Adding OSB floor

Build Your Own Summerhouse – Part 2, Floor Frame

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STAGE 2 – Floor Frame

So it’s been a while since I wrote stage 1, I’ve mostly been working on other projects, I can’t blog them fast enough. I guess I won’t be running out of things to write posts about any time soon. Creating the floor frame relies on your having already read/completed stage 1.

Ground Cover

Some people might want to skip this step and to be fair, it’s non-essential. However, it will mean life is a lot easier later when you don’t have to maintain the space underneath and around your summer house.

Essentially you’re aiming to cover all of the ground under and around the floor frame of the summer house with some kind of weed control fabric. I just bought reasonably cheap stuff as it’s not going to be walked on or really have to withstand any wear.

Start at one side and work your way across leaving a good 6-8 inch border at the edges and a similar amount of overlap between the pieces. Depending on the height and position of your concrete blocks you’ll either run the fabric between them or cut holes in it so it lies flat. Use plastic fixing pegs to pin the fabric in place.

Floor Framing

Make sure you don’t skimp on the materials here, this is going to be holding your whole building up! I used 50mm X 150mm C14 graded tanalised timber. Also, make sure you treat any cut ends to stop the water getting in and causing rot. Any sort of end grain treatment meant for tanalised wood will do.

The first image shows a close up of one of the corners of the floor frame. Start by cutting your longest pieces to length, there will be 5 of them, 3 to go flat on the blocks and two that form the long edges of the frame. Screw the edge pieces into the pieces that lay on the blocks from the bottom to make an “L” shape. Then it’s simply a case of cutting the short pieces to length, spacing them evenly and fixing them in place. Mine are 600mm between the centres. For all of this use screws suitable for outdoor use and of a decent thickness and length. I used 5mm X 100mm Goldscrews from Screwfix.

You may find that you didn’t quite get your blocks perfectly level causing small gaps between some of the pieces in the middle. Don’t worry about it, just add some small shims in the gaps and screw down tightly against them.

Floor Boards

Squaring the frame

Before you move on to adding the floorboards you need to make sure your frame is square. You’ll already have it most of the way there by eye and the sheet material you use will also help with this but there’s another more accurate way that works for all rectangular frames. To square the frame, simply measure diagonally between two opposite corners and then compare that measurement to the opposite diagonal. To adjust simply brace one corner and tap in the other with a soft mallet.


Once everything is squared up you can add the floorboards. I used standard 2440mm X 1220mm sheets of damp resistant OSB. The 600mm centres on your floor joists should mean this all fits together nicely. If you’re wanting a deck area at the front like mine then you only want to board the area that will be inside.

Lay the first board in place and snap a chalk line along the joist positions to help with screwing it down. Put screws in about every 400mm and use screws with a partial thread to pull the boards down tight. The last board will need trimming and it’s actually best done in-situ if you have space; screw it down and then use a straight edge and a circular saw with the depth set correctly to trim off the excess.


There are many ways you could insulate your summer house. It could be argued that you don’t need to at all but this heat reflective vapour barrier is cheap and effective, so why not. Other options would be rock wool and Kingspan (other foil backed foam board are available)

Unroll and cut enough that it overlaps the edges of the boards and then fix directly to the edges of the frame with staples. I folded mine over at the ends to help strengthen it. Keep doing this with a slight overlap between each piece, perhaps 50mm, until you’ve covered the whole floor. Use aluminium tape to seal the join lines.

And that’s it. One thing I should mention at this point is you’ll need to leave room on the joist nearest the front for the ends of the decking. I didn’t think ahead when I was putting down my boards and had to screw in the off cuts to make a ridge. This joist will take a lot of the weight that’s transferred down from the roof so it may be a good idea to double the thickness here anyway.

Stage 3 coming soon (ish)…

Also published on Medium.

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