Deck blocks versus poured footings
A precast deck block is a concrete pier you set on the ground and build on top of. It is the cheapest foundation a deck can have, it can be installed in an afternoon, and it is the wrong choice on most sites where people use it.
What a block actually provides
A typical block has a bearing area of about 1 square foot, a cradle on top for a beam or joist, and a hollow or solid body. It spreads load over that area and holds the wood above the soil. What it does not do is resist uplift, resist lateral movement, or reach below frost.
| Deck block | Poured footing | |
|---|---|---|
| Bearing area | ~1 sq ft | Set by design, 1.8–3 sq ft typical |
| Below frost line | No | Yes, by depth |
| Lateral resistance | Friction only | Soil bearing on the sides of the hole |
| Uplift resistance | None unless strapped | Post embedded or base anchored |
| Labour | Minutes per block | Dig, mix, cure |
| Cost per point | $8–$15 | $25–$45 in materials |
Where blocks are defensible
- No frost. In climates without ground freezing, the frost heave mechanism does not exist, and that removes the main argument against them.
- Low, freestanding structures. A ground-level deck, a shed base, a walkway — something that does not depend on the house for support and does not carry a guard.
- Well-drained, stable soil. Sand and gravel drain; clay holds water and moves.
- Temporary or movable. A structure that will be taken apart does not need a permanent foundation.
Why they fail, in order of frequency
- Frost heave. Water in the soil under the block freezes, expands, and lifts the block. It comes back down in spring, but rarely to the same place. After three winters the deck is out of level and the guard posts lean.
- Settlement on soft soil. One square foot is not much bearing area. On clay or fill, one block settles more than its neighbours and the frame racks.
- Lateral movement. A block resists sideways force by friction alone. A deck attached to a house pushes against its blocks every time someone walks on it, and over time they creep.
- Wood at grade. The beam sits in the cradle a few inches above soil. Rain splashes, debris collects, and the bottom of the beam stays wet.
If you use them anyway
- Excavate and compact. Set each block on 4–6″ of compacted gravel, not on topsoil or grass.
- Keep them below the structure, above the water. The block should not sit in a depression that holds water.
- Strap or clip the beam. If the deck is in a wind area, the beam needs a positive connection to the block, not gravity alone.
- Use ground-contact rated wood for the beam, and keep the wood at least 1″ clear of the block's cradle bottom so water can drain.
- Do not hang a guard off them. A guard post needs a footing that resists overturning, which a 1 sq ft block on the surface does not.
The honest summary
Where the ground does not freeze and the structure is low and freestanding, blocks are a legitimate engineering choice and are widely used. Where the ground freezes, or the deck is attached to a house and carries a guard, they are a false economy: the money saved on concrete comes back as a re-level in five years, or as a guard that leans.
What we are not saying
- It does not give a code position. Some jurisdictions accept blocks for accessory structures below a size threshold; others require footings below frost for anything attached. Ask before you dig.
- It does not size a block. Bearing area needed depends on load and soil, and the block you buy has a fixed area.
- It does not cover helical or screw piles. Those are engineered foundation systems with published capacities, and they solve the frost problem differently.
Questions people actually ask
Are deck blocks code compliant?
Sometimes. Many jurisdictions accept them for low, freestanding, unroofed structures and reject them for anything attached to a house or above a certain height. The rule is local, and it is usually written in terms of attachment and height rather than the block itself.
Do deck blocks heave in winter?
Yes, wherever the ground freezes. Frost heave lifts the block, and the return in spring is rarely exact, so the deck goes out of level over a few seasons. That is the main reason cold-climate decks use poured footings.
How many deck blocks do I need?
One under each beam bearing point and each post, at a spacing the beam can span. Because the beam span is a table question, the count follows the beam design rather than a rule of thumb.
Can I put a hot tub on a deck on blocks?
No. A filled hot tub is a concentrated load of several thousand pounds, and both the block's bearing area and the beam are outside their range. That is a designed foundation.
What goes under a deck block?
Four to six inches of compacted gravel on undisturbed soil. Not topsoil, not sand alone, and not a paver laid on grass — all three settle.