ply over 22mm t&g chipboard

[adam01]

Hello

I'm going to be laying just under 4 sheets of 4' x 8' (2440mmx1220mm) 12mm ply over 22mm t&g chipboard. Some questions

1) Screws or nails? I'm thinking screws (total wood depth 12mm ply + 22mm t&g =34mm. I was thinking these? http://www.screwfix.com/p/turbogold-double-countersunk-screws-4-x-40mm-pack-of-200/14973

Is that the right type?

2) What spacing for screws, every 150mm or is that a bit ott?

3) Along one edge there is a thin strip of t&g. It's not particularly firm (I will attempt to shore it up) however I can see there will be a gap between it and the ply. I want to fill the gap. I was thinking of using gripfill

4) If I use the screws I linked, do I need to either pre-drill or pre countersink?

5) Any other considerations?

Thanks

 

[ree]

What is the chipboard laid on?

Are there any pipes or cables under the floor?

What will be the final floor covering?

Item 3. is confusing - do you have a pic?

 

[adam01]

Chipboard is laid on a suspended wooden frame base which sits on a lower existing concrete floor (internal garage). It is sound and firm.

No pipes or cables under the floor. There are some pipes but they are much much lower.

Final floor, will be fiber based screed over the ply and Karndean finish.

I don't have a pic. What I mean is, the main chipboard boards don't go right to the edge of two walls (i.e uncut pieces) so some thin strips were put in on two of the edges. They are not fixed very well to the joists, and are not quite level either.

 

[JohnD]

Have you considered throwing the chipboard in the skip, where it belongs, and just laying a proper ply floor?

 

[dazlight]

If your fibre screeding over why not use 6mm plywood.

But if using 12mm ply then 150mm centres and 100mm on perimeter of each board. Make sure you prime the plywood before you screed it.