I get this question a lot, short answer… Yes. But there’s a reason very few contractors use over pressure treated redwood (or pt) for mud sills and bottom plates.

Let me first say that I have remodeled homes in the San Francisco and Marin areas that were over 100 years old, built entirely of redwood, and are still structurally sound. This is because the redwood is naturally rich in acids, which prevents it from rotting and prevents insects from eating it, since they are the two worst enemies of houses, the redwood makes sense. Until you look at the price tag. The fact is that many redwood areas were heavily logged and those that were not are now protected. As a result, the price of redwood is very high compared to other framing products and they are now using younger growing trees which equates to more knots, kinks and sags.

So, to cut costs, pressure-treated lumber was introduced as a lumber that could mimic the attributes of redwood but for half the cost. Pressure-treated lumber is usually Douglas Fir or sometimes Hem Fir and has many toxic chemicals injected into it (the most potent being copper or zinc napathane). These chemicals are what prevent rot and pest infestations, but are also known to cause cancer by contaminating groundwater when disposed of in landfills. In California, pt waste parts must be disposed of in a biohazardous waste facility. Recently, toxin levels have been reduced by approx. fifty%.

It is also worth noting that it is never acceptable to place untreated Douglas fir lumber in direct contact with concrete. There are several ways to treat Douglas fir and make it resistant to rot and pests. The #1 way to do this is to apply copper napathane in a spray can or regular brush-on liquid. You can find this at almost any neighborhood hardware store, but be careful doing it! It will stink for a day or two, so don’t apply it directly to living spaces. Be sure to read any and all factory instructions regarding the application of any sealant or rot preventative.

So to answer the question, yes…absolutely, redwood is a great product for clay sills. But if you’re going to use redwood, save some money and get ‘rough cut’ lumber, this will be a bit larger so you’ll have to cut it widthwise, but the price difference is worth the extra work.