Part 2 – How to pour a shed slab. This is part 2 of a simple guide to what you might need to do or what you could encounter when organising your sheds concrete slab. Go to part 1part 3

Site preparation

Clear the site of any rubbish, vegetation, household outdoor items etc. Anything that is in the near vicinity of the sheds site that can be moved, then move it. The more space you have to work safely in the better.

Mark out the slab area with spray marker or stakes.


Scrape of leaves, grass and top soil to level the site.

Put down your base material crusher dust/sand. Six inches of well-graded gravel or crushed stone material properly compacted will give good results. Compact this layer. Using string lines mark out slab precisely locating where bored piers or pad / block footings under columns will go. Dig thickened edge beams where required and trench beams through the shed if specified. Trench beams are sometimes required if you intend to have heavy machinery in your shed like hoists, lathes etc or where additional strenght through the slab might be required. Chamfer the edges of the fill and place waterproof membrane taking care not to collapse your base material or puncture the membrane.

Form work.

Locate, level, square and securely fix your formwork into position. Formwork is the timber boards that shape the slab and hold the concrete into position until it cures. It gives the concrete a form to shape to. Check your formwork is square by measuring across from corner to corner.

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Set Downs and Rebates

One thing to keep in mind when setting out your form work for your shed is whether or not you need to pour sections of your slab at different levels. If you have a garaport design shed, do you want the slab under the carport set down 50mm from the shed floor? Does your lean-to slab need to be set down from the shed or garage slab? Or should you have a slight fall at the lean-to outside edge for drainage if the lean-to is a roof only structure? What you do now can have a strong bearing on the water tightness of your finished building so if you are not sure, ask your shed supplier about set downs for lean-to’s and garaports. Rebates for sheeting – we do not recommend rebating the concrete at the edge of the slab for vermin proofing. Warranty conditions for wall sheeting suggests that rebated edges that sheets sit on can void your warranty.


Put your bottom steel or trench mesh in place followed by your top steel mesh sheets. Set these on bar chairs so that there will be 25mm or 1 inch of concrete above the steel. Tie the mesh together overlapping and do the same with the trench mesh. Be careful not to pierce your waterproof membrane. If you haven’t already, position any services (pipes or conduits) you require into place.

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