do you mean the back gate type thing?
i presume you do not mean the real back door to the house.
if you do mean the garden gate type thing then yes external ply is fine though it doesnt look too good in nmy opinion.
much nicer is a door made up of boards etc.
mazz you could do that, but as loon says you may get problems with the edges.
i always think that board type gates look like a council house.
making a ledge, braced and batterned door is a piece of pss. just cut them to length, lie them face down and put a z on the back. screw through the z with stainless screws careful not too long and hey presto a door.
it will look tons better as well. a builders merchant will even cut all the peices to the right length if you ask them.
most merchants will have a v type board in redwood that is made for this. it is nowmally about 15mm to 18mm thick. make the z rails out of something like 40mm to add strength. put the cross rails where the hinges go.
no need for glue just make sure you use redwood not whitewood. bnq only sells whitewood boarding.
stainless screws and screw for the back. if your z rails are 30mm and the thickness of the boards are 15 then you want to sink the head slightly. i would use something like 40mm 6s or 7s.
Mazz,
I have made a few exterior malasian 3/4 inch ply gates for customers, for one thing they do not shrink or expand , three 6x1 prepared should be enough for the top, middle and bottom rails screwed with 40mm size 8 or 10's counter sunk galvanised screws, (stainless better but expensive,) give the rails a coat of clear cuprinol before screwing on, or brown if you are going to stain the gate, no idea of what size the gate is but timber yard will advise you what size galvanised tee hinges to get. you may have to build up the middle rail at he lock end to take a suffolk latch or rim lock. good luck! (exterior ply is glued together with Phenol glue so it is boilproof and waterproof and does not delaminate) (just for you Panlid )
petengade wrote:Mazz,
I have made a few exterior malasian 3/4 inch ply gates for customers, for one thing they do not shrink or expand , three 6x1 prepared should be enough for the top, middle and bottom rails screwed with 40mm size 8 or 10's counter sunk galvanised screws, (stainless better but expensive,) give the rails a coat of clear cuprinol before screwing on, or brown if you are going to stain the gate, no idea of what size the gate is but timber yard will advise you what size galvanised tee hinges to get. you may have to build up the middle rail at he lock end to take a suffolk latch or rim lock. good luck! (exterior ply is glued together with Phenol glue so it is boilproof and waterproof and does not delaminate) (just for you Panlid )
Then wpbp ply was not used!! (water proof boil proof) you must use the correct plywood! it is stamped on the ply, have even built a boat out of it and still ok (BS1012 marine ply was too expensive) (40mm size 6 screws, what are you on? )
listen peter. you make gates out of plywood and you have the cheek to ask me what i am on
anyway i never knew wbp was stamped on the ply
by the way you are quite right. i meant 40mm 3.5s.(?)
im still of the old imperial so i would call them 1 1/2" 6s
the screws i buy are in metric but have the equivilant imperial size on the box for old gits like me.
(just for you Panlid 
)
though. Baords would be far more attractive. Try a gate kit Most DIY Stores do them.