Roof construction query

[daylightsolar]

Hi, we have a job coming up, the roof construction from the outside in is as follows.
Double pantile, felt, 40x35wide timber(approx), 50mm kingspan then 200x35mm trusses.
We were thinking of using 160mmx6 stainless woodscrews so we can get through the kingspan in to the trusses for a sound fixing.
Any thought on this or other methods that can be used?
Thanks a lot

Edit to add. I was under the impression that anchor screws need to be stainless, although i'm sure the star drive schletter screws we used on the last job weren't. Can anyone confirm whether the screws need to be stainless?
Thanks.

 

[SolarCity]

That seems like a strange construction. How are the 40x35mm timbers fixed?

 

[solarsavings]

It's a warm roof.

Fasten Master decking screws should be up to the job as they are suitable for pressure treated timber. They're long enough to get you through the counter batten and insulation into the rafters. http://www.*************/c/screws-nails-fixings/fastenmaster-timberlok/cat840174

you need to put screw fix into the link.

 

[Worcester]

Yep, only problem solarsavings, you can't put 8mm screws into a 35mm truss.

Are thy 35mm or 38mm At 38mm you can squeeze Eurocode 5 and use 6mm fixings if you predrill the holes.

The LABC don't even like 6mm, they'd have you use 4mm!!
Remember it takes 4 x 4mm screws to have the same strength as 1 x 8mm screw...

Stainless or otherwise, the

LABC guide says : "Brackets and fixings should also be suitably durable for their exposure"

MIS 3002 says: "4.4.3 External metalwork used for bracketry or mounting frames shall be constructed to correctly support the imposed static and wind loads, must be adequately ballasted or fixed into a suitable structural member and shall be adequately protected from corrosion for a typical life to first maintenance of at least 20 years (for example, stainless steel number 1.4301 or 1.4401 to EN 100881 or galvanised coating on mild steel as specified in EN ISO 14713:1999 for the appropriate environment, or equivalent)"

We use stainless - try fix8.co.uk

 

[solarsavings]

Uhm.......thought we'd already got past the we can't comply with the eurocode so the next best thing (at the moment) has to do.

The FM's are durable so can't see a problem there. The design of them will not pull the batten off the insulation when being tightened to the rafter, they will also allow the roof hook to be tightened down onto the batten. Those helical fixings are great but can be pulled through the batten. And you will need a long screw at 1 1/2 times the thickness you are fixing so it's going to need 150mm (the nearest suitable size)

Hopefully this link is allowed.
http://www.timbersolve.co.uk./osc.html

 

[gr7]

We always use A2 stainless screws, these are supplied by Hilti along with their pitched roof mounting system. We do pre-drill these into the rafters despite Hilti stating that this not necessary. Bear in mind that the system needs to stay fixed to the roof for the life of the system. BZP would probably end up corroding away to nothing before 25 years. It is sensible to spend a bit extra for the peace of mind.

 

[Worcester]

We always use A2 stainless. As the PV mounting fixings are in a more exposed postion than most other fixings (Tiles and slates are held on by Aluminium or copper, not galv'd nails)

Most house builders don't gaurantee for 25 years, you're lucky if you get a 10 year gaurantee and of course. Also galvanization is a sacrificial coating, meaning it will rust away eventually, whereas stainless is an inherent rustless material.

If you know it is going to rust away - should you use it?

 

[solarsavings]

Is stainless steel inherently corrosion proof?