I've got a sheet of 15mm plywood which I need to build a door out of. Basically I want to cut the door in one sheet and use the leftover for reinforcing the edges by adding a layer on each side.
Will this work?
How should I affix the frame? Screws or glue, or both.
I've built a plywood door for a shed before and ran into a few issues that I'll at least make you aware of even if it's not a direct answer to your questions.
I used a layer of T-11 Siding (plywood with grooves for the look of slats) over a 2x4 frame. I screwed and glued the plywood siding to the lumber frame and then lined the inside of the door with some 1/4" plywood for a finished look and strength.
What I ended up with were some very heavy doors. It seemed like a great idea in my head, but I ended up having to change the hinges I planned on for something more heavy duty. Also, the finished thickness of the door didn't allow me to use standard door locks. I ended up having to use a "shed" style T-handle latch and a regular lock and hasp to secure the door.
So, recommendations: Use one layer of plywood with a layer of reinforcements on the edges and maybe some diagonals for stiffening. Strong adhesive should be good enough, but you have to clamp it together, so you might as well use glues and screws. Pay attention to the finished thickness of the door to make sure it's what you want. Two layers of your plywood (one sheet, one reinforcement) is going to make a very thin door, and if it's full sized, I doubt you can stop it from twisting over time. Plywood over a frame of different lumber seems like the best option.
Here is a 4’x4’ door I made recently for a playhouse (bottom with the large X). It’s a layer of 15mm plywood with an exposed 2x4 lumber frame. It’s been holding up great so far.
jPhi1618's answer is good. I'd use urethane glue if you don't want to use screws. It's equivalent to what the plywood manufacturer uses to combine the plies in the sheet and should hold up to moisture and movement.
If you're ok with screws, countersink and pilot the strips for 1-1/4 gold construction screws (or something fancier). Use caution so you don't run them too deep or strip the wood in the underlying sheet.
On doors:
Hardware for non-standard thickness doors is available, but will eat up any money you saved making the door. Door knobs expect a door between 1.5 and 2 inches thick. So if you want to make it out of 15 mm plywood, you would need a layer 3 layers thick on both the hinge edge and the doorknob edge to use standard hardware.
This would be a non-trivial weight door if you went with 45mm throughout. Plywood is not very gracious about screws into the end grain. If you are hanging this door conventionally, I think you are going to have problems with the hinge screws pulling out.
The answer to this may be to use barn door hinges, which surface mount with either heavy screws or bolts. In this case I would choose screw hardware (see below) that matched the style of the hinges. I would suggest black iron with square washers, and black square head screws.
There are ways to make doors out of conventional dimension lumber,the plank and Z brace being most common. Do it this way, with a double layer of planks over either thin plywood, or oriented strand board. A layer of sheet material eliminates drafts through the door.
On screws:
Make a feature out of an necessity. Recently I did a stair railing out of 1x material. The bannisters needed to be heavier, so I glued 2 layers of 1x together, and screwed them with brass coloured screws with brass grommets. Stained with umpteen coats of empire red stain diluted 4:1 with water soluble varnish. The gold screws against deep red glaze is stunning.
