Building any type of outbuilding, including a garage, requires planning ahead and ensuring there is a sturdy base to build on. The best choice for this base or foundation is concrete, which will ensure the base is ready to hold the weight of the building on top of it. Pouring a foundation for your building is more than just dumping concrete on the ground, however. It requires planning, the right mixture of concrete and some reinforcements. In this article, you’ll learn how to do the task properly.

Determine the Size of the Outbuilding

Determining the size of the outbuilding will affect how the base is built and will also determine what fill is used, as well as whether or not you need elements to strengthen the concrete.

For most buildings, including a garage, the base would typically be 4” thick in the centre and 8” thick around the outer edges. This will help ensure the building won’t sink or become unstable. It’s also quite important to prepare the ground beneath before you pour the concrete. Plan to allow for 3” extra concrete around the building.

Check Local Laws

Construction is often covered by specific laws and you may need a planning permit before you build. In some cases, this is not required, if the building is small enough. However, you should always check with your local council to ensure you are not going to run into problems later.

Prepare the Building Site

To do this, you’ll need to carefully measure out the slab. Remember that it will be 3” larger on all sides, so this must be taken into consideration. You can place stakes at each corner and run string around them to give a good idea of where everything will be.

Once you’ve measured and ensured the space is correct, you’ll need to level out the area where you’ll be building. A builder’s level will help you determine how to level the area. Often, you’ll need to scrape off the topsoil and ensure the ground is not only level but also compact.

The area that you intend to fill in will need to be around 6” deep. You can then begin to fill in with gravel. While the amount of gravel will depend on the project, 4” is a good depth for most outbuildings. This leaves you with 2” of concrete below ground and 2” above.

Build Your Forms

The next step is to prepare the forms for your concrete. You can use old lumber to frame in the area that will have the concrete. This will be set atop the gravel. Take the time to dig a couple of inches around the main slab area, just outside the forms. This small trench can be filled with more gravel and will provide added stability for the foundation.

Use stakes or pieces of rebar to hold the forms firmly in place. Then you can dig down a little around the inner edges of the form, reducing the amount of gravel so the edges will contain more concrete.

You can also use scrap wood to brace the forms as concrete can be quite heavy and may push outward if the form boards are not secured.

Lay Your Waterproofing

Unfortunately, concrete has a terrible habit of leaching water upwards if it isn’t properly sealed. You’ll need to lay a waterproof layer before you add the reinforcements. This usually comes as a large piece or roll of material that you’ll need to trim and place within the form. Choose your waterproof layer carefully. Many people simply use 6mm plastic sheeting. The idea is to block moisture from seeping into the concrete and ending up in your floors and walls, which can cause water damage and rot. Over time, it will result in deterioration of the building atop and this can mean plenty of money in repairs.

Add Your Reinforcements

The type of reinforcement will depend on the size of the building. Anything smaller than a garage may use steel wire grids or mesh laid inside the form. The concrete will seal around this, making it less likely that the concrete will crack as it hardens and cures over time.

For any type of business that will be larger or manage heavy loads, such as tractors driving in, you’ll want to take the level of the reinforcement up a little. Instead of wire, you can use rebar, which is wired together to form a grid. Again, this is useful for preventing cracking and other types of movement related damage.

Pack the Fill

You’ll want to pack everything inside the area. The gravel should be firmly placed and not shift once the concrete is poured. This can be done over time by walking on the fill, or you can use a compactor to ensure it is properly pressed.

Order or Mix Your Concrete

At this point, you need to calculate how much concrete you’ll need. This can be done by multiplying the length by the width by the depth in feet. Remember that the trench needs to be considered, as well. From there, you can divide the total by 27 and add 5%. This will be the number of yards required for the slab.

If you are mixing on your own and need to produce a large amount of concrete, it may be best to divide the base into two halves with a form board and pour just half at a time, removing the board before the second pour.

It’s far easier to hire a company to bring in ready-made concrete for the pour. If possible, the mixer truck should get close enough to the build area to pour directly into the form. However, if this is not the case, you will need to have wheelbarrows on hand to rapidly move the concrete to the form.

Ideally, you’ll pour on a cooler day. Avoid wind and heat, if possible, as it can result in the concrete curing too fast. However, rain can also cause issues. The perfect day is cloudy, but not rainy, without too much heat or sunlight.

Prepare ahead of time and clear the path the concrete truck will take. It should be easy to reach the form.

You should also talk to the company about what mix type is best for your slab. They can recommend one based on what kind of weight the slab will deal with.

Pour the Concrete and Level It

It’s time to pour the concrete. Once the concrete has arrived, you want to ensure everything moves quickly. You’ll need extra hands, so be sure to have other people available to help.

As the concrete is poured, drag a screed (2×4 piece of wood) over the surface, with the ends resting on the wood form. This will help level the concrete. Repeat several times to ensure it is as flat as possible.

Flatten and Polish

Once you are certain everything is level, use the bull float to smooth the screeded concrete. This will take time because you need to do small strips at once. You should avoid more than three passes, as this can actually cause a weaker surface with too much water pulled up to the surface.

If you don’t have a bull float, you can use a trowel or other tool to smooth and polish the surface. This will give you a nice starting point if you plan to add flooring, or a good surface to use if you plan to use it as is.

Prepare the Edges and Add Grooves

You can round the edges once the concrete has begun to set, but is still soft enough to shape. This is done with a tool made for rounding and you simply pass it around the edges inside the form. This will help give the concrete a finished look.

On a large slab, you may need to cut grooves to prevent random cracking. Use a 2×4 to ensure you have straight lines to follow with the groover. From here, you can add any anchor bolts you need.

Remove the Forms

Once the concrete has cured, you can remove the forms. It’s a good idea to add mist over time or use plastic to ensure the concrete doesn’t cure too fast and crack. It can take a while to cure completely, but you should be able to remove the forms the following day. Wait for at least three or four days to begin building.

Building an outbuilding can be a lot of work, but the foundation is the most important part. Without a solid base, the building will never be as stable as you need it. Take your time getting this part right and you’ll have a well-done building in the end.

Contact Us

If you need concrete, at Forth Valley Concrete, we mix the perfect ratios for your needs. Schedule your concrete delivery now by contacting us today.