2

To try to keep the background short and to the point: I live just outside of Atlanta, GA and am looking to build a small (10'x12') on-grade freestanding deck behind my house. I'm not allowed to dig footings or do anything that couldn't reasonable be reversed. My yard is extremely hard-packed clay (difficult to dig with a pick/mattock... easiest to dig with a hammer and masonry chisel...). The house is essentially at grade; the back door sill is 3" above grade, with a 48x60" concrete pad in front of it that meets grade on the side opposite the door. The yard is sloped up from the house (i.e. sloped towards the house) on a 6% grade, so there's an approx. 7.5" difference in elevation between the diagonal corners of my proposed 10x12 deck. I'd like the deck to be as close to grade as possible to minimize the height closest to the door.

I have a fair amount of construction experience, but I've never designed a project like this myself, without either plans or someone much more experienced in the lead.

Plan so far:

My original design plan was for a 10x12' rectangle framed with 2x8" ground contact SYP common joists and end joists running in the 12' direction, connected with joist hangers to 10' headers of the same 2x8" SYP, and topped with 5/4 x 6" SYP decking running perpendicular to the joists. The plan was to rest this on solid concrete block (standard 8x16" blocks, in either 2" or 4" height, doubling up using construction adhesive where needed) that would be set directly on-grade, or slightly inset into the clay soil and evened out with leveling sand as needed.

So, my questions:

  • Thoughts on whether single 2x8 headers, set on solid concrete blocks, can support this?
  • The design load should be able to be carried by a mere four blocks, but that seems like far too little. I was thinking either three or four blocks per short (header) side, spaced evenly. Does this make sense?
  • Now, the complicated question: I really prefer the look of an overhang with the concrete blocks set inside the perimeter of the deck by a foot or so... and certainly not hanging out by 3" on the edges, which they'd be if I centered them under the headers. But that seems to really complicate the framing as I don't have the vertical space for beams underneath the frame. Any suggestions for how to support this on concrete blocks with an overhang on all sides and a minimum of additional framing? Would it be sufficient to just add blocking between all the joists in a line about 1' in from each header, i.e. directly above where the concrete blocks will be placed?

Constraints / Notes:

  • No footings, no excavation that can't easily be backfilled and hidden
  • As low to the ground as possible
  • This will likely be torn down in 3-4 years, so I'm not concerned about life span past that
  • While I want it to be safe and feel... not flimsy... it's only going to support 2 light grills, a small table and 2 chairs, and at the most (really big party, for me) four people. I'm more concerned about how it feels than whether it would meet code (if a deck this low had to).
  • I'm aware of the pre-cast deck blocks, but using them would require digging much more than I want to on the high side of the grade.

Thanks in advance for any thoughts/advice!

  • 1
    By "headers" do you mean beams or joists or rim joists ? Headers are part of walls spanning window and doors. – Alaska Man May 6 at 20:52
  • A deck can't be both grade-level and floating. The former means "resting on grade". – isherwood May 6 at 21:17
  • @AlaskaMan It might be a regionalism, but around here I've always heard rim joists referred to as headers, but maybe it's just incorrect nomenclature that's popped up. – Jason Antman May 6 at 21:29
  • @isherwood Thank you. I was mainly trying to optimize this for search. While I appreciate the edit, I really feel like the last part of my question - about the concrete block supports - is the most pressing for me. – Jason Antman May 6 at 21:30
  • Feel free to edit again. I was just looking for something more specific. SEO isn't a concern so much as value to the community. – isherwood May 6 at 22:37
0

Beams (what you called headers) usually need to be doubled. For that size deck I'm confident that they'll carry without undue flex or risk of failure.

Now, about the overhang... double the beams, which rest on the piers, and double the rim joists, which act as flush beams. Like so:

 ______________________________________________
|______________________________________________
|______________________________________________
| |          | | |          | |          | |
| |          | | |          | |          | |
| |          | | |          | |          | |
| |          | | |          | |          | |
| |          | | |          | |          | |
| |          |X|X|          | |          | |  X = bearing point
| |          |X|X|          | |          | |
| |          | | |          | |          | |
| |          | | |          | |          | |
| |          | | |          | |          | |

Note: The double joist hangers for the beam to rim connection will be inverted. The rim is carried by the beam, opposite of how the rest of the joists are carried by the rim. Mount it upside-down.

This will carry well for cantilever up to about a foot. Code usually allows for cantilevers of 1.5x the joist height or something like that, but you'll be ok here.

With this layout there's no benefit to adding piers at the center of the doubled joists. If anything, you'd add a third doubled joist down the center, meaning you'd have six piers spread between three doubled joists. I'm not sure that's necessary, though, since you'll have a beam spread of just 8 feet. You could also make the flush beams 2x10 for extra capacity.

| improve this answer | |
  • That sounds great to me, and I suppose seems pretty obviously correct to someone with a lot more experience saying "oh, yeah, that looks right" than figuring out right from scratch. Thanks so much! So just from a gut level, you think that a total of 4 piers would suffice? – Jason Antman May 6 at 21:37
  • 1
    It will from a structural standpoint. The only concern would be whether you expect them to sink into the ground when it rains. In that case you might put a wider base under the piers or add more. – isherwood May 6 at 22:36
  • This answer is on target. the loads on your deck for live and dead loads are about 65 psf and for a 10x12 = 120 sf deck that is 7800# / 4 pads = 1,950 psf, those pier pads are about 12" sq so 1 ft sq so ~2,000 psf and based on your comments about your soil it can handle a lot more than that – Ack May 6 at 23:29

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.