Q: What's the best way to build a 2-foot-high fence on top of an existing cement-block retaining wall? I was thinking of using joist hangers to connect the 2x4 posts to the wall's face.

— E. Hal, Sierra Vista, AZ

A: general contractor Tom Silva replies: I wouldn't do it. Joist hangers aren't designed for that purpose, especially on brittle concrete block. Besides, I don't think the exposed metal brackets would look all that good on a fence like this. Neither will 2x4 posts, which just aren't strong enough in any case. Your fence will be stronger and look better if you use 4x4 posts anchored in concrete.

To do this right, you'll have to fill the cores, the holes in the blocks, with concrete at each post location and anchor the posts in the mix. But first, get the posts ready. Use a ¾-inch auger bit to bore a 12-inch-deep hole in the bottom of each post. Then soak the ends of the posts in a water-repellent wood preservative — you won't get another chance to do this once the posts are set. The anchors will be ¾-inch threaded rods, each one cut 22 inches long. Spread polyurethane glue over half the rod length and screw the rod that far into each post's hole.

To fill the cores, wet the inside of the blocks and stuff wet newspapers at least 14 inches down the block-wall cavity. They will act like a plug and save you from mixing more concrete than you need. Now fill the cores to the top, agitate the concrete with a stick to eliminate air pockets, smooth the surface, and stick the posts' rods into the mix. Make sure to keep the ends of the posts out of the concrete; you don't want them to be stained by the wet concrete or to rot prematurely. Use wooden shims to create a ¼- to ½-inch space between the wood and the concrete. Finally, check that the post is plumb on two adjacent sides, and brace it temporarily on those two sides until the cement sets the next day.

Once you have all the posts in place, assemble the fence sections between them and remove the shims and braces. You'll now have a strong, good-looking fence that's even a little mysterious — nobody will quite know how it's staying up on the wall.
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