We have built 4x4 retaining walls around existing rocks and concrete that we were unable to get rid of. The idea was - and is - use something fill the gaps between the lumber and the rocks / cements. Given that some of the gaps are over 2 inches, what is the best - and easiest - way to fill these gaps?

4x4 retaining walls and gaps

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    Would have been easier to scribe the wood closer to the rock. But agree with Ed, mortar is the only suitable material, but it may crack away from the rock/wood slightly over time. With luck it should stay in place though and would still look fine with a hairline crack where it joins the rock/wood.
    – handyman
    Aug 16 '20 at 9:37

I have seen fine wood dust mixed with a putty of some kind mixed and placed inside gaps, if the wood is stained then also add a little stain to match the color.

Sanding down to the exact shape would have been the first step for me then doing the filling.

  • Thanks, I just searched around and found a recipe they use in Australia: 3 parts sawdust, 2 part sand and 1 part cement. I might use hydraulic cement. This is a low cost low effort experiment so I'll give it a shot. Thanks for the idea!
    – 0pt1m1z3
    Aug 22 '20 at 1:11

I would use mortar , if you wanted to get fancy you could add pigment and match the color of the rocks. The one in the corner looks to have some color. I have not done this exactly but have seen it done at first I thought he had chiseled the rocks but then I realized he matched the rock color and filled with mortar.


Mortar and putty are going to look out of place.

You could plant some ivy or vines near the gaps and let it climb up and cover the gaps. Nobody will ever know they're there.

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