Laying Concrete Garage Floor Slabs: What You Need to Know

October 29, 2013

Whether used to store the family car or an array of garden furniture and tools, ensuring that the floor of your garage is durable and secure is vitally important. Most garages will have concrete floors, designed to be long-lasting and robust. For those building a new garage, or renovating an existing one, here are a few things to look out for when approaching concrete floor slabs…

The first step to making sure your concrete slab floor is laid correctly is to prepare the base. Concrete can be lain directly onto soil, as even low quality earth has a high load-bearing capability (6 inch concrete slabs generally weigh around 35kg per square foot, while even low quality silt and clay has a bearing capacity of around 180kg per square foot). Skimping on this step will lead to a number of issues in the future, from cracks and shifts in the floor’s foundation to dampness, so make sure it’s not overlooked.

Start by removing any stones and other debris from the surface, before digging to get rid of the top 6 inches of soil. Try to keep the surface even throughout, fully compacting any filled in areas to ensure that slabs don’t bend and crack once laid due to an uneven base. Next, put down a layer of aggregate (at least 4 inches in thickness) and compact. Not only does this help to level out the concrete that’s laid on top of it, it also provides an insulating layer and allows moisture to escape. Vapour barriers (plastic sheets designed to stop water rising through porous concrete) can be fitted at this point.

Fit edging forms (lumber placed around the edge of the desired area) to set the parameters for the concrete slab. This can be easily achieved by hammering steaks into each corner of the proposed area, and nailing timber boards to the outside of each to form a rectangle, into which wet concrete can be poured. Laying down a mesh rebar is a good insurance option for reinforcement, but may not be needed if your preparation and mix consistency are correct.

While pouring the concrete, be sure to tamp it down with a concrete roller to fully compact the mix, before laying screed and flattening it down with sufficiently sized lumber. Continue to work the screed up and down the length of the floor until the whole surface is covered. This will give you the best chance of creating a robust and level finish that won’t be susceptible to cracks.

Finally, use contraction joints or grooves to compensate for expansion and contraction of the concrete in fluctuating temperatures. The concrete will require around 10 days to properly cure, so ensuring that it is covered and protected from the elements during this time is vital.

Laying a concrete floor – even in a garage – can be beyond many people’s DIY capabilities. Consider calling in professionals to carry out the work for you.