What screws to use for marine plywood box?

[TAEncBay37]

I have to join 12mm Marine plywood along the edge of a timber project and wanted to know if there were specialised screws that will do the job? I would pre-drill the holes, but thought there may be a particular screw designed for this type of plywood material that is prone to splitting along the edge when srewed. The project is a manifold box to house irrigation solenoid valves, which will be attached to the back of a shed and I would expect that 316 stainless steel screws would be best for outside as we are about 1km from the coast. Appreciate any expert advice on this!

[EricL]

Hello @TAEncBay37 

I suggest putting in an 18 x 18mm 5.4m H3 Treated Pine FJ DAR Primed LOSP on the inside corners of the marine ply. This will prevent the marine plywood from lifting and splitting. You could technically pred-drill pilot holes but the 12mm panel will not provide you with a great deal of strength.  

I also suggest using Selleys 250mL Aquadhere PVA Wood Glue Exterior Adhesive. It is suitable for exterior use in non-structural applications.

I've placed a sketch below to give you an idea of how to place the timber support. In regards to the screws, I agree with your choice of 316 stainless, but you might have to settle for Zenith 8G x 30mm Stainless Steel Long Thread Countersunk Head Timber Screws due to length availability.

However, I suggest visiting your local store and having a look at the stainless-steel screw selection.

If you need further assistance, please let us know.

Eric