Deck Set on Retaining Wall

[jacktripper]

I have a home with a retaining wall about 8' from the back of the house. The retaining wall is built with treated timbers and is about

4' high. I would like to build a deck on the second level of the house, facing the retaining wall. How should I support the deck?

Is it better to set the posts (1) on the higher ground beyond the retaining wall, (2) on the retaining wall itself, (3) on the lower ground short of the retaining wall, and if so should it touch the retaining wall (flush) or not?

Ideally, on the retaining wall or on the higher ground beyond the wall would be best use of space, so please let me know if those options are feasible, even if not the best plan. Thanks.

[Joe Bobst]

Get a set of plans from a qualified structural engineer/architect. You'll need them for your permit anyway.The retaining wall is there to keep the hillside from becoming part of your living space if there are ever prolonged rains in your area. He may suggest using poured concrete piers for supports on the outside of the wall as an added reinforcement for the retaining wall.. I think the forms for these are called Sonotubes? There are even some real trick ones that make their own footings. Whatever, for something tied to the house you want sturdy underpinnings. With competent advice and good craftsmanship your project will be just fine. Good luck.

Joe

[Harry K]

Just my opinion. If the wall has been there for several years with no signs of movement, I would build on the wall itself. Of course you have to consider local codes, nosy neighbors...etc.

Harry K

[ameijers]

Impossible to give a good answer without seeing the wall, and possibly digging around to see what is under it, and how it is tied into the hill behind. Simple retaining wall is unlikely to have footers deep enough for a tall deck like that. If hillside slumps (like after heavy rains), or the timbers rot out, outer edge of deck could easily collapse without warning. Unless you built the wall yourself and know what is down there, probably best to go behind it with conventional piling footers. In many areas, a deck that tall requires a permit and an engineering drawing, since a collapse could actually kill people (unlike a surface deck.)

Aside from noting that from your description, this proposed deck will make a big dark damp hole out of much of your existing back yard downhill from the retaining wall, I'd say you probably need to get professional assistance on the design from a deck or landscaping company. Codes vary by area, as does the substrata. Once you have a safe design, you can probably do most of the grunt work yourself.

aem sends....

[v]

??? Has the sky fallen yet today, the wall is only 4 feet high, doesn't seem like a big deal, and the span would only be 8 feet.

I would NOT support the wall on piers behind the wall, because that soil in turn relies on the wall for support. If you are not confident in your wall, then you should not rely on posts set in the soil held up by the wall.

I vote for the wall itself. Next for the soil in front of the wall, but that requires posts at least as high as the wall so what's the advantage.

-v.

[127.0.0.1]

in many places a 4 foot high structure requires a building permit

[ameijers]

OP said deck was to attach to second floor, which implies 8-10 feet below deck surface to fall. IOW, a 4-foot deck covering a 4-foot pit.

Yes, these do collapse and kill people. I've visited people with decks bouncy enought to scare me, and had to take them by the hand and show them what was failing underneath, or underengineered. They didn't grasp it was a danger, even though it was bouncy with 3-5 people. 30 people at a party would be risking a lawsuit, and maybe dead friends.

aem sends....

[v]

In many places a structure 1 foot or 1 inch high requires a permit (i.e. there is NO height threshold for a "structure"). But that doesn't mean there is a likelihood of death. A remote possibility, sure, hey kids drown in 5 gallon pails. It wasn't the permit part that annoyed me, it was the "death" part. You could just as well say "don't try to cross the street without a police escort, because it could result in death."

-v.