Q: The wooden spacers separating the concrete sections of our patio need to be replaced. Each piece is 7/8 inch wide by 48 inches long. The depth varies because each piece has decayed at different rates. It doesn’t look like any of the pieces were nailed or secured.

What are our options for replacing and how do we go about it? If we use wood, what kind, and how does one prepare the dirt underneath to keep the pieces secure and flush with the concrete? Are there possible materials other than wood? Rubber maybe, or silicon? By the way, we live in the hills of Oakland, Calif.

A: The way we see it, you’ve got a couple of choices. Replace the wood or fill the voids with a mixture of sand and pea gravel. Rubber or silicon is not an option for a retrofit with joints of this width. Wood and sand/gravel each have pluses and minuses. So, read on and pick your poison.

The pieces of wood between your concrete slabs are expansion joints. Because you live in the Oakland hills, you have clay soil. Clay expands when it gets wet and shrinks when it dries. As the ground moves, so do the concrete slabs. The wood between the slabs not only looked good when it was new, but absorbed the stress of soil movement.

It’s a good bet that your expansion joints are redwood. Redwood lumber has varying amounts of heart and sapwood. The heartwood is red and contains tannins that make it resistant to insects and rot. Sapwood is light and susceptible to bugs. This accounts for the different rates of decay. These days redwood is replaced by pressure-treated fir. It has the same bug- and rot-resistant qualities at a fraction of the price.

To prepare the joints, begin by removing all the wood. Then dig out the earth between the joints to a depth of 1 1/4 inches. Finally give the joints a good dousing of water with a garden hose. Try to clean the sides of the concrete slabs as well.

If you decide to go with wood, have a lumberyard cut pressure-treated two-by-fours 3/4 inches wide to fit the 7/8-inch joint. This allows for some expansion of the wood. Cut them to 4-foot lengths to make them easier to handle and lessen the chance they’ll warp. Paint the raw edges with wood preservative. While the soil is wet, pound the wood into the soil flush with the slabs with a rubber hammer. It may be necessary to pin the wood to the ground at points. If so, drill a hole in the wood and fix the boards by driving 16d finishing nails into the ground to hold the board in place.

Potential problems are the wood warping and popping out of the joint or irregular widths of the joints, which requires using a block plane to custom fit the boards to the joints.

To our mind, a much easier solution is to fill the joints with a mixture of course sand and pea gravel. Sand mixed with gravel provides a solid mixture, although the joints may require a little refilling after a period of time due to settling.

If you go with sand and gravel, begin by removing the wood and loose soil from the joints and wet the joints to compact the loose soil. Fill the joints with a mixture of sand and pea gravel — two parts gravel to one part sand. Fill the joints half way with the sand/gravel mixture and spray it with a garden hose to compact it. Fill it to the top and spray again. You’re done.

All things considered, we’d go with the sand/gravel solution instead of hassling with wood.

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