Fog, Damp Air and Wood Problems in Bay Area Homes

What is dry rot, and why is the name misleading?

Dry rot is wood decay caused by a fungus, not by an insect. The word dry is misleading: decay fungi generally need moisture to start. Once wood has been damaged, it can look dry, crumbly and brown, which is where the name comes from. The cause is usually a water source that kept the wood damp for a long time.

People often picture rot as something that happens to wood sitting in standing water. Dry rot gets its name because the wood it leaves behind often feels dry, cracks across the grain into cube-like pieces and crumbles under a screwdriver. By that time the fungus may be inactive and the moisture that fed it may be long gone. The damage remains.

What matters for a homeowner is the chain of events. Wood gets wet and stays wet. A fungus colonises it and starts to digest the structural fibres. The wood weakens and changes colour and texture. Then something gives way: a stair tread, a sill, a section of fascia. The process takes time, and it is usually slower than the speed people fear and faster than the speed people notice.

It is worth separating decay from termites, because the two are easy to confuse and they overlap. Termites are insects, and they eat wood. Decay is a fungus, and it breaks wood down. They can occur in the same piece of timber, and termites like moist, softened wood. That overlap is part of why one inspection report covers both, which we explain below.

We describe the general mechanics of rot in hedged terms because the exact speed and path depend on the species of fungus, the type of wood, the moisture level and the temperature. We will not give a local figure for how long wood takes to rot, because we do not have one and nobody should promise one.

How does the coastal Bay Area climate set wood up for decay?

The area has dry summers and wet winters. Rain from November to March accounts for the large majority of the year's total, and summer nights bring sea fog with overcast mornings and sunny days. For exterior wood, that means a long wet season followed by a dry one that is often damp at night.

The climate here does two things. First, the winter rains arrive in a concentrated season. Wood that is exposed to rain, or that sits where rain runs and pools, gets repeatedly wet. Second, summer is not simply dry. Sea fog rolls in at night and the mornings are overcast before the sun comes out. That can mean damp surfaces long after the last storm, particularly on the shaded north side of a building and in places that get no direct sun.

We are deliberately not quoting inches of rain or degrees. The figures we have seen come from secondary summaries, and the pattern is more useful than the numbers. Dry summers and wet winters, with fog on top, is the version that matters for wood.

The result is a house whose exterior wood dries out slowly. Paint and caulk crack in the sun, water gets behind them in winter, and the wood beneath does not dry before the next rain. That is a standard recipe for decay. A coastal house also tends to be exposed to wind-driven rain on the side facing the weather, which pushes water into joints that stay dry on the sheltered side.

None of this means a house in a foggy neighbourhood will rot. It means that maintenance matters more, and that small leaks have more time to do damage than they would somewhere that dries quickly.

Where does decay tend to show up on a coastal house?

Look where water lands and lingers: window sills and casings, door thresholds and frames, eaves and fascia, deck and stair posts, trim near the ground, garage door jambs and the subarea. Fences touching siding, planters against walls and blocked downspouts add moisture. These are general patterns, not findings about any specific street.

A good way to examine your own house is to follow the water. Where does rain run off the roof? Where does it hit the ground and splash? Which surfaces face the weather and which are shaded? Wood in those places is where an inspector will start, and where you can look yourself.

The table below lists spots that come up again and again. It is based on general inspection practice. It is not a count of local findings, and your house may not have any of these problems.

Some of these spots hide the problem. A sill with fresh paint can be soft underneath. A stair stringer can look solid from above and be hollow where it meets the ground. Poking suspect wood gently with a screwdriver is a common way to check, and a spongy or crumbly feel is a reason to ask a professional to look. Do not pull large sections away yourself if the area may be structural.

Who inspects for dry rot, and what does the licence cover?

In California, the Structural Pest Control Board's Branch 3 covers termites, wood-boring beetles, dry rot and fungus. An inspection for wood-destroying pests and organisms therefore looks at decay as well as insects. The resulting report lists findings, conditions conducive to infestation and recommendations.

California's structural pest system has three branches. Branch 1 is fumigation, Branch 2 is general pest, and Branch 3 is termite, which is the one that includes wood-boring beetles, dry rot and fungus. So an inspector with a Branch 3 licence is working inside the legal field that covers rot.

The written report from such an inspection is the Wood Destroying Pests and Organisms report, and it includes decay among its subjects. Our guide on old San Francisco houses and termite inspections goes through how to read one. Briefly, expect separate listings for active and past findings, for conditions that could lead to infestation, and for areas that could not be inspected.

What an inspector does not usually do is carpentry. The repair is a separate trade, and the same company may or may not offer it. The inspector's role is to find and document the damage and to point to conditions, usually moisture, that caused it. Our page on dry rot and wood damage explains how that service fits together, and termite inspection covers the inspection side.

You can ask any company for its licence number before work begins. Structural pest businesses also register with the county Agricultural Commissioner in each county where they work. We cannot give you office details, so we say only that: register with the county Agricultural Commissioner.

How is a rotted area usually repaired, in what order?

The usual order is to find and stop the moisture, open up the damaged area, remove or treat the affected wood, let the surrounding framing dry, then replace and seal it. Skipping the first step is the common mistake: new wood placed against a still-leaking area can decay in turn.

These steps are general trade practice. They are not a protocol, and the details change with the house, the wood and the extent of the damage. If a piece of wood is structural, such as a post, beam or sill, the repair may need a contractor and possibly a permit, and we do not give local rules for that.

First, find the source of the water. A rotted sill under a window may come from a failed seal, a clogged gutter or a leaking pipe. If the source remains, the repair is temporary. Second, expose the damage. That may mean removing trim or siding to see how far the decay goes. Rot often extends further than the visible surface. Third, remove the damaged wood back to sound material. Some repairers apply a wood hardener or a preservative to the edges of the sound wood, and some replace whole sections instead. Which approach makes sense depends on the location and how much strength is needed.

Fourth, let the area dry before closing it. Wet framing sealed in new paint is a repeat problem waiting to happen. Fifth, replace and seal. Primer on all faces and cut ends, a good seal at joints and a clear path for water to drain are the details that decide how long the repair lasts. We cannot give a durability figure because it depends on too many things.

Termites should be considered at the same time. When a house has decay, a technician may check for insect activity in the same area, since damp, softened wood is attractive to some wood-destroying insects. See subterranean termite control and drywood termite control for how those are handled.

• Find and stop the moisture source.
• Expose the damaged area and see how far it goes.
• Remove or treat the affected wood, back to sound material.
• Let the surrounding wood dry.
• Replace, prime and seal all faces and joints.
• Check for insect activity in the same area.

What can you do to slow decay between inspections?

Keep water away from wood and let wood dry. Clear gutters and downspouts, keep soil and plants off siding, seal and repaint exposed trim before it fails, ventilate the subarea, fix leaks promptly, and look at the shaded sides of the house each autumn before the rains start. Small habits do more than large repairs.

The best protection is dull and regular. Gutters and downspouts carry roof water away from the walls, but only when they are clear and when the water ends somewhere harmless. A downspout that empties against the foundation puts that water exactly where you do not want it. Extending it is a small job.

Paint and caulk are the skin of exterior wood. When they crack, water gets in. Re-sealing the joints at windows, doors and trim before the wet season is the easiest way to extend the life of the wood underneath. Fog does not stop at the front of the house, so check the north side and the shaded corners, which dry last.

Soil, plants and fences that touch siding hold water against the wood. Leave a gap. In the subarea, make sure vents are not blocked and that pipes are not weeping. Ask whoever inspects the house to note the condition of the subarea; this is one of the areas where problems hide.

If something looks wrong, a technician can take a look, and (855) 974-4733 is answered all day and night; the same day is possible when a technician is free. We pass calls to an independent local company, and what happens on site depends on what is found.