Deck footings are the most important part of a deck, for they support the entire deck. If a joist fails, its bad but can be repaired. A deck footing failing, and your entire deck will need to be rebuilt. Getting the deck footing right is essential to enjoying your deck for years to come.
What is the required Deck Footing Depth?
Deck footing must be a minimum 6” (150mm) below the local frost line. We all experience winter differently and so the frost depth varies also. Here are a few major Canadian cities and there required footing depth.
|Canadian city||Deck footing depth||Concrete Footing Size|
|Port Coquitlam||18”||8”x8” Pier|
|Kitchener||48”||8”-16” Weight dependent|
|Moncton||48”||Sized for 6”x6” post|
|Saint Johns||48” or bedrock|
|Medicine hat||60”||10” Diameter+24”x24”pad|
|Red Deer||60”||10” Diameter|
|Ottawa||60”||8” column on 24”x24” pad|
|Quebec||60” frost line|
|Grande Prairie||72”||10” Diameter|
|Winnipeg||72” (1.8m)||10” column+30”x30” pad|
As you can see in Canada, the required footing depth varies from 18” to 96” below grade. Our vast countries has many different frost levels. This varying in frost level affects how deep the deck footings will need to be dug in.
Now that you have the correct footing depth, the following are six other important considerations for deck footings.
Deck Footing Distance from House
The joist span determines an attached deck’s footing distance from the house. The deeper the joist is, and tighter joist spacing, the farther away the footings can be from the house.
Maximum Deck Footing Distance From House
|Nominal joist size Not incised S-P-F||12” joist spacing||16” joist spacing||24” joist spacing|
Where do you put footings on a deck?
All deck footings are placed under the deck’s beam to support the deck’s weight. Both flush and dropped deck beams are supported directly above footings. A flush beam is integrated into the joist system with hangers holding the joist flush with the beam. A dropped beam, is immediately below the joist, with the joist attached to the top of the beam. Either way, the footings are below the beam either directly or support by a post.
Deck post and footings must be spaced correctly according to the deck size. But there spacing can be adjusted for aesthetic reasons. For example, equally spaced footings visually look better on a deck. You are also allowed to overhang the beam past the beam post and footings. The amount varies by jurisdictions, but a quick rule of thumbs is maximum 2’ (0.6m) with the correct backspan, which again varies but often between 1 to 6 or 1 to 4.
Whenever in doubt, go with the smaller ratio. Better to overbuild then underbuild. I have never had an inspector fail a deck inspection because it was too strong. Overbuild is always better then underbuilt. Or, more importantly, better to have a deck last longer than expected then collapse under you.
Interestingly reducing beam span by increasing beam overhang increases deck strength. The individual footings still can only support the prescribed beam span, but the reduced span between footings reduces beam deflection. Beam overhang is a good thing but comes with its own limitations.
A drop beam supported by footings is commonly used to support an attached deck joists. The Canadian Wood Council prescribes that footing spacing is determined by the size of deck supported, but the calculations are base of maximum spacing of 8’ (2.4m), 6’ (1.8m) and 4’ (1.2m). Footings spacing can decrease, but the supported weight is calculated based on the rounding up to the bigger spacing.
For example, if your deck footings are 7’ (2.13m) on centre under your deck. You do your calculations off 6’ (1.8M) spacing. Allowing you to build better than the minimum but not under the minimum.
Using non-incised wood, a 2 ply 2×8 beam can support a deck 15’-16’ (4.6-4.9m). Same footing spacing but a 2 ply 2×6 beam can support a deck between 8’ (2.4m) and 14’ (4.3m) deep. A smaller deck with footings between 6’ (1.8m and 8’ (2.4m), a single 2×6 beam can support joist less than 7’ (2.13m) deep.
Reducing the footings’ spacing to 4’ (1.2m) will support all depth of a deck up to 15’ (4.6m). As you see, the closer the footings spacing, the smaller of beam you can use.
Should deck posts be set in concrete?
Setting a deck post in concrete will increase rotting as water will pool on the concrete rotting the post. Deck post should be anchored to a concrete column with a galvanized saddle preventing uplift. The saddle also raises the post slightly above the concrete out of any pooling water.
In the sad event that you ever need to replace a deck post, it is much better if it is not set in concrete. The post resting in an above grade saddle will make your job much easier. Allowing you to shore up the beam and remove the damaged post and replace all without digging as the deck post is not embedded in the concrete.
There are some exceptions and are far between that allow deck posts to be buried. But the exception is not for the post to set in concrete but surrounded by gravel, which allows water to drain away from the post preventing rotting. But this is rare, so always check with your city before designing a deck with buried posts.
Should deck footings be above grade?
Yes, concrete footings should extend a minimum of 6” (150mm) above grade and whatever distance below the frost line. Setting the deck footing above grade keeps the post and all wood elements out away from ground contact.
Whenever a deck substructure is less than 6” (150mm) from the ground contact wood should be used. Most appearance grade treated lumber is not treated for ground contact. Ground contact is a minimum of 0.40 PCF. Often to achieve this level of treatment requires the wood to be incised, reducing its structural strength and appearance.
But in short, must cities require deck footings to extend 6” (150mm) above grade. Helping ensure your deck post, stay dry and out of the mud and water, lasting for many years.
Deck Footings must be placed on undisturbed soil.
A word of caution with deck footings near a newly built house the excavation for the foundation may be cut disturbing the soil around the house. Regardless of deck footing depth, all deck footings must be on undisturbed soil. A house less than six years old, the ground around your house may not have fully settled yet. So, placing deck footing in the uncompacted ground around the foundation will possibly sink as the ground settles.
A rule of thumb is that foundation excavation will be cut back at 45° from the bottom of the hole. Meaning a standard 8’ basement, excavated 6’ below grade will be disturbed out 6’ from the foundation. Rarely affecting deck foundation as most deck footings are 12’ (4.8m) away from the foundation but something to keep in mind with a newer house. Make sure the ground is settled or compacted under your deck footings.
Can I use deck pier blocks instead of footings?
Deck pier blocks should only be used to support a freestanding deck. A deck attached to a building with a foundation, the house needs to be similarly supported with footings below the frost line. Using deck pier blocks on an attached deck will cause the deck to pull away from the ledger as it rises and falls with seasonal frost.