The roof construction on our new extension will consist of:

Upper Deck (18mm Ply)
120mm Kingspan
Vapour Barrier (resitrix alutrix 600)
Sub/Lower Deck (18mm Ply)
Joists
Plasterboard

We were going to these plastic tube fixings, rather than just metal fixings, to prevent thermal bridging:

https://www.fixfast.com/products/fasten ... ner-system

The tube fixings would be applied to the to upper deck which is 18mm ply. Obviously we would have to drill a hole in the upper deck ply for each fixing.

I'm wondering if we can use these plastic tube fixes if we are using torch on felt for the final roof covering. Is there a way to protect them from melting under the heat? Will they be fine just as they are or is there something we could cover them with to protect them whilst the felt is being applied?

One other concern is the gap these tube fixes leave within the insulation once inserted and screwed down. Should we fill that gap with something to prevent condensation building up, if so, what can we use for that?

If I use metal fixings that won't leave any gap, is thermal bridging something to worry about, especially if the fixes are only applied to the joists and not the lower/sub deck?

Thanks

why torch on felt and not GRP

Two ideas, 

1,you will need to countersink those tubes into the ply or you will have a little hump in your roof covering, so why not countersink a little extra and fill flush with a bituminised thick paint type of stuff. 

 

2, why not use the insulation with ply glued to it purposely for roofing. 

 

Kingspan also do an insulation board especially made for torch on felt. No ply needed. 

Edited December 31, 20196 yr by Russell griffiths

I wouldn’t use the fixing at all 

I use a lot of these for fixing insulation to the underside of concrete floors 

I definitely wouldn’t stick felt on anything other than a garden shed 

8 minutes ago, Russell griffiths said:

2, why not use the insulation with ply glued to it purposely for roofing. 

sips panels +grp covering 

example

https://www.sipsdirect.co.uk/product/144mm-panel/

Your flat roof is a standard detail apart from the extra layer of ply above the insulation - this is not required. Kingspan, and other PUR manufacturers, have insulation that will accept torch-on membranes and it looks like the fixings are OK. Remember the flame is directed at the roll of felt to melt the bitumen layer on it, not onto the insulation. Torch-on systems seem to have lost favour due to fire risk but if appropriate safety measures are taken it is quite acceptable.

best practice places osb or ply on the top side of the insulation to help secure it. i wouldn't fit an external insulation layer without it and unless the felt is being fitted to a flat roof abutting a hose hair covered slated roof there is minimal risk of fire. i would these days, however, go with a monolayer rubber roof for ease of fixing and longevity.

Best practice? Please read MIs, BS and BBA certs, no ply/osb above insulation. Lots of fire safety info on internet re torch on systems.

 

Moving to single ply membranes still no need for ply/osb above insulation (it’s in the wrong place!). Normally rubber/PVC single ply membrane is mech fixed or adhered direct to insulation.