Drywall took over American residential construction in the late 1940s and 50s. Before that — and for a while afterwards in higher-end work — interior walls were plaster applied over wood lath: thin horizontal strips nailed to the studs, with wet plaster pushed through the gaps to form "keys" that lock it in place.
Where you will find plaster locally
- Martinez — the oldest housing stock in our service area. Downtown and Alhambra Valley are full of it.
- Orinda — the 1930s and 40s village-core homes in Sleepy Hollow and Glorietta.
- Downtown Pleasanton — the Main Street historic district.
- Diablo — resort-era houses from the 1910s and 20s.
- Rivertown, Antioch and pockets of Saranap and Old Town Danville.
Four ways to tell
The push-pin test
Push a pin into the wall somewhere hidden — inside a closet, behind furniture. Drywall accepts it easily. Plaster resists hard, and if it goes in at all it will feel gritty rather than soft.
The knock test
Knock along the wall. Drywall sounds hollow between studs and solid on them. Plaster over lath sounds consistently dense and dull almost everywhere, because there is a continuous rigid layer.
Look at an existing hole
Behind a switch plate or an outlet cover, look at the edge of the cut. Drywall shows a paper face, a gypsum core and a paper back — usually half an inch. Plaster shows a solid grey or white mass, often three-quarters of an inch or more, sometimes with wood lath visible behind it.
Check the build year
Pre-1945 is almost certainly plaster. 1945–1955 could be either. Post-1955 is almost certainly drywall — unless someone did something unusual, which in custom Contra Costa homes is not that unusual.
Why the difference matters for repairs
A drywall patch relies on screwing new board to framing and taping the joint flat. Plaster does not work that way: it is a monolithic layer keyed into lath. Cut a rectangle out of it and you break keys well beyond your cut, weakening the surrounding plaster. Screw a drywall square into the hole and you have a rigid panel of a different thickness in a wall of different material, with a seam that concentrates every bit of movement in the house.
Done properly, a plaster repair either re-keys new plaster into the sound lath, or — where the plaster is failing across a whole wall — skim-coats the entire plane so there is one continuous surface again.
Should you replace plaster with drywall?
Sometimes, but it is not automatic. Plaster is harder, better at sound, and often finer than what would replace it. If it is broadly sound with localised damage, repairing is cheaper and better. If it is failing across whole rooms, has been patched a dozen times, or is coming away from the lath in sheets, re-boarding may genuinely win.
What you should be wary of is a contractor who reaches for demolition immediately. In a historic home, that is often the easy answer rather than the right one.
Not sure what you have? Send a photo of the edge behind a switch plate — that one picture usually settles it.