Laying a concrete pad is a handy DIY skill that can be put to use in a variety of ways around your home from driveways and paths to floors. In this guide we will take you through the process of laying a concrete pad for a 1.8m by 1.5m garden shed.
When you are deciding where to place your shed you need to allow enough distance from hedges and fences.
Boxing is the outer framework that is pegged in place on the ground that holds your concrete in place.
This can be made out of any old timber you have lying around. If the timber is not straight it can be easily straightened with your pegs. Ensure to use enough pegs to support the boxing so when the weight of the concrete is loaded up against it, it will not ‘blow out’.
Using 100x25mm rough sawn untreated timber, nail the boxing timber together and place on your selected site. Measure the diagonals to make sure they are both the same to ensure your boxing is square. Use your stringlines and dazzle to mark out where the perimeter of your concrete pad is going. Do all necessary soil removal prior to placing your boxing timber and base course down.
A concrete pad can be level with the ground or raised above it. For this job we are laying the pad level with ground.
This shed requires a total depth of 150mm. This depth can vary depending on the job or terrain. Generally speaking, a floor slab or a driveway will be 100mm minimum basecourse and sand, and 100mm concrete. Before commencing work, check with your local council as you may require a permit.
Peg the boxing timber in place. Make sure it is level so your shed sits flat, and then fix the pegs to the boxing timber.
Rake and remove any loose dirt.
For this shed we are going to create a rebate. This is a step around the edge of the concrete pad which the shed will sit into. We do this just for the purpose of water tightness.
Attach pieces of 20x25mm timber to the inside of the boxing timber to create the rebate in the pad that the walls of the shed slot onto.
Add 75mm of basecourse and compact it.
Add 10-20mm layer of sand and give it a rake. This is to stop the polythene being punctured by the basecourse.
Place a sheet of polythene over the sand and tuck it in neatly around the edges. This will act as a moisture barrier.
If you are laying mesh, use small concrete blocks or mesh chairs to hold it o the polythene, so it sits in the middle of the concrete slab.
A concrete pad this size doesn’t require mesh but anything larger or heavier will.
Mix your concrete in a mixer according to the instructions on the pack and then pour it into the boxing timber. Spread it out and then give the boxing timber a tap with your hammer to settle the concrete and get rid of any air bubbles.
Machine mixing - this is better for larger quantities.
Hand mixing - hand mixing is appropriate for small volumes.
Start in the corner and work your way along the edge. Ensure to occasionally lightly tap the side of the boxing timber to remove any air bubbles. For a shed this size, you can place all your concrete in and move straight to the finishing stage.
For the first stage of the finishing process, use a straight piece of timber to screed the concrete off. It is best to have a little excess concrete in your slab so you have a little extra to work with which will make for a more uniform finish.
Next, use a magnesium (mag) trowel. This stops drawing the water up and out of the concrete too soon.
How long you wait for this to dry is entirely weather dependant. You may need to test the concrete a couple of times to ensure it has dried enough for you to hit it with the steel trowel but not so dry that you can’t work with it.
The final stage is with a steel trowel for a nice, smooth finish. You can repeat this final step to create a smoother finish if that’s the look you want.
A pad this size will need around 48 hours to cure but keep an eye on it. Concrete that dries too fast can crack and shrink, so in sunny conditions you might want to give the concrete a light mist with your hose to prevent this. Alternatively you can also use wet sacks to retain the moisture.
