Before erecting a shed or greenhouse, it is a very good idea to lay down a pad of concrete for it to sit on. This serves not only to give the shed/greenhouse a firm footing, but will also help to reduce moisture seeping up from the ground into the timber.
Once you know the dimensions of the shed, etc, that you are erecting, you need to place some setting-out strings to contain the concrete at the size you require. Setting-out strings are simply lengths of 25mm thick timber held in place by nailing them to pegs driven into the ground on the outside. This will create a "mould" for the concrete and help to create nice clean edges. Make sure the timber you use is as wide as the depth of the concrete pad you are planning.
Now you have your formwork in place, you need to start filling it. If the soil you are laying the pad on is very soft or high in clay content, you will need to lay a layer of fine hardcore (crushed building rubble, stones, soil, etc). Lay this to about 50mm and make sure it is well compacted. Use a small garden roller or simply compact it with a spade.
Now using a general purpose concrete mix (1 cement, 2 sand, 3 course aggregate), start to fill the formwork. For more info on concrete mixes, read our Concrete Guide . Spread out your concrete as you go and tamp it down with a levelling board. Tap the sides of the formwork periodically to create a cleaner edge. Continue to add layers of concrete until you reach the required depth. Smooth off the surface with a wooden or plastic float.
Cover the pad with a clear plastic sheet to keep frost out and to stop the concrete drying out too quickly. If the concrete dries out too quickly, it will not set properly and may crack and crumble. If it looks like it is drying out too quickly, spray it lightly with water to damp it down. Once it has set properly, you can remove the formwork and the pad is ready to take your chosen structure.
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