When all the paint begins to peel from this bedroom ceiling and mushrooms are found growing on the basement floor, the old house owner knows that there is a moisture problem. Unfortunately, by the time these obvious signs appear, a great deal of additional but unseen damage has probably already been done.

Moisture is an old house's worst enemy because it creates the climate needed for rot to grow. Rot is caused by fungi that can only grow when the moisture content of the wood is higher than 25 percent. Once the rot fungi have taken hold they need no additional moisture because the process of decay produces water.

Decay can be stopped if the wood is dried, the weakened material replaced with sound wood, and the original source of the moisture is eliminated.

Quarterly inspections of the old house are the best way to detect incipient moisture problems and the conditions that would encourage them to develop. Armed with a detailed plan of the house, a flashlight and an ice pick, make a systematic review of the house.

Begin with the roof. Check all the flashing (the sheet metal used to waterproof the seams of the roof) and the gutters. In the fall and winter, leaves and ice may clog the gutters and valley of the roof. Excess water can be forced under the flashing and drip down into the ceilings and walls.

The shingles should also be checked. All wooden shingles and all roofs eventually have to be replaced.Monitoring their condition and replacing damaged shingles before they do damage to the structure of the house is much cheaper than replacing an entire roof and the wood that supports it.

Examine all of the joints on the exterior wood trim. Moisture can easily enter the wood at these points. Even painted trim is not immune to rot. Paint can seal in any moisture that enters and can also keep the wood from drying out. Poke the wood with the ice pick. If the pick goes in an inch or more, there is a problem with rot.

If there is a crawlspace underneath the house, crawl in and look around. Moisture condensing on pipes or joists (those are the wood beams that support the floor) could be the start of a problem. Use the ice pick. If there is no evidence of rot but there is moisture, plan to spend part of the next day covering the ground in the crawlspace with polyethylene sheeting and insulating the pipes.

The kitchen and the bathroom are the worst danger spots inside the house because of the plumbing fixtures. Check for leaks under the sinks. Replace the caulking around the tub and the sinks if it begins to crack. Use the ice pick to check the floor and walls near the commode; slow leaks there may not be noticed until the floor is severely damaged.

Be sure to use the ice pick to check for water damage underneath the tub. One old house owner who had just moved into a Victorian gem, filled with clawfoot bathtub and began to soak. As the tub sank noiselessly into the basement, she slid onto the bathroom floor.

An old leak had started the process of decay. She and the bath water were too much for the rotten floor joists to hold.

After the inspection, note any signs of potential trouble on the house plan. It may be necessary to consult an architect or carpenter about serious problems or conditions. If the problem is being caused by something as simple as clogged gutters, setting up a maintenance schedule will be the answer.

The original design of the house may be the cause of the problem. In that case, redesigning the faulty element or adding protective flashing may need to be done. In any case where there has been damage the source of the moisture needs to be found and the eliminated, all of the damaged wood replaced, and the repair checked regularly to make sure the problem does not reappear.

Beverely Reece is associated with the Preservation Resource Group, a firm that conducts preservation workshops for homeowners here.