Plywood Strength?

[Tim Good]

In my bunk room there was a length of 12mm ply. It could hold body weight clearly because it was a bunk.

I recently removed it and put in an 18mm piece which I now intend to use as a work bench. You'd think at 5mm ticker it would be more than enough. However when I bare down on it, it barely feels like it would hold my weight! It flexes quite a lot and doesn't feel rigid like the 12mm on there originally.

The ply is good quality from Robbins Timber who advised it was correct.

It is laid with the grain / layers along its length. If I cut a new piece across the layers would be make it substantially stronger?

If anyone feels I should have a totally different wood for a work bench then please recommend.

Thoughts?

 

[mtb]

you could glue two pieces back to back using a good quality waterproof glue
what frame are you using or is it just meant to fit into the space used as a bunk, if your making a new frame then you can create strength and make it rigid. The frame is just as important as the top
the picture shows the side view of the bench I made for woodworking

 

[Stork_III]

How many plies does it have, the more the better (and the more expensive)? The grain of alternate plies is normally at right angles. Is it the same wood as the original?. Often inner plies of decorative plywood are of inferior less dense wood. A backing frame will stiffen it considerably, even just with members across the width.

 

[Concerto]

Adding some supports will stiffen the plywood, irrespective of quality. Try to have an outside frame and supports every 500 to 600mm along the length. The thicker the supports, the more rigid it will become. If you can use 50mm square timber it would be very stiff, but 38mm square would be adequate. This might raise the working height slightly, but that would probably be a good thing to stop stooping whilst working.

 

[estarzinger]

^^ yea, just add some ribs underneath. That will dramatically increase stiffness without adding unnecessary weight.

but why 18mm would feel less stiff to you than 12mm is a bit of a mystery . . . . fewer plys, less stiff wood species, voids in the plys . . . .could be any of a number of things basically indicating a lower quality plywood.

 

[ghostlymoron]

It depends on how its supported. If 'simply supported' at each end it will deflect much more than if supported on all 4 sides or if supported in the middle as well.