Q&A / 

Garden Wall Repair

Bernadette Snoussi lives in London, England.

She's got a problem with an older decorative garden wall that's in need of repair.

Here's her description, but I think the wonderful photo she sent does a superb job of telling the tale:


Could you please tell me whether it's possible to apply new concrete plinths over the top of the existing damaged ones on this little boundary wall (to avoid further damage when removing broken one)  and if so, would they be applied with concrete or some other form of adhesive?"

Bernadette is talking about repairing the missing chunk of concrete cap. Photo credit: Bernadette Snoussi

Bernadette is talking about repairing the missing chunk of concrete cap. Photo credit: Bernadette Snoussi

Here's my reply:

Bernadette, you sent a wonderful photo. It really helps me understand what you're up against.

First, you can repair this by adding concrete to replace what's missing. Be sure the stones in the concrete are NO LARGER than 12 mm in diameter for this smaller, thinner pour.

The hardest part will be building the formwork required. You'll need a flat piece of wood to prevent the concrete from falling into the semi-circle below. The sides of the form should overlap the existing cap at least  4 inches on each side to ensure the replacement concrete is in the same plane.

Pay attention to any slight sloping of the sides and use clamps to ensure the side forms do not move while you're pouring and working with the new concrete.

I'd use a hammer drill and drill into the sides of the existing concrete to install some steel pins. Be sure this steel is primed with a great metal primer and you put on two finish coats of paint on the steel. These pins are inserted into the existing cap and will help lock the new repair with the old. The steel only needs to be about 6 mm in diameter and 100 mm long. Insert the pins about 50 mm into the existing concrete.

Here's your biggest decision to make:

The replacement piece of concrete will look like a scar on your face. It's going to be ugly.

Do you want to hide it? If so, you can then stucco the entire cap along the length of the wall. The issue with this is that the mellow aging, moss, and character of the cap will disappear.

You may be able to get the patch to blend in very well if you read ALL of my past columns about tuck pointing brick and matching mortar. The secret is using the same sand and aggregate in the concrete that the original masons used.

Ninety days after the repair is made, you then do an acid washing of just your repair area to remove the cement paste from your new sand and aggregate.


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