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I'm planning to build an L-shaped storage box to keep outside, and I'm interested in your thoughts on suitable materials.

It'd look like two steps of a staircase, with the top of each step liftable like a storage trunk. I'd also like it to function as a bench, so my partner and I can sit on the top step, with our feet on the lower step.

I'd like to try to keep the things inside the box dry, I can easily line the inside with plastic or sealant, but the materials will need to be hardy. It'll be protected from most rain by the high walls behind and to one side of it, the low wall to the other side, and the overhanging roof that'd be totally above it.

It'd be roughly 1,100mm wide with the steps' run & rise being roughly 400mm each. It'd need to take the weight of two humans; let's say a total of 200kg, in case a heavier friend joins us on our steps.

I'm handy, but haven't done any furniture design — I've been googling about for information on the load-bearing capacity of treated chipboards available here in the UK (perhaps 10mm or 16mm P5, P6, pr P7?) — but I realised that my first approach — a 2x4 frame in pinewood, with treated & painted chipboard faces — might not be the best approach!

Do you have any advice on materials? I'm comfortable going for more expensive materials to allow easier workability, or longevity. (I have woodworking tools, but no workshop. My local wood supplier will cut sheets to any size, and may also be comfortable with cutting the L-shaped pieces)

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    I'm not familiar with what materials you have in Europe, however chip board and and pine 2x4s are not suitable for outdoor use. They will degrade quickly. In the US we would use a PVC material that is supplied in the same dimensions as most framing lumber. Also Pressure treated lumber (PT) for sheathing exterior plywood with cement based siding, or marine grade material such as Starboard.
    – RMDman
    Jul 30, 2023 at 15:15
  • This is instantly very useful! Thank you! I had a hunch this would be a poor choice of materials — though I am wondering how wooden sheds cope? Let's see if anyone more familiar with what's available here can help me out; I'll take a look around the internet to see if folks have UK-specific equivalents to the materials you've mentioned.
    – JP.
    Jul 30, 2023 at 15:53
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    I think you'd want, at a minimum, a pressure-treated plywood—if that's available in Blighty. Chipboard will just expand and fall apart if it gets wet.
    – Huesmann
    Jul 31, 2023 at 12:27

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Even if the roof above overhangs this storage box, I would build it with the assumption that it will at times get wet from above. It would be good to plan to have the top as much as possible be made out of single pieces with slight overhangs of top over the sides and then to have drip edges cut into edges at the bottom of the sheets. This is so the water will drip away at the edges and not loop back and get into the storage below. Also, if you put ever so slight slants on the top panels that are barely perceptible but would make the water roll off predictably to the side that you choose, this will help to keep the contents dry. There are marine grade plywoods and marine finishes that are very resistant to moisture, these would be great for the top. Also consider having rot resistant or impervious material on the bottom/feet. The plywood top would benefit with a stiffening half-lap timber frame beneath it. I would suggest avoiding manufactured wood unless it was designed for specifically outdoor or marine use; however the quality of the material and finish is vastly more important at the top and bottom that in the body. If you need to cost engineer, focus on the top and bottom. Personally, I would not line the interior with plastic as it might cause condensation and mold in changing weather conditions. Instead, try to the think of the top as a roof and the sides as sidewalls and try to imagine how moisture would roll-off. The box only needs to have gaps tight enough to keep insects out.

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I'm considering something similar, for in a porch but where the rain will blow at it. I'm also in the UK.

I'd make mine from planks of smooth planed pressure treated timber. This is generally softwood, but lasts well so long as it's not immersed for long periods.

I want mine white, so I'd cut the pieces, prime, undercoat if necessary, paint with exterior gloss, assemble, and possibly touch up the paint. I couldn't paint the back once assembled in place, and don't want to get paint on the brickwork where it's going to go.

I want mine ventilated so would leave gaps between the planks on the sides. Anything I wanted to protect from moisture would go in a lidded plastic storage box inside - bought before I measured up and cut.

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