Warm Roof and Fixings Advice

[jakeR]

I am in the process of refurbishing a 1980s timber framed house and will be replacing the existing (knackered) tiled roof with a pitched warm roof finished with 13/3 corrugated metal. 

I've attached a drawing of the proposed layup of the roof (which I hope is clear enough!). The roof trusses will be left exposed and the bottom layer of plywood will be fireproofed and lacquered and will be left visible from the inside. As the roof trusses are a pretty skinny 38mm, I'm planning to screw a wider vertical batten above the plywood into which the layers above will be fixed, as I don't fancy my chances of hitting the trusses with screws through 200mm of insulation.

 

I'm looking for some advice on the fixings to use and how many. I was planning on using thermally broken fixings as shown in the purple in the drawing, going through the counter batten, osb deck and PIR insulation- which would need to be about 250mm long. However, I'm wondering if I can just fix the OSB deck in this manner and then just screw the counter batten into the OSB? If it's sufficient to just fix the counter batten to the OSB, then can anyone give me a ballpark figure for the spacing/number of screws needed or how to work this out? Any tips on thermally broken fixings and screws for the battens also would be much appreciated. Many thanks.

 

(Apologies, if this has been covered before and thanks for all the amazing advice from everyone I've read so far!)

[Iceverge]

I would consider omitting the top layer of OSB entirely. It's doing nothing really.

 

I would just diligently tape the joints in the ply too and forget about the vapour membrane.

 

You could spec the trusses with a wider top chord from the Factory to avoid the need for the additional batten. 

 

In any case I would use 2x layers of 100mm PIR, joints staggered foamed and taped. Don't fret about thermally broken fasteners. They'll be almost zero performance difference to a metal one as the cross sectional area is so small over 200mm.

 

The counter battens and batten is good for tiles but for metal cladding you could save some work and just put a 45deg (allowing for drainage) 70*20mm batten instead. The recess in the corrugation making up the rest of the ventilation. ( I personally think the corrugations alone are enough.....)

 

 

Have you a need for overhangs and eaves? Do you have a detail for these? 

 

 

[jakeR]

Hmmm, very interesting, thanks! The top layer of OSB was really just for walking round on while I did the rest of the roof but it would save a grand or so not having it. Perhaps I can omit the top breather membrane as well? The idea behind using the Alutrix Vapour barrier is that the roof is watertight from this point which will help in construction.

 

I'm replacing the roof on existing roof trusses so am unfortunately stuck with the top cord, hence the additional batten.

 

I would much rather not use the thermally broken fixings as they are expensive and look a bit fiddly to use. I was worried about thermal bridging but don't really know what difference this will mean in the real world, so maybe normal screws will be just fine.

 

I've not seen 45 degree battens before so that could be good.

 

The roof currently has a reasonable overhang at the eaves (I'll post some photos) which I will probably keep. I was hoping to do something clever where I can trim the trusses back outside the building so that the overall thickness of the roof doesn't look too chunky from the outside.