I have this castle built in my garden out of plywood. The problem is that it keeps leaking. I've tried silicone which doesn't last, and also a window sealant, but since the plywood expands and contracts it also lets the water through.

The join in question is a horizontal plank joining a vertical one in the middle. So it's a floor joining to a wall.

  • When you say 'castle' do you mean like a fort/treehouse-like structure? Do you have photos? Mar 18, 2015 at 11:58
  • A photo or diagram would be immensely helpful.
    – rob
    Jun 24, 2015 at 17:47

3 Answers 3


I used to build a lot of wood boating projects, i.e. deck boxes for life-jackets, hatch covers etc. Anytime two boards were joined, I used a spline the full length and filled the gap (if one existed as in a curved hatch cover) with 3M 5200 sealant. Epoxy is used extensively in boating projects which are almost always outdoor project subjected to intense sunlight. Using fiberglass over edge joints will also keep out the water.


You will want to install siding, essentially add a layer of material that repels the water away from the plywood.

You can start by wrapping it in plastic housewrap and then nail wooden shingles to the wall. Start from the bottom and overlap them.


Without a photo it's a little difficult to be absolutely certain what your problem is, but I'll try to cover the ones that I think apply to your situation.

It sounds like you built your floor, then butted the walls on top of the floor, and any rain that catches the edge of the floor seeps under the wall. Regardless of which solution you choose, you should keep in mind that it's always a good idea to slope any exterior horizontal surface (even if only slightly) so it can shed water rather than allowing it to pool.

My first inclination is to say that you should have instead installed the wall frames out to the edge of the floor, then install your plywood to the outside so the bottom of the sheet extends below the top of the floor, and ideally below the bottom of the floor. This creates a drip edge so water follows the wall below the floor surface and the water cannot infiltrate into the structure. If your plywood does not extend past the bottom of the floor, you can install aluminum flashing to help carry the water below the floor.

The join is a horizontal plank joining a vertical one inn the middle. So it's a floor joining to a wall.

Again, without a picture I can't be certain what your problem is, but this comment makes me think half of your floor is inside the structure and the other half is outside the structure, as in a porch. A possible solution in this case is to frame the wall and install right-angle flashing on the floor where the wall frame and floor meet. Caulk or otherwise seal it to the floor, then attach your plywood sheathing to the wall so it covers the vertical part of the flashing but does not extend all the way to the floor. This allows it to slide up and down with the seasons and prevents the bottom from absorbing pooled water on the floor and rotting.

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