How to Lay a Concrete Slab for Your Garden

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G'day and welcome to my guide on how to lay a concrete slab for your garden shed.

Nowadays garden sheds are very easy to assemble and don't require DIY skills at all. However, laying the slab can be a bit more challenging and concrete is hard to fix if you make a mistake. This guide is intended to help you to plan your job carefully and execute the job with a great outcome.

Tools you might need: Measuring tape, hammer, shovel, long spirit level, bolt cutters, saw, stringline, wheelbarrow, edger and  power mixer. 

Once you nominated a good, reasonably level site for your shed, measure and mark out the area, stretch stringline between pegs then clear the area and remove any vegetation. You'll need a 100mm deep slab for your shed. It is recommended that you extend 150mm behind the edge of the slab so it gives you enough space for preparing the box frame.

Your site needs to be level, firm and free of vegetation. If you have a rain water tank you might want to sprinkle water on the surface to achieve further compaction. You can also add a layer of gravel and tightly compact into the surface.

Your box frame will keep the concrete in place while it sets. 30mm x 100mm treated pine is ideal for the frame. Place the box frame on the site and level it. Hammer in pegs on each side (1 peg/1000-1500mm) and nail the frame to the pegs. Make sure the ground is roughly level underneath your frame and there are no gaps between the ground and the bottom of the frame where concrete can seep through.

The next step is to add steel reinforcing to prevent the slab from cracking (I personally like to lay building plastic underneath the mash but it is not essential). Cut the reinforcing mesh 50mm less than the size of the box frame to make sure it will not be exposed and will be protected from rusting.

At this stage you are ready to pour the concrete into your frame. You can source your concrete from the following mixing methods:

  • Hand mixing: Don't underestimate the time and energy that this method requires. I choose this method when I erected my first garden shed, it was a 2000mm x 1050mm slab and I regretted the decision after 10 minutes. If you only need a small slab, it definitely will be the cheapest solution. Mix the concrete on a smooth, clean surface not on dirt.
  • Power mixer: Mixers can be hired from your local tools hire shop. After you turned it on first add the aggregate, than the sand and the cement and let it mix thoroughly. Then add the water and let it mix for 3 - 4 minutes.
  • Ready-mixed: You will need to order 1 m3 or more generally. If you tell the supplier the size of the slab they'll work out the amount needed as well as the cost. Make sure the track can access your site. If not, you'll have to use wheel barrows to transport the concrete to the site. You'll need a few helpers with this job to get it done in a reasonable time.

Once you have the concrete in the frame, spread it evenly with a shovel. Make sure the mash is fully covered and the edges are well filled. Then put a board across the frame and move it in a sawing action to level the concrete. Then apply a firm tapping action, moving the tamper forward by its size each time. Next, round the edges with an edging tool to give the slab a more professional look.

All you have left is to remove the box frame a few days later and your slab is ready for your garden shed.

Thank you for reading through my guide, I hope you found it useful.

Don't forget to have fun during the process.

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