I love building ground level decks. You can work directly off the ground, no climbing up and down ladders. They are less expensive as they do not require a railing. As a bridge between your yard and house, they are at a convenient height, requiring minimal stairs if any, allowing easy access from your house to your yard all while providing an excellent defined outdoor living space.
In Calgary, Alberta, where I live and build decks, a ground level deck, is considered anything less than 24” above grade, in most states, this is even higher at 30”. Not sure why Canadians get 6” less, maybe we are just a little more cautious. Anyways your question is, “Can you build a deck directly on the ground?”
Yes, you can build a deck directly on the ground, but with some conditions. A deck on the ground must be built with ground-contact wood, and the substructure needs to be adapted for minimal height. Even though technically a ground level deck is elevated up to 24”, here we will discuss decks built less than 8” high.
An advantage of a deck less than 8”, it doesn’t require stairs. The height is lower than the maximum riser height of 8” (150mm). The selection of material is critical in building a deck on the ground level — not so much the decking but the substructure. The wrong joist and beam material in contact with the ground will quickly rot, making your deck, garbage.
Substructure Ground Contact
If your decks’ joist is touching the ground, they need to be sufficiently treated to allow for ground contact. Even if you are not burying your joist in the ground. A deck less than 8” with 2×8 joist will be in contact with the ground.
Treated lumber since 2004, is commonly treated with Alkaline copper quaternary (ACQ) or copper azole (CA) but the level of treatment is key for ground contact. Most treated wood from your local building store, like Home Depot, Lowes or Timber Town, will be non-ground contact wood.
Wood for ground contact needs increased levels of treatment. Dirt is a natural sponge, holding water, increasing decay of the wood. To achieve a higher level of saturation often requires incisions on the wood. So, a simple way to tell if it can be in contact with the ground, is it incised? If its pricked full of treatment, probably good to go. But always good to check the level of treatment stamp, just in case.
- Non-ground contact 0.15
- Ground contact 0.4
- Non-ground contact 0.1
- Ground contact 0.21
- Non-ground contact 0.6
- Ground contact 0.15
Wood is considered in contact with the ground if its within 6” (150mm) of the ground. Making almost any deck less than 8” high, it’s in contact with the ground. Unless you are going to dig out the ground 6” below your deck, wood treated for ground contact will be needed.
Limited Ventilation for joist
A deck so close to the ground won’t have good air movement. Water will become trapped under your deck, saturating the joist, causing the joist to decay prematurely. Another reason ground contact joist needs additional treatment. Deeper penetration protecting against higher levels of saturation for the wood to begin to rot.
Most decking is not designed for ground contact and needs to have at least 6” of separation. Being especially true for pressure-treated decking as it is not treated enough for ground contact. Ideally, a ground-level deck will have 2×8 joists providing enough clearance for the decking. Any lower increasing the chance of mould and fungi growing on the decking, especially the underside. Even with more resistant composite decking, fungi and mould will still grow on if not allowed to dry out regularly.
To make the deck close to the ground will require flush beams or digging trenches for the drop beam. Beams flush with the joist will require joist hangers, an additional cost, but will keep the beam off the ground and out of water.
Digging a trench for the beam will also allow you to use deeper beam material, increasing strength while making joist installation easier. The additional clearance under the joist reduces frustration with joist rocking on the ground. With the trench, make sure it’s wide enough for the beam to dry. Also, be careful that water doesn’t pool in the trenches destroying the beam.
Deck Support Options
Another thing to consider with a low deck is support. Will the deck have footings or be a floating deck? A floating deck will move as the ground freezes and thaws but will be easier to build. A low deck with footings below the frost level will be stronger but will require more dirt removal.
Floating deck substructure
Think of a floating deck as a thick blanket on the ground. Building the deck directly on the ground with minimal adjustment to level the deck. As the ground moves up and down, so will your deck. Just like your blankets at night. Flush beams built into the design will help to prevent the deck from buckling, but it will still tilt as the ground moves. The ground may look stable, but it really isn’t.
Footings Increase Stability
Adding footing under the beam will increase the decks’ stability. Require more digging but giving the deck a firm foundation.
With a flush beam on concrete piles, an inch or two above ground will keep the beam out of water. Be careful to check all the heights before pouring concrete for you will have little room to adjust the height.
For a dropped beam, dig the trench a couple of inches deeper than the beam, then dig holes deeper than the frost level. In Calgary, this is 4’ deep but Google it for your area. Install the required sono tube for the top is level with the bottom of the beam height. A deeper trench will increase airflow, drying the underside of the beam and prevent water from pooling beside the beam. Once the concrete is cured, install the beam un-top.
Footing will provide a firm support for the deck, transferring it below the frost line, keeping your ground level deck unaffected by the seasonal freeze, thaw cycle.
Use 6×6 treated post for dropped Beams
Using a 6×6 post for a drop beam has the advantage of knowing they are safe for ground contact. A fence post is designed to be buried in the ground. A 6×6 post can safely support ten feet of deck with a concrete pile every five feet.
Ground Level Decks’ don’t Require Permits
That being said, code is a guideline for quality construction. If you want your deck to last, it should be built to code. The span limitations for joists are the span limitations for joists. It doesn’t matter if it’s inspected or not. A 2×8 can support only so much weight. The code is there to provide a reference point to build a quality deck. Permit or no permit, build it right.