External Ply

[007]

I am building a structure in my garden which has a curved surface. For this I was going to use layers of thin plywood to build up the curve. The edge of the plywood will rest on a layer of gravel. What type of plywood would be best (WBP or marine)? Will either survive being in contact with the ground (gravel)? What treatment should be applied to the wood to preserve it (anything other than Cuprinol Wood Preserver)?

Thank you for any advice you can offer.

 

[sidecar_jon]

marine ply is shockingly expensive but will last longer. Personaly id use WBP and soak the edges in paint, especialy where it touches the gravel. If your going to laminate it watch what glue you use, PVA wont last long.

small tip from boatbuilding. Bend the first layer but dont fix it perminantly, put the next layer on with glue and screw a batton round the curve let it set and repeat with other layers, otherwise its easy to get it all "wamberly"

 

[PowerTool]

How thick are you planning on making it ?
i.e. is one or more sheets of flexible mdf an option ?

 

[sidecar_jon]

can you get waterproof MDF?..... if you can, im gona build a boat!

 

[007]

Thanks for the feedback!
OK, I'll give you some more details..
I got the idea from here
http://www.blackanddecker.com/projectcenter/DocumentView.aspx?DOC_ID=p_2_102_23915_23954_24035.html
except that the curved surface I am constructing is concave rather than convex as show here. Its for the edge of some decking which is built on 2"x6" joists which rest on a layer of gravel.
So I was going to use 1/4" ply, probably 4 layers, then finish with a layer of 3/8"ply, if necessary (not sure why the last one needs to be different, but that's what it suggests at the above website).
Was thinking of using Evostik Weather Proof Wood Adhesive with screws - is this ok, or can you recommend an alternative?

The minimum curve radius is just over a 1m, but the curve traces out an ellipse. Total length is under 7m, with a height (with the ply resting edge to the ground) of 150mm. I was hoping to get it all from one sheet.
PowerTool, what is your suggestion re. flexible MDF? Is this better than plywood in damp conditions?

sidecar_jon, thanks for the boat building tip. How would you do this for a concave curve?

Thanks again for your help and advice!

 

[sidecar_jon]

same way.... ive done it with fitting ribs in wooden boats. Laminated ribs. basicly the thin ply will try to buckle and not make a "fair curve". so you screw a tempory batton on top of it clamping it together. Remove the batton when its dried. Often useing cling film to stop the rib beign glued to the planking.

Im not sure why the top layer has to be thicker, sounds overkill to me!.... as an idea our local wood yard does off cuts, the strippings to make planks the right dimensions. They are thin and might look better than a blank Plywood face to your deck. Just and idea, glue them over the ply as last layer for a planked effect.