Sanity Check on Flat roof please.

[Warrentdo]

So I have a flat roof.

GRP - 18mm OSB3 - 150mm PIR - VCL - 18mm OSB3 - softwood firings  - 200x47 C24 joists.

worse case bottom of GRP to 40mm inside joist is 290 mm

screws at 200mm ctrs which will make the VCL more like a sieve.

im also thinking of drilling a pilot hole 5mm down to the joist (make the VCL worse).

3mm gap between each board and 10mm gap around the edges (wall)

 

Questions are.

Is hitting 47mm 250mm away achievable?

I would think that when board share a joist I would have to double joist?

 

Any thoughts/advise?

Warren.

[ProDave]

The screws won't make the VCL like a sieve as the screw will remain filling the hole.  If a screw does miss the joist, leave it in place, don't pull it out, and fit another one and try again, making the appropriate adjustment in your aim. 

[saveasteading]

All these warehouses you see have about 10 screws per m2. 

Occasionally you might see one leak, easily fixed, out of 20,000 or so fixings. 

They are fully sealed for vapour, water and air.

Make sure you use the right screws for the purpose.

Roof cladders mark the fixing line on the cladding to ensure they start at the right point.

290mm is a long screw and, yes, likely to miss.

What about fixing the top osb to the joists, then the grp to the osb?

[Warrentdo]

3 minutes ago, saveasteading said:

What about fixing the top osb to the joists, then the grp to the osb?

Yes full fixing firings to joists, then temp fixing osb to firings, then slide PIR in, final fix top OSB to Joist, then GRP over the top.

[saveasteading]

6 minutes ago, Warrentdo said:

Then GRP over the top

Ok i see. The grp is already there  and the rest is being squeezed in?

Why does the grp need new fixings?

[Warrentdo]

2 minutes ago, saveasteading said:

Ok i see. The grp is already there  and the rest is being squeezed in?

Why does the grp need new fixings?

No, this is a new roof, the GRP is on top 🙂

[saveasteading]

"Slide pir in" confused me. So no need for the very long screws?

 

There are fixings for every purpose. A specialist supplier can advise what will grip osb (a wide screw with a coarse thread i would think) and also seal the hole.

 

Very good roof screws have integrated washers that not only cover the hole, but wind down the thread to  make the seal.

 

As to the vcl...the long screw could rip it and you wouldn't know. But if your weight is on the area, the load should keep it all in place.

[Warrentdo]

2 hours ago, saveasteading said:

"Slide pir in" confused me. So no need for the very long screws?

 

Sorry for confusion. not slide, place otherwise I would have to support the top layer of OSB with sky hooks and I've run out.

Everything is new. Everything is placed on top of each other, in order, and fixed if required. Big Screws needed to stop it blowing away in wind, earthquake or anything else.

Warren