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Warm roof deck fixings


andyscotland

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(hopefully) quick question. I need to get fixings for the upper deck of my warm flat roof.

 

I have 18mm OSB (nailed down) then 130mm PIR, then 18mm OSB. My engineer specified the nails for the bottom deck but there's nothing mentioned for the top one.

 

The GRP manufacturer says I need 40mm penetration into the joist - so 206mm min overall length.

 

I've seen various fancy "warm roof deck fixings" - helical nails, plastic tubes with a screw at the bottom etc. All fairly expensive, coming in about 50p per fixing.

 

Can I just use very long ordinary woodscrews? E.g. these self-drilling countersunk 6 X 240 which work out about 25p each online.

 

Or am I missing something and I need a fancy specialist fixing for some technical reason?

 

As well as the cost, I'm thinking it'll be easier to keep a screw vertical into the joist below where a long nail might drift and miss it?

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a wee question  have you actual done this yet?

i,m wondering why you did not just use sips panels for the roof ?

anyway 

there are plenty of long fixings available specifically for sips panels which would do for this  

from the site you posted  i would use the  ForgeFast Construction Screw Tan 8.0 x 240mm nice big flat head

If you have already fixed bottom osb and now you just fitting pir then osb --do you really need to screw it down  that hard  ?--your going to bond a grp roof on to it which will stick it all together 

use some no nails or something on both sides of pir andthen  maybe just a few screws to keep top sheet in place while it all sets --then on you go with the grp laid up roof ,

but std osb is not what your grp roof people will spec --it should to be correct roofing osb  ideally tongue +groove type 

it is resin impregnated to stop rot problems 

and you must not have any moisture on this decking before you lay grp 

 

my roofer lays t+g boarding same day as he does the roof lay up to make sure its all dry  

and if its that big a job --he will lay what he can grp coat in a the day -

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@scottishjohn a few reasons for not using SIPS. Main one is most of the roof is existing garage joists in unusual fan-out shape, had to optimise position of several rooflight openings and lightwells on site and then set out trimmers/doubles etc to suit. So easier to build up a layer at a time in situ. Also no heavy plant on site (and no access for it).

 

I've already fixed the bottom layer of OSB. There's a polythene VCL to go on top before the PIR so was concerned adhesive fix might not be sufficient.

 

The Forge Fast construction screws work out about 95p a screw...! Would you definitely recommend them over "normal" ones?

 

Yes, the OSB deck is T&G roofing OSB and aware of the need for it to be dry.

 

The boards and the PIR have been stored outside (well covered and with no visible sign of leaks). I was considering laying the deck and covering with properly lapped / battened breather membrane (I have a good bit spare) for a day or two to make certain any dampness has a chance to migrate out before I GRP. Then uncover and laminate in a single day.

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1 hour ago, jfb said:

i am surprised that the roof buildup doesn't include a ventilation gap (counter battens over the PIR) that the final layer of OSB is fixed to - is that a normal build up?

 

Yes - with a warm roof (insulation above the structure) there's no requirement to ventilate. You make a "sealed parcel" of insulation, top deck and waterproofing sitting on top of the structural roof.

 

At least, that's what I understood from advice from insulation and GRP manufacturers (and put on my building warrant drawing) so hope it's correct!

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11 hours ago, andyscotland said:

(hopefully) quick question. I need to get fixings for the upper deck of my warm flat roof.

 

I have 18mm OSB (nailed down) then 130mm PIR, then 18mm OSB. My engineer specified the nails for the bottom deck but there's nothing mentioned for the top one.

 

The GRP manufacturer says I need 40mm penetration into the joist - so 206mm min overall length.

 

I've seen various fancy "warm roof deck fixings" - helical nails, plastic tubes with a screw at the bottom etc. All fairly expensive, coming in about 50p per fixing.

 

Can I just use very long ordinary woodscrews? E.g. these self-drilling countersunk 6 X 240 which work out about 25p each online.

 

Or am I missing something and I need a fancy specialist fixing for some technical reason?

 

As well as the cost, I'm thinking it'll be easier to keep a screw vertical into the joist below where a long nail might drift and miss it?

i've seem a warm deck flat roof be blown off due it the insulation not being properly fixed down. very often the top layer osb is not put down as seen 'to be not necessary'

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12 hours ago, Simplysimon said:

i've seem a warm deck flat roof be blown off due it the insulation not being properly fixed down. very often the top layer osb is not put down as seen 'to be not necessary'

 

Yeah that was my concern about a purely adhesive fix. We're not cliff-top, but we are relatively exposed for an urban site (on top of a hill) so would rather be sure it's solidly fixed down.

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36 minutes ago, Temp said:

Beware very over long screws need more torque and can have problems shearing off heads.

Thanks, hoping that would be less of an issue in this case as only the top 18mm and bottom 60mm will be in timber, the centre section should turn fairly easily in the PIR?

 

Or is the torque just a function of the distance between tip and drive head?

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42 minutes ago, andyscotland said:

 

Yeah that was my concern about a purely adhesive fix. We're not cliff-top, but we are relatively exposed for an urban site (on top of a hill) so would rather be sure it's solidly fixed down.

the one i saw was in the middle of edinburgh, so not exposed at all.

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1 hour ago, andyscotland said:

Or is the torque just a function of the distance between tip and drive head?

No it's just the length in the wood that matters. I had an issue with stainless steel screws shearing off until I got the pilot hole size right - but stainless isnt as strong as regular steel.

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I would look at https://www.screwfix.com/p/timbascrew-wafer-timber-screws-gold-6-7-x-200mm-50-pack/6787j .  50p each, wafer head and you will get 50mm embedment into the OSB / joist which will be plenty.

 

Can you check from underneath to make sure the fixings are locating OK?  It is quite easy to miss a 45mm wide target especially if you are butting board edges over a joist.  It may be better not to line up the boards with the joists - just lay in staggered rows and screws along each joist.

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@Mr Punter thanks. 

 

They're T&G boards, long side perpendicular to joist and staggered rows. Only butt joints are on diagonal lines of the roof hips but those joists are tripled so plenty to hit.

 

I've deliberately held off doing the VCL and ceiling battens underneath so I'll be able to see from below that all the fixings have made it into the timber. Also considering making up a wee  guide block to start the screw at perfect right angle to the deck.

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On 30/08/2019 at 09:48, andyscotland said:

I've seen various fancy "warm roof deck fixings" - helical nails, plastic tubes with a screw at the bottom etc. All fairly expensive, coming in about 50p per fixing.

 

Can I just use very long ordinary woodscrews? E.g. these self-drilling countersunk 6 X 240 which work out about 25p each online.

 

Or am I missing something and I need a fancy specialist fixing for some technical reason?

The point of plastic tubes is to thermally break the fixing, whereas long wood screws represent hundreds of little thermal bridges.

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3 hours ago, mvincentd said:

The point of plastic tubes is to thermally break the fixing, whereas long wood screws represent hundreds of little thermal bridges.

Hmm good point. I wonder how significant this will be.

 

Looks like with the tubes I'd need to drill a hole for each one, and drill a 50mm X 2mm

countersink/rebate for the head (according to one supplier it would otherwise be visible through the GRP). That's quite a lot of work for each fixing. And there's still a partial thickness bridge from the screw through the bottom 40ish mm of the insulation.

 

My u-value calc for the roof includes an allowance for nails or screws through the insulation, tempted to think the time and effort for the tubes wouldn't be entirely justifiable?

 

 

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On 31/08/2019 at 08:41, Temp said:

Beware very over long screws need more torque and can have problems shearing off heads.

you can always pre drill 

but yes i understand that why good quality screws and driver  with torque setting is important -also using in old beams where wood is dried out will increase the load --so if old beams maybe pre -- drill  

which is why i suggested the 8mm not 6mm type

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  • 4 weeks later...

@Simplysimon @Mr Punter the wafer head screws have worked a treat, very firm grip on the boards and really clamping it all together.

 

Got most of them into the joists ok first time but a few missed (I could actually tell just from the difference in torque at the end).

 

So I now have some 6mm holes through the construction. Not sure how worried I should be about this?

 

Can't see they'll make much odds to the insulation performance.

 

They have punctured the VCL between bottom deck and insulation. But that is belt-and-braces to follow the GRP manufacturer instructions. There's another vcl still to fit on the bottom of the joists, sealed to the walls, which will be intact.

 

Should I do something about them, or leave them?

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35 minutes ago, willbish said:

You're not thinking about taking all the fixings out so you can replace or tape the damaged VCL are you?!

 

Lord no, that would be insane! Would be sure to miss some again putting it back on so it'd be a never-ending job ?

 

I took out the screws that missed as they went in and replaced a centimetre or so away. So the holes they left are empty.

 

Almost all of the holes that missed still scraped the joist so would probably be tricky to tape reliably from below, as they're right in the corner of joist/OSB. But as I have another VCL going on the bottom of the joists/above the ceiling I'm not overly worried. At least I think I'm not. ?

 

I could try and squirt a bit of expanding foam in to close the hole in the insulation layer, but I think that'd be pretty messy given they're such narrow holes.

 

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