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I am making this small pigeon loft. It rains a lot here so I used some marine plywood that won't rot and clear coated it.

I was going to do the corner framing in douglas fir, but the color clashes with the plywood at the corners.

So, I was wondering, you can get this plywood in very thick sizes, like 40mm thick. Is there any reason why I can't cut the plywood into small thick strips of 40mm*40mm fence posts of about a meter high to replace the douglas fence posts at the corner?

I don't see why I couldn't do it, but I have never seen anyone cut plywood just for framing material. Is there a reason why I shouldn't do it? Thanks.

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I don't see why I couldn't do it, but I have never seen anyone cut plywood just for framing material.

In an interior context this is something that's done occasionally. The first application I'm aware of is an all-plywood workbench which was published in one of the major woodworking magazines some years back. The design mimicked the structure of a conventional solid-wood bench where the plywood was sawn into strips on the table saw and laminated to make the structural members (careful cutting yielding 'free' mortises in the legs and tenons on the ends of the stretchers.

There are some good reasons I can think of why you wouldn't see this in an exterior application and one of the major reasons would be one of cost. It's not the only one and may not be the most important.

The thing about laminating up plywood to make a structural member is the amount of edge this exposes. Basically you make an all-edge face for every piece you glue up. And it must be remembered that every second ply is oriented at 90°, which means end grain facing outwards for approximately half the surface. I realise you're planning on using a clear finish, but I think this is likely to be an issue in the long term.

Exterior finishes require careful application to provide initial protection (no pinholes or thin/missed spots) and then regular upkeep for the lifetime of the piece to maintain that protection. Skip or put off a single season of the upkeep schedule and water could get to the all-edge faces of the plywood, leading to swelling or delamination, or both, not to mention the colour changes (greying or dark stains).

I was going to do the corner framing in douglas fir, but the color clashes with the plywood at the corners.

It's a pigeon loft, how much does this matter? I do understand wanting things to match, at least be fairly close, but does it really matter for something like this? If your inner OCD does insist on close or matching there's always the option to stain or apply a coloured finish. Or you could....

Paint it
A better option is paint. Obviously this ensures complete uniformity of colour without any experiments or test pieces, but in addition paint is the best protective finish. This will be noticeable over a span of many years if that's the hoped-for lifespan of the project.

Alternatively...

Don't apply anything
And let the wood weather naturally. After a couple/few years the ply and the fir would come together to a weathered grey colouration which will naturally tie the disparate materials together.


1 Since then a number of similar benches have been published by various creators. One doesn't have to be too cynical to believe the rest copied the idea of the first one, or indeed one of the existing knockoffs, even if they present the idea as their own.... I'm looking at you Paul Sellers.

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  • Oops... I gave you credit in my answer, but failed to properly do so before hitting "send". Rectified.
    – FreeMan
    Commented Sep 15, 2022 at 14:20
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Graphus brings up some very valid points about all the exposed "end grain" of plywood when laminating your own posts. This is a valid reason not to do it.

A couple of options to avoid all that exposed "end grain" come to mind as alternatives to your simple, flat lamination:

  • You could make a simple box out of your plywood with mitered joints. This would leave the only exposed "end grain" at the top and bottom. If you wanted to be really sure, you could miter caps onto the top & bottom, as well.

    i.e. Make a 45° bevel on each edge of the plywood and make a tall, vertical box that ends up looking like a post. There should be plenty of glue surface, and, at 40mm (~1.5" for those on this side of the pond) thick there would be plenty of room for driving screws through it, too.

    The box would look something like this awesome ASCII art of an "exploded view" showing the top-down view of the individual pieces of plywood:

    _________
     \     /
  |\  \   /  /|
  | \  ---  / |
  |  |     |  |
  | /  ---  \ |
  |/  /   \  \|
     /     \
    ---------
  • If you need an 80x80mm post*, using thinner 20mm plywood, cut 4 strips at 40mm wide. Laminating these on their faces gives you a 40x80mm solid post.

    Two additional strips at 80mm wide attached at 90° to the existing lamination will cover all the "end grain" and give you your full 80x80mm post. Leaving two strips of "end grain" only 20mm wide on each face, which would be easier to hide and/or seal.

    Something like this:

----      <-- one layer of 20mm ply, 80mm wide
||||      <-- four layers of 20mm ply, 40mm wide
----      <-- one layer of 20mm ply, 80mm wide

All in all, though, I would generally agree that this is probably overkill for what you need, unless you really, really need to have all the color actually match and paint won't do the trick for you.


*Of course, adjust all measurements to fit the plywood you have/are buying and the dimensions of the posts you're intending to make. This is an example with simple math.
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    ASCII art FTW!!
    – MattDMo
    Commented Sep 15, 2022 at 14:20

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