Wednesday, 21 June 2017

Building a New Garden Shed - Starting to Concrete

June 1st, 2016

The existing concrete floor was at two different levels. It was partially cracked also and didn't have a vapour barrier which meant the air in the shed was always damp, with lots of condensation on the underside of the corrugated roof on cold mornings, which inevitably dripped down over everything. I decided to break up the cracked section of concrete which resulted in a large pile of rubble. I would also lay a new floor on top of the remaining concrete section, but make it thicker at the edges for structural strength. The new shed would be several feet longer and wider than the existing one. This resulted in having to excavate lots of soil which needed to be spread out over the garden, under trees, into hedges and basically anywhere I could find space.

Marking Out

I marked out the 4 corners of the shed with 12 mm rebar and marking paint. Firstly I hammered two pieces of rebar into the ground to mark the front wall, knowing the length of the planned shed. Then I roughly positioned the 2 bars for the back wall knowing the width of the shed. Knowing the length and width of the shed and using Pythagoras's Theorem, this gave me a measurement for what the diagonal length should be between opposite corners. Using two long measuring tapes, I altered the position of the two back pegs until both diagonals were equal.

Building the Formwork

I used 4 x 2s for the form work. For the long walls, 4 x 2s had to be joined with short 4 x 2 scraps. At the corners, nails weren't driven home so that the formwork could be easily disassembled with a crowbar. Thinner boards can be used instead of 4 x 2's, but 2 inchthick timber requires less pegs because it doesn't flex as much over a long span. Pegs to support the formwork were driven into the ground at corner points  marked out by the re-bar as described in an earlier post and also at two intermediate positions for each wall.
I wanted to make the concrete around the perimeter of the slab stronger for structural strength. So I dug a trench all around the edges and filled this with rubble from the broken up floor. This would make the concrete slab about 5 to 6 inches thick around the perimeter. Next I laid out a sheet of 1200 gauge polythene sheeting and on top of this I would add a layer of concrete about 3 inches thick on the existing 5 inch concrete floor. The floor would be poured in six, 1m wide strips.

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