How to pour concrete deck footings

If you're hoping to install a garden shed, prefabricated garage, pergola or greenhouse, or need a solid base for your decking joists, you might find yourself having to pour concrete footings. Concrete deck footings can be used to secure your structure while also protecting it from groundwater. The number of footings you need should be calculated according to the total weight they will have to support.

Please note that, depending on the type of structure you want to install, you may require planning permission. For example, you will need permission for decking in the following scenarios:

• the decking height is over 30 cm from the ground;

• the decking covers more than 50% of your garden along with any other outdoor buildings;

• the decking extends beyond the front wall of your home.

Steps

1. Analyse the soil before pouring the footings

2. Space out and size your footings

3. Choose a form and dig the holes

4. Insert your forms and pour the concrete

5. Let the concrete cure and clean up

6. Concrete footings: tips to follow and mistakes to avoid

Pouring concrete footings means you can avoid the hassle of digging and moving multiple cubic metres of earth. But before you start, you do need to know what type of soil you have. Well-draining soil is ideal as water won't build up in the holes used to pour the footings.

On the other hand, if you struggle with drainage, you may need to install French drains in the surrounding area. Surface soil is always loose which is why you need to dig down beyond the frost level to provide the base for your footings. The type of soil you have depends on where you live.

The base structure of your deck, shed or greenhouse will generally be made up of joists of different sizes. The size of these joists determines both how far apart your footings need to be (generally 50 cm to a metre) and how big they need to be.

Footing sizes

Ideally, your footing should be wider at the base in a kind of elephant foot shape as this provides better support.

Here are the minimum sizes required for footings:

• 400 cm² at the base (200 x 200 mm or Ø 250 mm)

• 300 cm² for the main tube section (180 x180 mm or Ø 180 mm)

Choosing a footing form

Wood can be used to make square forms while wax-coated PVC tubes will create circular forms.

It's also possible to use cardboard form tubes. These lightweight and inexpensive forms are made of strengthened cardboard and come in a range of sizes and shapes including squares. Cardboard tubes for concrete can be purchased in lengths of two to three metres and are easy to cut down to size using a handsaw. The advantage of these forms is that they can be left in place where they won't be seen. After pouring the concrete, you can cut away the top part of the cardboard and break it down for recycling as opposed to getting lumbered with PVC to get rid of.

Digging your footing holes

Depending on the size of the footings, you can use a post hole digger, or electric or petrol post hole auger to dig the holes. Alternatively, you can use a shovel. Bear in mind that a power auger does require some machine-handling experience – especially if you fear it might get stuck on a boulder or stone.

Footings work by compression and a larger base will absorb the load more easily.

• To create a wider base, cut 10 cm from your PVC or cardboard ø250 mm form and place it at the bottom of a hole with coarse gravel at the bottom.

• Pour concrete up to the edge of the form and tamp down as best as possible. Even out the concrete with a trowel and place two or three pieces of re-bar in the centre.

• Then cut a length of about 300 to 350 mm from your ø150 to 200 mm form. This will make up the main part of the form.

• This part should be placed on the base you just poured while it is still wet. The tube should protrude from the ground and all the forms should sit level with each other.

• Be sure to check that that everything lines up perfectly, especially where the joists will lie.

It's important to work in stages. More likely than not, you won't be able to pour all your footings at once. Ideally, the footing should protrude slightly from the ground to stop groundwater from rising. If the level of your footing doesn't allow this, widen the area around the footings by a few centimetres.

• Clean up your workspace and take any waste to a recycling centre. If you have any cement left, store it off the ground in a dry spot.

• Clean up all equipment while the concrete is still wet; trowels, mixing trays and cement mixers should all be cleaned with water.

• Do not remove the forms until the concrete is completely cured.

• Make the most of the time while the concrete is curing to clear up the area. Spray on a natural weed killer then lay tarp or a weed control fabric on the ground, cutting holes for the footings. This way, you won't get weeds coming through the floor of your shed or decking.

• Once the concrete is fully cured, you can remove the forms. Be sure to remove any nails to prevent injuries to yourself or others.

Sizing and spacing concrete footings

• The weight of your structure might not justify pouring footings measuring over 150 mm in diameter but it's worth noting that it is quite tricky to pour ø 100 mm footings.

• Remember that your footings and posts will work and look better if they are arranged symmetrically.

Avoiding mistakes

• If you are creating a foundation for a deck, don't forget to factor in a slope. You might also need to ensure the structure will be level with the entrance to your home, for example.

• Be sure to leave room for adjustments in order to wedge in your structure foundations. You can always level things out later on.

• If your footings are too tall, deal with the issue before the concrete cures fully.

Required skills

Pouring a concrete deck footing can be done by any DIYer. However, be sure to work methodically and to follow all steps carefully. The main skills involve mixing concrete or using a cement mixer, taking accurate measurements, and handling a shovel or auger.

Time required

Depends on the number of footings, their volume and your chosen method. The work should be done over two days and you will have to wait for the concrete to set.

Number of people required

1 person or 2 people to simplify the task

Tools and equipment

• Post-hole auger

• Shovel, mattock and spade

• Wheelbarrow

• Laser level and chalk line

• Spirit level and plumb line

• Aluminium straight edge

• Folding rule

• Cement mixer

• Trowel and hammer

• Tarpaulin sheets

• Waxed PVC forms or cardboard tubes

• Re-bars

• Sand, gravel and cement

Safety wear

• Protective clothing

• Safety shoes

• Dust mask

• Safety goggles

• Ear defenders

• Protective gloves

Please note: this is a non-exhaustive list; be sure to match your personal protective equipment to the job at hand.