Garden shed plans

A concrete slab for your shed

This is the original metric version of the plan (millimetres/centimetres). See also Garden shed plans : Imperial (feet/inches).

A concrete slab gives you a solid foundation for your shed. But if you don''t want to use concrete, you can build a wooden floor instead.

Site preparations

Assuming you already have a level site, start by removing the top 20-30mm of soil.

Your slab will be 50mm thick (not including the trench around the edge) and you want it to rise 20-30mm above the level of the surrounding ground to stop water running into the shed.

Boxing

Use untreated 50mm by 25mm timber floor boxing. 50mm is the minimum thickness for concrete in nonstructural applications. For your shed, you need to dig a small trench around the edge as showen in Figure 2.1. This trench will mean that the concrete around the edge of the slab, where the walls will sit, is thicker than the concrete in the center. It will stop the slab cracking under the weight of the shed. The trench should be about 150mm wide at the top.

Pouring the slab

For instructions on mixing and laying concrete, refer to our article All about concrete.

Important: We recommend you set anchor bolts into the edge of the concrete immediately after you pour the slab. This can be quite tedious, but it is the best way to attach the frame of the shed to the floor in our opinion. Although there are other ways to attach structures to concrete, they all have their own draw-backs and require specialised tools. Masonry anchors and concrete nails are not generally recommended for new concrete.

Because this is a relatively small concrete slab, you should have time to place all the bolts after you pour the slab but before you work the surface of the concrete with your trowel. In larger jobs the bolts would have to be placed first and the concrete poured around them. You can buy special anchor bolt holders if you do choose to place the bolts before the slab is poured.

Refer to the plan and make sure you don't place any bolts where the doorway will be or below a vertical beam. Start about 100mm from the corner and place the anchor bolts about 300-400mm apart.

Alternative: wooden floor

A suitable alternative to a concrete slab is a wooden floor with skids. For a wooden floor, lay four 2.2m lengths of treated 100mm by 75mm timber down on your flat site. The timber needs to be treated to a grade that is suitable for continuous contact with the ground (ask your supplier).

Nail five 1.5m lengths of 50mm by 50mm timber at right angles to the larger beams. This will form a platform which you can nail floor boards onto. Your shed can be screwed down to this floor.