Does the home have a wet crawlspace or a missing crawlspace vapor barrier? This could contribute greatly. Is the roof sound? Is the building on a concrete slab? If so, the carpet and pad may be absorbing moisture into the building. What type of heating system? The mildew on the walls is probably the result of uninsulated walls (colder than the dewpoint) having long-term contact with the moist air in the living area.

Q: Two years ago, when I bought my home, the inspector stated in his report that the 10-year-old cedar roof had 15-20 years of life remaining. It is now in need of immediate replacement due to poor workmanship and materials. The inspector responded in writing recently stating that he did not actually walk on the roof during his inspection because he does not walk on slippery, wet cedar roofs. The roof is very mildly pitched.

A: Cedar shake roofs last about 18 years nowadays, if maintained. Thirty-year cedar roofs are a thing of the past. Your inspector obviously did not have spiked shoes called "corkers." Corkers make walking on even moderate pitches comfortable when soaking wet. Cedar roofs are slippery even when dry. Many professionals will not go onto a cedar roof without corkers. If he won't walk on wet roofs, I guess he only works about two months of the year here. Make sure you understand an engineer/inspector's policies and procedures before hiring. Doing a roof inspection from the street is acceptable only when the roof is inaccessible, brittle (i.e.; slate or asbestos), or the pitch is great.

Q: My water utility is requiring "backflow preventors" on my residential irrigation system and wants annual certifications. What gives?

A: Backflow preventors have cousins you may be familiar with - air gaps, double-check valves and anti-siphon valves. Their basic purpose is to prevent water in contact with a potential source of contamination from being drawn back into the system. This can occur when there is a large draw of water such as a fire hydrant test, or a water line shut off temporarily for repairs.

The plumbing code requires these devices on any fixture with "cross connections." Your dishwasher is probably the most familiar. Many sink-mounted dishwashers gurgle when the water is draining from the machine.

There were numerous deaths from backsiphoned water in Chicago not long ago. It was traced to an exterior sprinkler system that did not have an operating backflow prevention device. Many amateur-installed sprinkler systems do not have these very important safety devices. The lack of backflow preventors could potentially kill you and your neighbors. This code has existed for many years, but the prevalent thinking now is to have ongoing checks performed to assure compliance. Humble pie

Two weeks ago, during my discussion of reduced pressure and flow in a plumbing system, I stated that reducing pressure would increase flow, using the work of Bernoulli as the basis for this assumption. Bernoulli's principles of fluid dynamics show that as pressure is increased, velocity is decreased. This is true in jet engines, but not true in something with a constant cross section, such as a plumbing system. Thanks to Ron Dupuis and Roger St. John for correcting me. Please continue to keep me on my toes.

The Expert answers readers' questions every Saturday. Send questions to Ask The Expert, Seattle Times, P.O. Box 70, Seattle, WA 98111, or call 206-464-8514 to leave your questions on Ask The Expert's recorded line. E-mail address is dhay@seattletimes.com