How to align screws to hit purlins through insulation

[Tigercubrider]

https://www.axminster.co.uk/axminster-drill-guide-500458

 

[d000hg]

So you can't do it. Full of hot air. No surprise there.

I'd rather hire someone who tells me they are concerned about something than bodges on and makes a mess.

Or someone who spends all their time posting put-downs on an internet forum. Which is it, do you charge your customers for posting crap on DIYnot, or do you not have many customers because your know-it-all attitude puts people off?

Do feel free to save yourself some time by not answering my posts since they seem to upset you so.

 

[Munroast]

I would have thought if you are boarding the top of the joists with 20mm planks, screw the planks down, then the roof can just attch to these, then no accuracy is needed when screwing the rest on top

 

[StephenStephen]

Just a humble diy-er here - but if I were doing it, I'd screw the subdeck to the joists, screw the top deck through the insulation to the subdeck, glue the roof covering down and, clamp it with timber screwed onto the vertical outside edges of the roof.

 

[paulrockliffe]

Personally I would work out where the first hole should be and then try it out. If I missed I'd maybe look a bit closer from the side and move up or down the roof and be confident I'd then hit. Same at the other end and run a line between them and you won't miss many.

You can bring a mark up as you build up the roof too.

If it's critical and you have time to burn you can use a mix of measurement and maths to get the exact point and make a guide to get your pilot holes bang on perpendicular, but that shouldn't be necessary.

You say the deck is your ceiling, but are you not covering it in something, if only so the light is reflected downwards? If so you've less to worry about.

 

[Tigercubrider]

https://www.axminster.co.uk/axminster-drill-guide-500458

https://www.amazon.co.uk/ABEST-Adju...M2PAW7P3079&psc=1&refRID=EPMXNHZWJM2PAW7P3079

 

[Lower]

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.

OK, your image showed a ceiling. If the sub deck is the ceiling then i can understand why your roofer is worried as they wouldn't normally have to worry as much about missing with the odd screw.

He should be able to mark out the centre lines of the joists ok. To be sure of hitting each joist i'd pilot hole each hole with a drill jig which could be something like the jig in my link below. Or he could just drill a vertical hole in a scrap section of joist and use that as a jig until the hole wears too large and starts to slop.

https://www.amazon.co.uk/Wolfcraft-...RTDSFKM6FG9&psc=1&refRID=ATA0CCC2ERTDSFKM6FG9

 

[Tigercubrider]

and choose screws that won't come through

 

[paulrockliffe]

and choose screws that won't come through

I think the problem with that is if the screws don't come through, they won't go into the purlin either. You could screw from the inside if you really had to, but that has it's own complications so is unlikely to be the best option.

 

[d000hg]

I would have thought if you are boarding the top of the joists with 20mm planks, screw the planks down, then the roof can just attch to these, then no accuracy is needed when screwing the rest on top

Just a humble diy-er here - but if I were doing it, I'd screw the subdeck to the joists, screw the top deck through the insulation to the subdeck, glue the roof covering down and, clamp it with timber screwed onto the vertical outside edges of the roof.

Seems you two are on the same page. You're saying since the sub-deck is fairly robust, we don't need to worry about attaching to the joists/purlins. Work out screws long enough to not quite go all the way through the sub-deck, so they are ~15mm into it for instance?
I suppose this is the same premise as glueing - a larger number of weaker bonds are as good as a few massive screws.
The concern is just that the PIR is quite thick at 120mm, but it does make sense when considering the glue option by comparison.

Personally I would work out where the first hole should be and then try it out. If I missed I'd maybe look a bit closer from the side and move up or down the roof and be confident I'd then hit. Same at the other end and run a line between them and you won't miss many.

You can bring a mark up as you build up the roof too.

If it's critical and you have time to burn you can use a mix of measurement and maths to get the exact point and make a guide to get your pilot holes bang on perpendicular, but that shouldn't be necessary.

Sure, I guess just trying it is worthwhile - maybe it's less of an issue

You say the deck is your ceiling, but are you not covering it in something, if only so the light is reflected downwards? If so you've less to worry about.

No, it's by design a wooden roof as part of the cabin/barn aesthetics.

Ours won't be used like this or be as dark, but this is the general effect - we are not having a solid wood floor to avoid it being too ^woody^.

 

[d000hg]

I think the problem with that is if the screws don't come through, they won't go into the purlin either. You could screw from the inside if you really had to, but that has it's own complications so is unlikely to be the best option.

Screwing from the inside they now have to be the full length of the purlin depth + the whole roof. We're talking about 14" screws or something daft Well unless you pre-drill and recess but this sounds a horrible job!

 

[StephenStephen]

I'm confused - If you were to screw the top deck through the insulation to the subdeck, why wouldn't you make the screws longer than needed? - that way at least some of them would likely go into the joists, especially if set out to do so, and I can't see what harm they'd do sticking down a little into the void above the ceiling.

 

[paulrockliffe]

Sure, I wouldn't recommend it, just another perspective. I think worse-case you'll not make so much mess of your ceiling that you can't fill the holes and have it disappear into the character of the ceiling. Though I'm more inclined towards the screwing into the subdeck suggestion, you don't need to fix right through if you have more wood in your sandwich.

 

[paulrockliffe]

I'm confused - If you were to screw the top deck through the insulation to the subdeck, why wouldn't you make the screws longer than needed? - that way at least some of them would likely go into the joists, especially if set out to do so, and I can't see what harm they'd do sticking down a little into the void above the ceiling.

There's no void.

 

[StephenStephen]

There's no void.

Yes I realise I was looking at the diagram rather than the photo - actually there's no ceiling!