Wood rot, termite damage, and structural decay are common concerns for homeowners in San Francisco and the surrounding areas, particularly in the Victorian, Edwardian, and mid-century housing stock that defines the region. The marine fog climate, salt air, and persistent moisture create ideal conditions for fungal decay and wood-boring insects year-round, making wood deterioration a practical reality rather than a rare emergency. This guide explains what these problems are, how to recognize them, what they mean for your home's structure, and how to approach repair honestly.
Whether you are dealing with soft siding, a bouncy floor, a deck that flexes, or damage discovered after a pest inspection, understanding the problem before you call a carpenter will help you make informed decisions about repair versus replacement and about which professionals you actually need. This is a reference page meant to leave you better informed than when you arrived.
Dry Rot and Wet Rot: Not the Same Problem
Dry rot and wet rot are two different fungal decay problems, and the distinction matters because they behave differently and require different approaches. Dry rot is a fungus that needs sustained moisture in the wood (typically 20 to 30 percent moisture content) but does not require standing water or active dripping. In San Francisco's marine fog climate, this sustained dampness is present year-round in exterior wood, making dry rot a constant structural concern. Dry rot spreads through wood along the grain and can even travel across masonry to reach new timber, which is why it can affect a larger area than the original leak. The wood affected by dry rot develops a distinctive cubical cracking pattern as it shrinks, and the texture becomes dry and crumbly when you probe it. A paint film can look perfectly intact over dry rot because the fungus works from inside the wood outward.
Wet rot, by contrast, requires a persistent direct water source: active leaking, standing water, or constant spray. It stays localized to the area where water is present and does not spread through sound wood or across masonry the way dry rot does. Wood affected by wet rot typically darkens and becomes soft and spongy to the touch, almost like compressed cardboard. Both conditions can hide under sound-looking paint and finish, which is why visual inspection alone will not tell you the true extent of decay. The paint film can appear normal because the damage is beneath the surface. Probing with a screwdriver or awl is how you determine whether wood has lost its structural section, and that physical testing is the only reliable way to know what you are dealing with.
Termite and Beetle Damage, and How It Differs from Rot
Termites and wood-boring beetles cause damage that looks and behaves differently from fungal rot, though the two problems often occur together. Termites hollow out wood from the inside, following the grain, and typically leave a thin shell of sound wood at the surface. They also leave mud tubes on masonry surfaces as they travel between the soil and the wood they feed on. Wood-boring beetles, by contrast, create visible exit holes in the wood surface and leave a fine powdery residue called frass. Both types of insect damage weaken the structural section of a member, but they do it in different ways and at different rates. The key point is that insect damage and fungal decay often coexist because the same conditions that invite rot (sustained moisture and poor ventilation) also attract insects.
Identifying and treating an active insect infestation is the work of a pest control operator registered with the California Structural Pest Control Board, not a carpenter. When you have a pest inspection done, the report will tell you what damage is present and what treatment (if any) is recommended. A carpenter then removes and replaces the wood that has been damaged, working from that pest report. We do not apply pesticides, fumigants, or chemical treatments, and we do not claim that an infestation has been eliminated. Our role is to rebuild the structure once the pest operator has done their work. This division of labor protects you because each professional is doing what they are trained and regulated to do.
Reading the Signs Before You Open Anything Up
Before you open up walls, ceilings, or decks to see what is underneath, there are several signs that point to wood decay or insect damage. Look for paint that blisters, peels, or holds moisture at window sills, door thresholds, and the base of exterior trim. Press your thumb firmly against fascia, soffit, and trim boards; if the wood gives easily or feels soft, that is a sign of decay. A screwdriver pressed into wood should require real effort; if it sinks in easily, the wood has lost section. Check doors and windows to see if they have changed how they close or open, since settlement and swelling from moisture often go hand in hand. Walk across your floor and notice whether any section feels noticeably soft, bouncy, or more flexible than the rest; a deck board that flexes more than its neighbors is another warning sign. Finally, a persistent musty or damp smell in a basement, crawl space, or enclosed area often indicates moisture and decay.
None of these signs means you need to panic, but they do mean you should have a carpenter look at the area. What you see on the surface almost always understates the extent of decay rather than overstating it. Paint and finish can hide significant damage, and the only way to know how much sound wood remains is to probe carefully and, if necessary, open up a small section to see what is underneath. A good carpenter will show you what they find and explain what needs to come out and what can stay.
What Decay Does to a Floor, a Deck and a Wall
Understanding how wood members carry loads will help you understand what decay actually does to your home. A floor system works like this: floor joists (the beams that run perpendicular to the wall) carry the weight of the floor and everything on it; they sit on a rim joist (the board that closes off the end of the floor frame) and a sill plate (the board bolted to the top of the wall); the subfloor nailed to the joists spreads the load and ties everything together. In a deck, the ledger board (the board bolted to the house) and the deck posts and beams do the same job. A load-bearing wall holds up the floor or roof above it through its studs and header. When wood in any of these members loses structural section to rot or insects, it loses the ability to resist bending. The first thing a homeowner notices is not collapse; it is stiffness loss, which feels like bounce or flex in a floor or deck. That bounce is your signal that section has been lost and the member is working harder than it should be.
The progression is gradual. A joist that has lost ten percent of its section will feel slightly softer; at twenty or thirty percent loss, the bounce becomes obvious. At some point, if decay continues unchecked, the member fails, but that point comes after a long period of increasing flex and movement. The ledger connection where a deck attaches to the house is especially important because it carries a large concentrated load and is often in a location where moisture collects. A failed ledger connection is the most dangerous condition in a deck, which is why ledger inspection is worth doing every few years in this climate.
Repair or Replace: The Actual Criteria
The decision to repair a decayed member or replace it depends on four practical factors. First, how much sound wood is left? If the member has lost less than about twenty percent of its cross-section and the decay is not at a critical bearing point, repair is often the honest answer. Second, where is the decay located? Decay at a bearing point (where the member sits on a wall or post) is more serious than decay mid-span because it affects how load is transferred. Third, can you actually close the water source? If the leak or moisture problem that caused the decay is still present, a repair will fail again. Fourth, how do the costs compare when you account for access? Sometimes opening up the wall or floor to replace a member costs far more than sistering (bolting a new piece of wood alongside the old one to restore section and stiffness). A good carpenter will quote both options where both are real and explain the trade-offs.
Epoxy consolidants (products that harden soft wood) have a real but narrow use: they can stabilize non-structural profiled trim and molding that is worth saving for its historical or aesthetic value. They do not restore structural capacity and should never be used as a repair for a joist, beam, or load-bearing member. Filler or epoxy over structural rot is not a repair; it is cosmetic work that hides a problem and leaves you with a member that will continue to fail. A repair that is honest costs more upfront but protects your home and your investment. Replacement costs more in labor and materials but gives you new wood with a full lifespan ahead of it. The choice depends on your situation, and a carpenter who explains both options is giving you the information you need to decide.
Who Does What: Carpenter, Pest Operator, Engineer
Wood decay and structural damage involve several different professionals, each with a specific role. A pest control operator registered with the California Structural Pest Control Board inspects for insects, identifies active infestations, and applies any necessary treatment. A carpenter (that is us) removes and replaces damaged wood, repairs and closes water leaks where they affect carpentry, and restores structural sections. A licensed structural engineer sizes beams, calculates load paths, and specifies repairs when the structure itself is compromised. The San Francisco building department permits and inspects structural work, including beam replacement, load-bearing wall removal or alteration, and certain deck and stair work. Requirements and thresholds vary, so you should verify with the building department which work on your project requires a permit and an engineer's review.
San Francisco Carpentry is fully insured and EPA Lead-Safe Certified, and we do the carpentry work: removing decay, replacing damaged members, sistering joists, installing new framing, and finishing the repair so it lasts. We do not treat insects, apply pesticides, or offer pest control services. We do not size beams or provide engineering opinions; where your project requires a structural engineer, we will tell you that. We do not test for or remediate mold. What we do is listen to what you have found or observed, explain what the damage means, show you the options, and rebuild your home honestly. If you have questions about your specific situation, call us at (628) 285-5429 for a free estimate.