How to align screws to hit purlins through insulation

[d000hg]

We're building a warm roof on our cabin. The main roof structure has substantial purlins running along the roof at 80cm centres, boarded with 20mm planks to form the sub deck and covered with a VCL.

We have 120mm PIR boards which will be sandwiched between the sub deck, and a top deck (OSB or marine ply).

So we plan to screw through the OSB,PIR and boards into the purlins I guess using 150mm+ screws.
How can we ensure the screws hit the purlins? Even a slight angle and at that length they'll miss and poke through the roof.

Warm roofs are not so common in the UK but are elsewhere... Is there a trick to it? Our contractor was discussing with me he's not confident doing it by hand.

 

[datarebal]

glue it

 

[d000hg]

glue it

All layers? What sort of glue can provide long term bonding wood to foil?

Interested to hear thoughts on this from anyone else. @datarebal are you in the trade and have experience doing this approach?

 

[GoodDIYjob]

All layers? What sort of glue can provide long term bonding wood to foil?

Interested to hear thoughts on this from anyone else. @datarebal are you in the trade and have experience doing this approach?

Our boards were bonded (by the roofer) using a flat roof PU adhesive like this.

https://www.roofingsuperstore.co.uk...GXCQS8VVa4IY1kccpsRCw8wzAxhrDmKQaAk3aEALw_wcB

 

[d000hg]

Just thinking a bit more, what about the VCL though? If the insulation is bonded to that and the outer deck to the PIR, the whole roof is attached to a piece of plastic sheeting which doesn't sound right

 

[^woody^]

Our contractor was discussing with me he's not confident doing it by hand.

A contractor is not confident of drilling or screwing vertically after marking out? Oh dear.

 

[d000hg]

I give you a 160mm thick block of wood and I've marked on both sides where the screw should go and you're telling me on the side of a roof, with your electric screwdriver, you can guarantee to screw absolutely true in both direction so it hits the target, when you cannot see the thing you're aiming for - you just have to eye-ball that the screw is absolutely perpendicular to a flat surface.

Every 1 degree you are off you are missing by 3mm.

Why don't you go try it out and show us the video. I have no idea how easy or hard it would be but when someone who does this for a job says they think it is tricky, I listen. Could be an interesting challenge - put an old sheet of OSB on a 6x2 so you cannot see the 6x2 and you have to screw through the OSB and timber and see how far off from centre the screw comes out.

 

[d000hg]

Come on mr professional, show us your chops

 

[Tigercubrider]

Drills do exist with a bullseye spirt level on the back, and a normal one on the top.
You can also get drills that fit into a jig that is like a plunge router.
This ensures that the drill goes down exactly where it should.

 

[^woody^]

Why don't you go try it out and show us the video

Why don't you just employ someone who knows what they are doing?

I'm not the UK Hole Drilling Champion 2019, and my hole drilling skills are probably no better than any decent contractor, but I would suggest that hole drilling is not the same as the science of designing vehicles for interplanetary travel.

 

[d000hg]

Thinking more it's even worse as the roof isn't flat. You've got to ensure you are drilling exactly vertical against a sloped surface as the purlin is mounted vertical. I challenge anyone to guarantee to drill accurately without even a visual reference that sort of distance.

@Tigercubrider are those the sort of things one might find in a hire shop? We were discussing a router-type jig but weren't sure if they existed - or what they are called. Would it let us set the thing on a sloped surface and rotate to get it vertical, I tried googling but could only find variations on a plunge to ensure you are drilling perpendicular which isn't helpful.
Spirit levels on the drill would help but I think for this precision a mount of some sort is going to be far easier in the long run!

 

[d000hg]

Why don't you just employ someone who knows what they are doing?

I'm not the UK Hole Drilling Champion 2019, and my hole drilling skills are probably no better than any decent contractor, but I would suggest that hole drilling is not the same as the science of designing vehicles for interplanetary travel.

15 degree board on a vertical 6x2, you have to drill through freehand and come out neatly in the middle without looking round the side as you do it.

Put up or shut up. Easy to talk big in your cosy sofa with a mug of tea...

 

[Lower]

Presumably the roof is only sloping in one direction?

Roofers do this sort of thing all the time. The screws won't be long enough to go through the ceiling itself (which i presume isn't installed yet anyway), so if they miss the joist they take it back out and have another go.

A few holes of the size that you get from the tip of a screw won't be significant in the vapour barrier (the pir foam itself is an effective vapour barrier) and if you're paranoid about it you can always tape over any holes.

 

[d000hg]

Thanks @Lower but because it's a warm roof, the sub-deck IS the ceiling - nice planed boards. Now your point still stands because a few holes will barely be visible at height but it sounds a time-consuming chore

Would a roofer use any guide or are they just used to doing this particular task - drilling accurate vertically by eye - since they do it every day whereas a regular builder wouldn't? An aid of some sort seems like it would be much quicker - set a line across the roof where the purlin is, then just move along with your pre-angled drill?

It's not a HUGE roof - 11x7m building. Pitched so yes two sides each sloping 15degrees in one direction only.

 

[^woody^]

Put up or shut up

The general concept is when one has not got a scooby, employ someone who does. And pay for someone who knows, not someone with a perchant for a cup of PG and a Fyffes, and who cannot hold a drill perpendicular to a surface.

Now you may not have a clue about drilling a hole, but I assure you, that it's really not that difficult and it's not down to little old me to prove anything to you.

I do wonder though, how on earth you are going to manage running this business, when every single thing seems an issue to you.