Concrete vs Block vs Timber: Which Retaining Wall for a Steep Bay Area Lot?

The quick comparison

Poured concrete: the strongest option

Reinforced concrete is the usual answer for tall walls, driveway walls and walls that carry a surcharge. A cast-in-place wall has a continuous footing, steel reinforcing and a solid face. It's the thinnest of the three options for a given strength, which helps on small lots.

• Best for walls over four feet and for hillside work
• Handles loads from driveways, buildings and slopes above
• Can be faced with stone, stucco or textured forms
• Needs forms, rebar and a concrete delivery plan
• Needs weep holes and a drain, because it doesn't leak on its own

See our concrete wall page for the details.

Reinforced block: strong and flexible on access

Concrete masonry units, usually called cinder block, are stacked with vertical steel and filled with grout. They make a strong wall that can be built by hand, with no formwork and no concrete truck. That's why block shows up so often on tight Bay Area lots.

• Good for medium and tall walls with engineering
• Can be carried into stairs-only or narrow-access yards
• Needs fully grouted cells and steel to work as a retaining wall
• Can be stuccoed or faced for a better look
• Needs a drain and weep holes

Many failed block walls were built without steel or grout. See our block wall page for what a proper wall includes.

Timber: good for short walls and tight budgets

Pressure-treated timber, rated for ground contact, works well for short garden walls. It's easy to build, easy to carry and has a natural look. It also has a limited life when it's buried and wet, which is exactly what a retaining wall asks of it.

• Best under about four feet and away from structures
• Needs ground-contact rated lumber and corrosion-resistant fasteners
• Needs deadmen or tiebacks for taller walls
• Needs a drain, like every other wall
• Not a good fit near buildings in fire zones

Our wood wall page lays out the tradeoffs.

Mistakes we see with each material

Concrete

Too little cover over the rebar, especially near the coast. No weep holes. Forms that bulge during the pour and leave a rough face. A footing that's too shallow for the soil.

Block

Empty cells with no steel or grout. Backfilling with clay. No drain. A cap that isn't sealed, so water gets into the cells and rusts the bars.

Timber

Untreated or poorly treated wood. No deadmen on a taller wall. Buried posts that sit in standing water. Soil piled against the back with nothing to drain it.

Almost every mistake on this list is about water or steel. Get those two right, and any of the three materials can do its job.

How to choose

1. Start with height. Taller than four feet, think concrete or reinforced block.
2. Think about what's above the wall. A driveway or a house means engineered concrete or block.
3. Think about access. If a concrete truck can't reach the site, block may be the practical answer.
4. Think about the look. Concrete and block can both be faced with stone.
5. Think about the long term. Timber is cheaper up front but usually needs replacing sooner.

On steep lots in places like Daly City, reinforced block and concrete are the usual choices, with timber for the low steps.