Thread: Deck support post on retaining wall

The rear deck and side deck meet near the retaining wall. Three 4 x 4's were utilized at this area for support. Movement has occurred, thus front post is not plumb. Seems to be improper for a deck post to rest on a retaining wall. The third picture shows buckled siding where it appears that settlement and/or movement of the retaining wall has applied pressure to the wood trim.

This home was constructed in 2002 and the deck must have been an owner installation. 6 x 6 deck posts were utilized at the front deck and looked ok.

Your thoughts are welcome.

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It looks like the retaining wall has tipped, causing the buckling of the siding and the out of plumb posts. The deck will most likely need to come off to fix the retaining wall.

You could use the retaining wall as a support if done properly.
1st the retaining wall must have been built to hold a structure. It doesn’t appear
this is the case because hydraulic pressure has already made it move.
2nd the posts used needed to be isolated and secured to the retaining wall, and
be properly secure to the deck.

Originally Posted by Jeff Langhorn

You could use the retaining wall as a support if done properly.
1st the retaining wall must have been built to hold a structure. It doesn’t appear
this is the case because hydraulic pressure has already made it move.
2nd the posts used needed to be isolated and secured to the retaining wall, and
be properly secure to the deck.

I am going to call for further evaluation by a PE. There were some fairly large cracks in the retaining wall and the horizontal cracks can indicate future failure.

Here is what the NC IRC says about retaining wall design:

R404.1.3 Design required. Concrete or masonry foundation

walls shall be designed in accordance with accepted engineering practice when either of the following condi

tions exists:

1. Walls are subject to hydrostatic pressure from
groundwater.
2. Walls supporting more than 48 inches (1219 mm)
of unbalanced backfill that do not have permanent
lateral support at the top and bottom.
Included are retaining walls located on residential
sites and any retaining wall systems that cross
oyer adjacent property lines regardless of yertical
hcight.
3. Retaining walls that support buildings and their

accessory structures

If the deck is secured to the house, it looks like the siding was never removed. That's another no no.

The deck area to the right of the retaining wall appears to have a higher elevation over finished grade - cannot tell from photo perpective the projection or if a cantiliever (depth of field?). I presume a walk out basement or similar is below, suggesting this deck has an elevation far more than 4' over the area below and more likely exceeds 8' (on the right of retaining wall).

If finished grade below is substantial - it would require far more substantial support than mere 4x4 posts.

I do not see a cap or plate to distribute point load to block wall or pier, nor a means to retain/restrain position or resist forces.

Originally Posted by H.G. Watson, Sr.

The deck area to the right of the retaining wall appears to have a higher elevation over finished grade - cannot tell from photo perpective the projection or if a cantiliever (depth of field?). I presume a walk out basement or similar is below, suggesting this deck has an elevation far more than 4' over the area below and more likely exceeds 8' (on the right of retaining wall).

If finished grade below is substantial - it would require far more substantial support than mere 4x4 posts.

I do not see a cap or plate to distribute point load to block wall or pier, nor a means to retain/restrain position or resist forces.

Good day Mr Watson,

I have attached a side view of this deck and support posts. Main support posts were 6x6's with cross bracing (bolted), the 4x4's were only utilized at the retaining wall.

Sidney

I agree with Jeff a retaining wall if designed properly could easily support the deck. The short 4x4s are improperly attached at the top and bottom, but the real issue is not the deck. The retaining wall looks to be the most critical issue in your photo. It has some serious issues that need to be addressed.

The retaining wall looks to be 6'+ high. Without proper tie backs into the earth behind it, the wall will do what it is doing. There's too much ground load behind it to keep it standing indefinitely. I always recommend a water tight finish cap on top of block walls like that; aluminum or overlapping treated 2x, etc. Without a cap, water just seeps into the wall and contributes to wall erosion. That wall has seen better days. I would write the wall as a collapse hazard.
Partially supporting the deck on the wall is not a problem when conditions are good.
4x4 Posts are NOT attached to either the deck or wall from what I can see. Don't wishy-wash is some 'improperly attached BS'
The main posts also look to be sitting at grade, not elevated and without brackets.
A good carpenter can fix the porch in a couple days. The wall is a bigger issue.

Don't forget, once a structure such as a deck is attached to a dwelling it becomes subject to the codes pertaining to the dwelling itself (in my area at least). Thus, if the deck rests on a retaining wall such as the one in question, the wall becomes subject to applicable residential building codes as well. This includes things like, but not limited to, proper footings, wall support for type and height (as if it were a basement wall), and so on.