Bonding a warm flat roof

[Adrianlondon]

Hi all

Sorry or butting in on your thread but I'm now at the stage when it is time for me to decide what to do regarding the roof insulation.

For background purposes, I am having a 6m x 6m single story extension. The joists will be 9"x3" The firings will be 140mm down to 1mm which is a drop of approximately 1:45. This can be reduced if necessary (maybe 80mm down to 1mm)

The joists are going from left to right on my extension but the firings are going in the other direction. Unfortunately I cannot budge from this.

I live in NW London.

If there are any other questions you'd like to ask me to be able to come up with a more precise answer, please do ask.

So, before reading this thread I was all ready to go out and but some XR4120 insulation and sandwich it between some 18,22 or 25mm plywood. I thought that the foil underside of the insulation would act as a Vapour Control Layer.

Then I thought I'd get some TD4126 and not have any plywood on top and lay the EDPM directly onto the ply of the TD4126.

After reading this thread I thought I'd go with the option of using TD4126........that was until I looked up the price of the Insofast!!!!!

If I'm right in thinking, they are about £1 per screw. That is VERY expensive (for a screw). I will need about 14 sheets of TD4126 so this will increase the cost of the roof by about £170 (after I've damaged a few!!!)

What I'm really not sure about is the benefit of using TD4126 when I've got to put 12mm ply on top whatever. Would I not be better using XR4120 instead and then maybe (to be extra safe) put 22mm ply on instead?

I'm also not very understaning of the 'top cap' part of the roof.

I'm sorry for appearing stupid but I'm totally new to all of this so please don't bite my head off!!

 

[Adrianlondon]

Insofast fixings must be accurate to hit the joists and firrng pieces when they go in, so that they do not puncture the VCL above the bottom OSB deck above the insulation..Hope you understand that.

I don't understand that. Can you please explain. Surely if the Insofast has to go from the top of the external plywood all the way through to the joists then it must go through the VCL.

 

[theoldun]

Mr. London, You are going to put us to a lot of writing, however, your first post first.
On a 6 metre run minimum fall is 1 in 80. 1 in 80 = 0.0125mm per metre by 6 metre = 75mm of fall
Tapered firrings should only run with the joists not over joists. A fall over the joists should be formed with reducing rips Example if fall is 75mm a 80mm rip should be fixed to joist at top of fall and a 5mm rip should be fixed to joist at bottom of fall. You can then either string a line and measure down to each joist to see the depth of each rip or work each rip out mathematically.
You should be able to get all the other information you require from our previous posts.
Regards second question. Yes Insofast are going to pierce VCL, that is why they should be fixed through bottom decking and into joist thereby reducing the risk of interstitial condensation If you miss the joist and hit the bottom decking above insulation you increase the risk of condensation and the bottom decking is not strong enough to hold fastners. No we will not bite your head of fellow, we go out of our way to help people who are prepared to help their selves.
Will tell you where your problem will be. Depth of facia, but that is a story on its own.
Any further questions then come back.
Forgot to say that if you are having a single ply covering EDPM then you require minimum of 12mm top cap between insulation and covering.
Yes Insofast are not cheap,so feel free to use what ever you wish. We only told you what we use.
Regards oldun

 

[Adrianlondon]

Thank you for the info.

Regarding the Insofast, you'll see that I've started a separate thread trying to find out the difference between the two.

Regarding the EDPM, I will indeed be having a 12mm external plywood on top of the TD4126.

There is still a part of me that thinks I should use the cheaper XR4120 and then put 18mm plywood on top (with nothing underneath - insulation touching the VLC.

Regarding the piercing of the VCL, I would have thought that it wouldn't be the end of the world as I'm having the foil backed 12mm plasterboard as my ceiling. The VCL should be just for added protection.




Adrian

 

[Fractal]

Oldun, is there any reason the insulation needs to be foil-faced for this?
I've seen some Thermawall TW53 for £4 a sheet which would work out miles cheaper than the Celotex.
Thanks

 

[theoldun]

Oldun, is there any reason the insulation needs to be foil-faced for this?
I've seen some Thermawall TW53 for £4 a sheet which would work out miles cheaper than the Celotex.
Thanks

Thermawall TW53 is for external insulation and it cones in 1200x600 sheets and only has a K value of 0.025W/m2 as against Celotex of 0.022W/mK. Have mo idea of compressive strength.
If it was us we would not attermpt to use it.
Regards oldun

 

[Adrianlondon]

Hi all

I've had to change the plans regarding my roof. The reason for this is to do with the total height being more than 3m

I now have to put the insulation in between the joists.

Originally the foil backed plasterboard was going to sit underneath the joists and then a gap between the joists and then the insulation etc

Assuming that the foil backed plasterboard still has to be under the joists (I need a ceiling after all!!!!) can someone please tell me what the 185mm of insulation sits on?

I read somewhere that someone used chicken wire to support the insulation but that doesn't sound very stable to me.

I can start a new thread if needed.




Adrian

 

[Fractal]

I'd like say a huge thanks to oldun and noseall for their valuable help.

I got the roof done over the bank holiday weekend apart from the fascia and detailing around the rubber and the BCO came around today and passed it and declared it a "very good job".

So thanks again for the great advice!