Should the high side of a concrete block retaining wall be sealed?

Question

I am mid-project in a DIY yard project. I have subbed out a large retaining wall to a civil / wall contractor. It’s 220 feet long, between 2-5′ in height, and makes 2 90-degree turns. It’s built to code with CMU blocks and has an overkill of rebar in it and through the 26″ wide and 14″ deep footer. I’m in Southern California, so it’s a dry climate. I plan to have the face of the wall stuccoed.

The wall just went up this weekend, but hasn’t been filled with concrete, yet. That is coming this weekend. Before he comes back on Saturday morning I wanted to get a few things cleared up for my own knowledge and to know what to ask for to make this wall last. The wall has a lot of room behind it now, but will be backfilled with 12″ of 3/4 gravel and then compacted and graded soil. No landscape fabric.

When reading on the Internet, the topic of sealing the back, “positive” side of the wall has come up quite a few times. People on the Internet say both you should NEVER seal a retaining wall and others say you should ALWAYS seal a retaining wall. The never crowd says that hydrostatic pressure will build behind the waterproofing and lead to a wall failure. The always crowd says that letting water into the blocks will lead to the blocks deteriorating and the stucco peeling off of the front face in a few years. Some places even seem to have local building codes that either require it or require you not to do it. My local building code does not even mention sealing the backside of retaining walls either way. What is reality – should you seal the back like a below-ground foundation or not seal it at all?

Thanks in advance for your help!

Answer 1

I think both the issues of (1) water penetrating the wall and causing problems from behind for your stucco and (2) hydrostatic pressure behind the wall physically damaging it perhaps to the point of failure are valid.

If you look at professionally-designed and built retaining walls, such as those in government buildings or under freeway overpasses, they always seem to have drain holes near the bottom.

It seems logical to me that the answer in your case is BOTH - seal the back to minimize issues with your stucco, but provide drainage holes to release any hydrostatic pressure by running short plastic drain tubes through every 3 feet or so along the length of the wall near the bottom.

Answer 2

Having researched the same question recently, I've found the same two recommendations to ALWAYS or NEVER seal the positive side of a retaining wall. I suspect they're both right, but don't always define what they mean by "retaining wall" or explain assumptions about material and climate.

In short, I believe you can and even should seal CMU retaining walls when applying paint, stucco, or other decorative finish. @Armand adds a good point that you'd also want "drainage holes to release any hydrostatic pressure."

But walls made of retaining wall blocks meant to allow water to pass through (through the blocks themselves or intentional weep holes) should not be sealed. This includes advice to avoid landscape fabric on the back of such walls, especially if they would get clogged by dirt.

For either type of wall, proper backfill and drainage (mentioned in an earlier version of the question) are required.

ALWAYS

The always crowd says that letting water into the blocks will lead to the blocks deteriorating and the stucco peeling off of the front face in a few years.

This Landscaping Network article on retaining walls supports your point and explains that moisture will seep into and through masonry.

"On the dry side it [moisture] will evaporate leaving behind mineral efflorescence or discoloring mildew. Moisture is the enemy of decorative veneer such as stucco, stone and tile."

It then explains that's why it is "essential to use a substantial waterproof membrane on the back of every retaining wall just as it is applied to the back of basement walls where the same conditions occur."

Waterproofing seems to be important to preserve stucco. And I understand you would only stucco "solid" walls made of concrete or Concrete Masonry Units (CMU).

NEVER

The never crowd says that hydrostatic pressure will build behind the waterproofing and lead to a wall failure.

In a video by Stanley "Dirt Monkey" Genadek at 3:54, he shows an example of a retaining wall failing due to clogged landscape fabric. The example wall hasn't been sealed, per se, but it's a similar argument that (certain types of) retaining walls should not be sealed.

Finally, I suspect many companies and professionals will have a recommendation that's correct for their regional market and customers. For example, perhaps it doesn't matter if you seal a CMU retaining wall or not if there's proper drainage. Maybe the advice also differs depending on climate.

Answer 3

It's likely moisture will seep through the CMU from the back (high dirt) side and get behind the stucco finish causing it to crack and flake, possibly mildew if it were really wet/damp. You could either live with that and refinish it as necessary. Stucco is somewhat permeable itself so it's possible that if it's a pretty dry climate or well-drained/draining soil, the stucco might fare just fine. So that's one option. Another option is going all out and waterproofing the backside of that wall that's holding back the dirt. You could use a fluid-applied waterproofing or some other type of waterproofing (sheet, drainage mat, cementitious WP), put weep holes at the bottom of the wall to allow water through to the low side and/or french drain running along the bottom of the wall and controlling the water drainage somewhat. AND be sure to put tamped gravel behind the wall to create a natural drainage area and to relieve hydrostatic pressure behind the wall! So that's option 2. Because I'm a little scrappy in my DIY and cheap in my wallet I would land somewhere in the middle and be sure to do the gravel behind the wall, and the weep holes. It's only a 30" wall so I wouldn't be too worried about the hydrostatic pressure. For me gravel and weep holes would be enough peace of mind, and if I had to do a little stucco repair I'm not too worried or bothered.