I have some decking for which I've built the frame, and the main "floor" decking already but haven't done the sides for yet. The decking goes right up to the edges of the subframe that it sits on.

For the sides, i'm going to screw some more decking boards horizontally onto the side of the sub frame. However, I'd like there to be a 5mm gap between the decking top and the side, to allow for some expansion, and also to allow for a consistent flat surface for the boards on the side.

How i was thinking i might achieve this is by putting a 5mm plastic strip between the boards and the frame, where i'm screwing it in. (I was thinking plastic just for ease of cutting it up into the required size) I would then glue the strips to the board, in line with where the screws would go, before putting it onto the frame and screwing it on.

Does this seem like a sensible approach? Does anyone know of a product like this? Or a simpler way to do it with some cheap material?

EDIT - i just thought of one solution: buy a bag of nuts and bolts (or just nuts if they can be bought without the bolts) where the nuts are 5mm thick. This should be cheap and easy. Then, yuse the nuts as spacers. Ideally i would screw through the middle of each nut, but this might be tricky to manage.

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    5mm is a large gap I usually use a 16 penny nail to space my boards. A gutter nail would be close to 5mm (the long nails to attach rain gutters to the house). they would be easier than cutting up plastic.
    – Ed Beal
    Apr 19, 2016 at 13:19
  • Thanks @EdBeal - do you mean that the gutter nail is 5mm thick? For spacing between the boards (also 5mm) i've been using a few thin slices of wood which i cut up, but here i think i need something i can put behind the boards which will go all the way down behind four or five of them, in line with the screw holes, ideally on either side of the screws. I'm not sure how a nail would work in that situation - how would i hold it in place? Apr 19, 2016 at 13:22
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    I usually use a nail on each end the head keeps it from falling through if i have a bowed board i will use several and straighten the board by starting with 1 end screwed in and pulling the board to the nails and screwing down some times it takes 4 or 5 nails to do this and maintain the spacing. The gutter nails are close to 5mm from memory not sure exactly how thick. they could be bent over since they are long making it easier to pull them after the board is screwed down
    – Ed Beal
    Apr 19, 2016 at 13:32

1 Answer 1


I've always just held the decking back from the skirt (which is fastened tight to framing) to create a gap, but this obviously results in an undrained channel. I can see the benefit of your plan for allowing small tree debris and other dirt to drop out.

Rip some 5mm strips of your decking material from scrap and cut them just shorter than the height of the skirt (so they don't show). Skip the glue (too slow and messy) and just tack them on at 16" intervals (or in line with your joists) with small galvanized nails. Screw on your skirt boards and be happy.

One thing to keep in mind, though: Unless you don't fasten your decking to the rim joist, expansion pushes the framing also, making your gap useless. In my experience, decking doesn't move enough to warrant such a gap. It may be nice to have it for aesthetic reasons, though. Sometimes flush butt joints are ugly.

  • Thanks. Maybe i will not bother with the gap and just screw them all straight onto the frame. I thought that the strip behind might make it easier to have them all nice and flat but maybe i'm overthinking it. Apr 19, 2016 at 14:48

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