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posted: 9/2/2012 5:32 AM

Algae on shingles is homeowners' nemesis

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Q. I have ugly black streaks/stains on the asphalt shingle roof of my house. They appear on the north side of the roof only, where the sun doesn't hit very much; the south side is fine. I assumed it was some kind of mold or algae, and there are a few small green clumps on the roof, but mostly there appears to be nothing growing on the black stained areas. I've seen the same condition on many other north-facing roofs in this area, so it must be a common problem.

I've tried one commercial product, Jomax roof cleaner and mildew stain remover, which claims to remove black streaks. I followed the directions precisely, but it did not work for me. It perhaps spread the stains a little and gave them a brownish tinge, but they are still there and look worse than ever.

Do you know of any product or procedure to remove the stains? What about Mold Armor (which I read about in your column) or Wet & Forget?

A. The staining of roofs by algae is a widespread phenomenon found in all but the most arid climates. Algae develops from a combination of moisture and spores in the air.

A number of cleaning products can be used to remove algae, including Roof & Deck Cleaner from Stainhandler (, Spray & Forget ( and Wet & Forget ( Both Spray & Forget and Wet & Forget claim to be environmentally safe, and readers have reported good luck with both. These products should be sprayed on the roof in warm weather by someone able to tackle the job safely: a general contractor, handyperson, roofing contractor, etc.

A reader recommended Mold Armor and said it did a great job clearing his roof of algae. You may want to try it. I have had no experience with it.

You also can try spraying the roof with a solution of equal parts fresh Clorox bleach and water. (One gallon of the solution will treat 50 square feet of roof). Spray from a ladder on a windless day, but do not use so much solution as to cause substantial runoff.

If you have metal gutters, run water from a garden hose through them during the spraying. Before spraying, thoroughly wet the vegetation below the roof and cover it with plastic. Do not rinse the roof, but rinse the plastic and soak the vegetation when done spraying the roof.

As with all products and methods for dealing with moss, lichen or algae, you must follow directions on the containers and allow plenty of time -- sometimes months -- before seeing results, depending on the extent of the infestation.

The algae is likely to return over time. If you want a permanent solution, have zinc strips installed just below the cap shingles at the top of the ridge. As rain hits the zinc strips, they release ions that are poisonous to algae, moss and lichen. You can buy zinc in rolls in some hardware, big box and building supply stores, or in more manageable strips from This process will also take several months to show results.

Please remember that it is dangerous to walk on a roof, particularly if it is wet or even damp. It's a job best left to people experienced in working on roofs, such as building contractors and roofers. Walking on a roof can also void any warranty, although these warranties leave much to be desired and can be hard to collect on.

Q. In an attempt to find the leak that has caused the parquet floor to buckle outside our bathroom, our contractor took apart the shower diverter that leads to the handheld shower. He thought the thread was not quite right, so he took it apart and took a picture of the inside (female) thread. He capped it and had us use the regular shower, then turn the control to the handheld to pressurize the cap and make sure nothing was leaking. I know he added/replaced something internal, then wrapped the male thread with very fine white tape and put everything back together. There was no disruption to the shower tile, wall or any plumbing.

He didn't use a water meter as the flooring was saturated. He took off the baseboard in the linen closet that backed to the plumbing in the bathroom, then cut a hole at the base of the wall. He put a camera up the wall and said all was well. In the hall, he took off the baseboard, the carpet, pad and parquet. We left it exposed for a week -- no apparent leak.

He has now replaced the parquet with the same height plywood, as there was not a match for 46-year-old parquet. In another few weeks, we will shop for replacement carpet and pad. I'm in no hurry, as I want to be sure the problem has been fixed.

We also seem to have a crack in the ceiling wallboard along a taping line below where the apparent supply leak caused the damage in the hall flooring. How and when do you suggest we proceed with this situation?

A. The saturated flooring indicates a long-term slow leak. The drywall tape damage is also an indication. A major event would have been noticed early.

Your contractor seems to have concentrated his efforts on the plumbing fixtures and found no evident problem, which is comforting. But you should check for a leak in the shower base, either through the grout joints, if they were not sealed or epoxy grout was not used, or around the floor drain -- often the culprit.

To check for these potential sources of leaks, let the water run for half an hour or longer to soak the shower base or, better, place a flat rubber stopper over the drain and fill the shower base to within an inch of the curb. Let it stand for several hours or overnight, if possible.

The reason you haven't found wet conditions may be that you haven't used the shower for a while or you haven't let the water run long enough. It also could be that the conditions leading to the leak have abated. The chemicals in the water may have sealed any leaks, but that's a stretch.

You may have to cut a hole in the ceiling below the shower while the water is running or standing to see if a leak appears and from where.

I once found, after exhaustive testing, that an unexplainable leak was the result of bathers splashing water onto the escutcheon of the shower valve. That water ran down the wall and spread on the ceiling below, damaging the plaster ceiling.

There are many mysteries in construction. I hope you get yours resolved.

Q. We live in a two-story brick home built about 1950. We installed whole-house air conditioning about 15 years ago. We have a temperature-activated whole-house fan in the attic (controls located in the attic). During the hot weather, I have been propping the attic access open about 3 inches while the attic fan is running to draw up some of the second-floor heat. Is this a good or bad idea? Sometimes the fan shuts down before the air conditioner, and the second floor then seems to get very warm. Is there a formula for working these two things together, or is it just trial and error?

A. You are causing the air conditioner to run more because you are sucking some of the cooled air from the living area and sending it outdoors through the attic. The fan shuts off first because you are cooling the attic, and its thermostatic control responds.

A better approach is to add insulation in the attic to slow the radiated heat gain to the second floor. You can have several inches of cellulose blown in. You will need to build a dam around the access panel to keep the cellulose from falling down through the panel opening. This can be done with cardboard or wood.

You should weatherstrip the access panel and place a 2-inch-thick piece of rigid insulation over it. Use polyiso insulation to get the benefit of the reflectivity of its aluminum skin. You can either put it on top of the cellulose insulation if it comes up to the top of the dam, or fit it within the coffer of the access panel, holding it with small pieces of trim. If the cellulose does not come up to the top of the dam, add fiberglass batts around the dam to insulate its exposed sides and reduce the heat loss through them.

Q. I read with interest your recent article about attic fans. I am considering installing one. My house has east- and west-facing gable ends and a tight ceiling such that there are few, if any, convective paths. It has a ridge vent and soffit vents.

If a fan were installed in the attic wall near the gable peak, would it draw substantial air down through the ridge vent, especially in proximity to the fan, thus interfering with efficiently venting the whole attic? Or is this not a concern? Thanks for your excellent articles every week.

A. Convective paths can also come from electric wires and plumbing pipes in the walls, if the holes through which they run have not been foamed.

You have soffit and ridge vents -- a very efficient system. Why add a fan that draws electricity and messes up the system? The sun accelerates the air movement without cost to you.

More on the humming house: Three readers have emailed me about their experiences with humming in their houses. One of them found a bee nest and another found a yellow jacket nest in the walls of their homes.

That reminded me of a similar situation where a beekeeper was called in to remove a bee nest, after which honey oozed out of the base of the wall and created a mess.

The third reader wrote: "The problem was an ungrounded outlet. I had insulated the attic in the garage and in doing so, I had shoved a batt of insulation up and had disconnected an electrical conduit joining connector. What happened then is that without the ground to that outlet, the grounded conduit to the water meter started to vibrate/hum. Perplexed, I had shut off the main breaker and the noise still persisted. With a $3 outlet checking device that plugs in and shows if the outlet is wired correctly, I located the offending outlet and found the disconnected conduit in the attic. I wanted to pass this info on to you and your readers. Thanks for a great and informative column."

I am thankful to all three of you for offering possible solutions. I am passing them on to the reader who asked the original question.

• Henri de Marne was a remodeling contractor in Washington, D.C., for many years, and is now a consultant. Write to him in care of the Daily Herald, P.O. Box 280, Arlington Heights, IL 60006, or via email at His book, "About the House," is available at and in bookstores.

2012, United Feature Syndicate Inc.

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