very shallow fall make sure your roof is really rigid (over-spec those joists) or the joists will sag over time and the roof may sag causing pooling or a failure of the roof finish. You've not said what the span is so now way to confirm.

 

[wannabebuilder]

Thanks for your reply freddymercurystwin

the span is 2.5m.

re:the height limit
I said building regs but I meant planning rules 2.5m max height if right next to property boundary

re: condensation
is there a potential condensation problem if everything is filled in with kingspan (no space between deck and insulation)

 

[freddiemercurystwin]

Sounds like you may be getting Planning and Building Control confused? Anyhoo a warm roof is just that: warm! Ideally you would fit all of the insulation above the joists so say 100mm but if your height is limited you will have to either lower the joists or accept less insulation. The risk of any condensation problems are tiny.

Your joists are undersized even at 400 centres. Increase them to 50x150 or bigger if you can, if not at least 50x125 (even then they may be undersized). For the sake of some modifications now to incorporating a rigid roof will far outweigh the problems of sorting out a sagging leaking roof in 5 years time. It may be worth considering wider joist if height is a real issue ie 75x125's as they would be OK although may be more difficult/pricey to source.

 

[wannabebuilder]

thanks again freddymercurystwin

let’s say i go with 50x150 + 18mm deck (i should be ok height wise) and then I use 100mm kingspan which leaves me with 50mm space between insulation and deck. How do I vent that area if there is no overhang so no soffit vents?

Is there another technique roofers use other than those ugly mushroom vents?

 

[freddiemercurystwin]

Err no need for ventilation, fiill your 50mm void with insulation that's the whole point. You can get a flat roof insulation from Kingspan that has the ply bonded to it already although not sure if you can get it in 50mm. http://www.insulation.kingspan.com/ireland/tr31.htm

 

[wannabebuilder]

ok too late for thermaroof.

could you just confirm that this would work ok and if i do it like this i won't have any condensation problems.

1.roof deck
2. 50mm sheeps wool insulation or Rock Mineral Wool (between joists)
3. 100mm kingspan rigid board (between joists)
4. plaster board with aluminum foil

or can you suggest anything else cosindering everything I said so far
i'll use 50x150 joists

thanks for your advice so far

 

[freddiemercurystwin]

Cannot guarantee you won't get any problems as your timber joists are a 'cold bridge' between the outside and the inside. Not a big risk but cannot guarantee condensation/damp patch free where the joists are. A continuous layer beneath would likely solve it (ie between the underside of joists and the plasterboard), not ideal frankly but better than nothing. This is why some insulation over the top is preferable. even say a 20mm continuous layer over the top would be better than nothing.

BTW make sure your rigid insulation is pushed right up tight to the underside of your plywood deck and held there with nails or battens, do not rely on friction. Seal any gaps with expanding foam. You really don't want any gaps opening up between the underside of the ply and the top of the insulation.

One other thing intrigues me though, if you only have 190mm to play with how are you gonna get any reasonable fall onto your roof?

 

[wannabebuilder]

back to the drawing board
I'll probably change it to have some overhang and install some vents. Re:fall - i'll see what i can do i might have a few degrees if not then i'll probably make a new lower base which is going to be easier than rebuilding all the sides.
It's all a learning curve