Can I build a deck this low to the ground?

Question

I want to build a deck off the back of my house. The door is near grade level, about 1' above. The grade doesn't have very large slope, so that means that the beam at the far end would be below grade:

Note: Forgive the crude pen-drawing i sketched out at work. Didn't really want to bother with plenty of workspace, pencils or rulers.

Am i allowed to use beams when they are this close to grade? i was thinking that metal bracket holding up the beam, and the lower part of the beam, can be filled with crushed stone:

My question is: is this allowed? Even if the beam is pressure treated, and even if it is surrounded in whatever is desired (from nothing, to earth, to crushed stone), is this allowed?

Tags: desk, beam, footing, crushed stone, below grade

Answer 1

Sure, you can do that. Rather than using a beam though, just use some of those pre-cast concrete pier blocks (like in this article ^archive).

If you really want to use a beam instead (it might be cheaper) you'll want to make sure the treated wood you use is rated for ground contact. You might have a hard time finding ground-contact rated 2x6's at a local home store, but they probably do carry ground-contact rated 4x6's that you could use instead.

Another option would be to use railroad ties for your "beam". just dig a trench, fill it with gravel, level it out, compact it, and set the railroad ties on top (note - railroad ties are much heavier than they look).

Answer 2

In Massachusetts, you can use wood for foundations as long as it is the right kind of pressure-treated wood.

“All lumber and plywood shall be treated in accordance with AWPA C22, and shall bear the label of an accredited agency showing 0.60 retention.” 780 CMR 5402.1.2

I think that means that your beam can be underground. You will need to treat cuts and holes with copper napthenate:

“Where lumber and/or plywood is cut or drilled after treatment, the treated surface shall be field treated with Copper Napthenate, the concentration of which shall contain a minimum of 2% copper metal, by repeated brushing, dipping or soaking until the wood absorbs no more preservative.”

The footings for your beam will need to be frost protected, i.e.,

“extend a minimum of 48 inches below finished grade at all points” (5403.1.4)

One alternative is to build a patio instead of a deck. A deck is defined by 780 CMR 5202 as an

“exterior floor system supported on at least two opposing sides by an adjoining structure and/or posts, piers, or other independent supports.“

You will need to add at least four inches of gravel or crushed stone on top of the compacted dirt. Gravel should be 3/4 inch or smaller, and crushed stone should be 1/2 inch or smaller (780 CMR 5504.2.1). Between the gravel or stone and the wood, you should put a six-mil (0.15 mm) polyethylene moisture barrier with six-inch overlaps between the pieces of plastic (780 CMR 5504.2.2).

(Warning: amateur reading of the building code.)

Answer 3

If your deck won't be too large, you could build a free standing deck frame with 2x6's.

2x6 joists would be resting on doubled up 2x6 beams. Use 8" builders tubes 4' below grade with metal post holders sunk into concrete (the ones you have in your sketch). If you're using 4x4 post holders, they will be 3.5" actual and you can sandwich 1/2" pressure treated plywood between two 2x6's to get a snug fit.

Answer 4

I know this is an old post but I will answer it in case someone wants the right answer.

All you need to do is make the rim joist your beam and attach the floor joists with joist hangers. So the beam will be at the end of the deck the same height as the floor joists. If you need a beam in the middle you can make the beam the same height and add floor joists on both sides of beam.

Answer 5

For anyone in the future, in most areas with not insignificant snowfall this is not code. Upwards of 6" of clearance above ground level (grade) is required to minimize moisture rot, specifically, after the buildup beam (min 5.5"), joist structure (min 3.5"), and boards (min 1") and variance (~.5").

Your deck will fall at a minimum of 16.5" from grade (the ground). The kicker of course is that at around 24" you're required guard rails leaving you with less than 8" of play.

And for those who may wonder, bolting the structure to the structure is not what classifies your deck as an extension to the house. It is proximity to the house (check codes). While I'm here, digging lower or adding dirt do not change the ground level (specifically grade).

If your door is under 16" from grade, you may want to look at concrete or stone patio.

Ontario is uniform in the populated regions (northern excepted), here's a decent guide from Markham: https://www.markham.ca/wps/wcm/connect/markhampublic/a7149174-b1c5-450d-b267-a67685b5378d/HomeownerGuide_Decks.pdf?MOD=AJPERES&CACHEID=a7149174-b1c5-450d-b267-a67685b5378d

Answer 6

You can build a deck where there are several beams parallel to the house, and the joists go between the beams. That puts the beams and the joists in the same plane. It's more work and you likely need multiple beams, but it works fine.