How to Build a Concrete Base in Preparation for a Garage

Last Updated: June 2, 2021 References

Before buying a concrete garage it is extremely important to lay a solid concrete base. This will ensure that the garage remains structurally sound, and avoid the garage door opening mechanism failing.

Method 1

Method 1 of 2:

Site Preparation

1
In order to make the shuttering easier and to ensure consistent thickness, prepare the ground and get it as flat as possible.
2
Assess the firmness of the ground. If the ground is too soft, use hardcore to firm up.

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3
If you have soft ground, consider using a steel mesh to reinforce the concrete.^{[1]
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4
Install a damp proof membrane if the site where you are erecting the base is prone to damp. Make sure the membrane is added to the whole of the base to avoid cracking.^{[2]
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5
If you are installing the base adjacent to a wall, use damp proof between the concrete and the wall, otherwise damp may transfer.

Method 2

Method 2 of 2:

The Shuttering

1
Set up your shuttering at least one or two days prior to pouring the concrete.^{[3]
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2
Make sure the shuttering is securely pinned in place with time stakes.
3
Walk on your shuttering and make sure it is absolutely solid, and doesn’t move.
4
Use a strong timber that will remain straight and not bow.^{[4]
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5
Ensure the height of the shuttering has been measured in accordance with your chosen concrete thickness.
6
Check that the shuttering is completely flat, level, square and over sized – about 150mm longer and wider to allow for any discrepancies.^{[5]
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7
Make a removable door within the shuttering, so that you can get access to the area with your wheelbarrow holding the concrete.
8
"The concrete pour"
9
Before you order the concrete, make sure your site is fully prepared.
10
Ensure you have a rake, tamp, shovel, float, spirit level, gloves, wellington boots, hose pipe and wheelbarrow ready to use.^{[6]
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11
Order the exact amount of premixed concrete to be delivered. Ask for a wet mix which will give you a little more time to spread the concrete.
12
Where possible, have the concrete poured directly into the base. If this is not possible, have it poured directly into the wheelbarrow.
13
Use the rake to get the concrete as level as possible.
14
Use the hose pipe to clean off any concrete spillages before it dries.
15
Smooth the concrete with a ‘straight edged’ tamp that is just wider than the concrete base. Guide the tam across the shuttering to remove any excess concrete.
16
Once the concrete is spread, re-insert the shuttering.
17
Smooth the outer edge of the concrete with a float (around 250mm).
18
Leave the shuttering in place for a few days before removing.
19
Allow the concrete to set for at least a week before installing your concrete garage.
20
If rain looks likely, cover the base with plastic sheeting.
21
"Important"
22
Don’t wash concrete down the drains as it will settle, causing the drain to block.
23
Laying a base is much easier when done in two’s. inches from the base of the the concrete

Community Q&A

Question

How thick is the base?

Community Answer

Check with your municipality. But, typically, the tamped base should be equal to your concrete's thickness (i.e. 4-inch base for 4-inch slab). Though, 6-inches of each is largely the new norm for vehicles in most areas now, with 4-inch only being for walkways, sidewalks, basements, patios, etc.
Question

So I don't need to use hardcore? Just level the earth, use shuttering and then metal reinforcement, cover with polythene and then concrete?

Community Answer

If the earth is hard, maybe. If it is soil, no. Once the hardcore is laid, it needs to be compacted using a 'wacker' - a petrol-driven machine that vibrates the hardcore into a solid mass. Hardcore is composed of different-sized crushed stone pieces, so it moves until all the gaps are filled. You can compact soil, but hard to get it level. The risk with just soil is that your concrete cracks where softer ground 'gives'. Don't forget, too, that with soil there are worms - however hard you ram it, they will appear, constantly weakening the structure.
Question

Can I construct a concrete base in small sections over a number of days?

Community Answer

It's very difficult to get a good finish when doing this since the joins between each section will be visible, and it is very hard to smooth the join if it's wet on one side and dry on another. If you have to do this, put in wooden separators, like the shuttering, and do one section a day.