Fixing top deck for flat roof

[Peter500]

I'm building a warm flat roof with 200mm insulation sandwiched between two sheets of 18mm OSB. I'm wondering whether the top deck needs to be screwed into the joists or whether the screws only need to penetrate the lower OSB deck?

I thought I saw somebody online say that they should go into the lower deck, but I've just seen a video where somebody only screws into the OSB. I think it would be very hard for me to hit the joists as some joist are only 35mm and shaped like a banana. I'm planning to use the thermally broken tube washer things.

 

[catlad]

It would be nice if you hit a joist now and again, but it isn't easy using 250mm screws.

 

[Peter500]

It would be nice if you hit a joist now and again, but it isn't easy using 250mm screws.

It would mean that the ones that miss would be sticking through and I'll be running services through those areas

 

[noseall]

I'm building a warm flat roof with 200mm insulation sandwiched between two sheets of 18mm OSB. I'm wondering whether the top deck needs to be screwed into the joists or whether the screws only need to penetrate the lower OSB deck?

I thought I saw somebody online say that they should go into the lower deck, but I've just seen a video where somebody only screws into the OSB. I think it would be very hard for me to hit the joists as some joist are only 35mm and shaped like a banana. I'm planning to use the thermally broken tube washer things.

We always fix through into the joists.

 

[Peter500]

I found this https://www.fixfast.com/skills-hub/correct-washer-and-fastener-combination , which seems to say the industry standard is a "12mm minimum penetration through the underside". I'm assuming that means 12mm into the OSB. I've found a screw/tube combination that will give me 17mm penetration, but probably a little less with the thickness of the vapor barrier etc.

If the screws miss then I will probably make it very difficult to make it air tight. It would be a lot quicker to do without trying to hit the joists, so I'll have a thick about what I do. I'll need to draw a diagram with the exact positions of the joists.