Screw length/spacing, flat roof 120mm insulation?

[John37]

Hello, this is my first post.

What screws do I need for a ground floor flat roof in a sheltered location, please?

The construction is 47x175 joists at 400mm centres with joist hangers on one side and 75x100 wall plate the other. Firrings are 50mm to nothing over 3.2m (at side of house) and 100mm to nothing over 6m (at back of house).

Above the firrings will be 18mm ply, vapour barrier, 120mm celotex, 18mm ply and GRP covering.

What screws and spacing do I need for (1) the firrings, (2) the bottom layer of ply, (3) the top layer of ply?

Any advice, please? Many thanks

John

 

[limerockconstruction]

Try Twistfix for the top deck fixings, I've used them a few times recently.

For 120 warm deck they suggest 195mm I think, but I got on much better with the 245's - they are a helical fixings with a small mushroom head on, they're not coming out once they're in!

 

[John37]

Thank you. From Twistfix website, it seems their fixings are suitable for thin, 6mm ply (e.g. as bonded to TD4000 Celotex), but since I will have a separate 18mm ply deck above the insulation, I think screws may be needed.

The total area of the flat roof is 100 sq m, so the costs of the fixings are significant.

I wonder if 6mm x 200mm spax woodscrews would be suitable? What spacing should they be?

 

[limerockconstruction]

I've put them on four flat roofs recently, all warm decks, all made up of 18mm base ply, insulation (varying between 120 and 150mm), then 12mm ply. I found that they're helical style made them much easier to fix.

Give them a ring, the guy I dealt with was called Ray, not sure if he was a regional contact.

I thought they were worthwhile the investment.