First fix in flat roof

[Pale_rider]

I need to do the first fix of a flat roofed extension and am wondering what is the best way to do this, with regard to the insulation. The roof has M8 rafters and then the decking above, the resulting gaps will then be filled with solid insulation (Celotex). I need to run cables for LED Downlights and a smoke alarm. I am planning on running the cables through holes drilled centrally in the rafters, clipping them to the side of the joist and then to the underside of the roof decking in an inverted "U" fashion, then leaving the ends hanging down where the lights will be, so that the builder can drill 100mm holes in the Celotex and insert the insulation between the rafters and over the cables, coiling them up so I can drill through the plastered ceiling later. Obviously the positions will need to be accurately noted beforehand. Does anyone see anything wrong with this routing method? I can't see any other way to do it as once the insulation is in place it will be impossible to do. The lights are all LED so there is no problems regarding the de-rating of the cable as it will be covered in insulation over all it's run.

 

[RyanCheetham]

Celotex is usually fitted flush to the underside of the rafters leaving an air gap above the insulation. If it was me I would be clipping my cables as high up the rafter as I could.

 

[Deleted member 9648]

Make sure the builder cuts holes in the celotex acurrately and of sufficient size to allow the downlight fixing clips to open up fully. It's a big time waster if celotex is in the way.

 

[shanky887614]

Make sure the builder cuts holes in the celotex acurrately and of sufficient size to allow the downlight fixing clips to open up fully. It's a big time waster if celotex is in the way.

makesure to use very few clips and leave plenty of slack around each downlight.

its a pain in the arse trying to cut out celotex then trying to fish the cables

 

[GB.Kayak]

I need to do the first fix of a flat roofed extension and am wondering what is the best way to do this, with regard to the insulation. The roof has M8 rafters and then the decking above, the resulting gaps will then be filled with solid insulation (Celotex). I need to run cables for LED Downlights and a smoke alarm. I am planning on running the cables through holes drilled centrally in the rafters, clipping them to the side of the joist and then to the underside of the roof decking in an inverted "U" fashion, then leaving the ends hanging down where the lights will be, so that the builder can drill 100mm holes in the Celotex and insert the insulation between the rafters and over the cables, coiling them up so I can drill through the plastered ceiling later. Obviously the positions will need to be accurately noted beforehand. Does anyone see anything wrong with this routing method? I can't see any other way to do it as once the insulation is in place it will be impossible to do. The lights are all LED so there is no problems regarding the de-rating of the cable as it will be covered in insulation over all it's run.

I am not in a position to quote 'chapter and verse' on the subject of drilling rafters but I have a very strong 'feeling' ... 'in my water' ... that while joists may be drilled, rafters may not. Generally speaking I suspect, as a 'structural' engineer, the nature of the loading on the rafters and their sizing is much more critical than that on ceiling or floor joists.

 

[ruston]

I suspect he means joists as it is a flat roof .

Then after reading it again - maybe not.

 

[GB.Kayak]

I suspect he means joists as it is a flat roof .

Then after reading it again - maybe not.

r, thanks ... I didn't make the connection! Perhaps Pale_rider, as OP, you would clarify the terminology ... my understanding was that rafters are sloping roof members though I am open to correction.

 

[ruston]

Yes. You are correct.

https://www.google.co.uk/search?q=r...Fwww.cabinltd.com%2Fconstruction.html;453;360

 

[uksparks]

I had to fit 10 down lights in a flat roof that were not there before, it had 100mm of celotex and minimal gap, I managed to get the cabled to where I needed them, but cutting the holes was fun, well, cutting it was easy, but then I had to put my hand up each hole and scratch away at it to make a cave inside the celotex, I managed to make a huge hole in it actually and lots of space around each light, but what a faff!

 

[Pale_rider]

Sorry guys yes it is a flat roof, rafters, joists whatever, I thought they were rafters myself but hey ho. I don't thing there is a structural issue it is just a flat felted roof above?

 

[GB.Kayak]

Sorry guys yes it is a flat roof, rafters, joists whatever, I thought they were rafters myself but hey ho. I don't thing there is a structural issue it is just a flat felted roof above?

P, in which case, so long as you stick to the rules in the on site guide on hole diameters, vertical positioning and pitch I would think that you would be ok.

 

[midgetman]

Clip high. Put your own drilled noggins in. Drop cable through this. Then they can butt celotex upto noggins.
Oh downlighters! Forget the noggins lol

 

[Midwest]

Here we go, never mind how to first fix, what about cutting gert great big holes in the celotex for the down lights? The flat roof is designed as a cold space something or the other. Cutting holes in the Celotex will allow warm air, from the room below into that cold space. Celotex and Kingspan advise against doing so, and eagle eyed building inspector may ask home owner how they have achieved u value with down lights in such ceiling.

 

[shanky887614]

Here we go, never mind how to first fix, what about cutting gert great big holes in the celotex for the down lights? The flat roof is designed as a cold space something or the other. Cutting holes in the Celotex will allow warm air, from the room below into that cold space. Celotex and Kingspan advise against doing so, and eagle eyed building inspector may ask home owner how they have achieved u value with down lights in such ceiling.

nope they dont question it around here.

if they do and the customer wants downlights there a flase ceiling would be needed

 

[ruston]

A few have been pulled here over it .

 

[GB.Kayak]

nope they dont question it around here.

if they do and the customer wants downlights there a flase ceiling would be needed

If it is in an area where there is a lot of warm moist air then the customer will suffer significant damp problems!

 

[Midwest]

nope they dont question it around here.

if they do and the customer wants downlights there a flase ceiling would be needed

I've spoken to my LBC, it's one of the things they look out for now. Won't do it myself and advice customers accordingly.