Q: We have a leaky slate roof and plan to put on a new roof of asphalt shingles. Can the asphalt be put on top of the slate or must the slate be taken off first?

A: You will have to take the slate tiles off first because you cannot successfully drive nails through them for the new shingles.

Q: The wrought iron hinges on my kitchen cabinets have been painted over several times, as have the brass plates and handles on my closet doors. What is the best way to remove all the paint from this hardware?

A: The best way is to take off all the hardware and soak it in a pan or bucket containing enough liquid paint remover to cover them completely. After about five or six minutes lift each piece out (wear rubber gloves), rub with a stiff brush to loosen the old paint, then drop the pieces back into the solution for another couple of minutes. When all the paint is off, drop each piece into another container and flush with solvent as recommended on the can of remover. Wipe dry with clean cloths and return to cabinets and doors. (A note of caution: Some removers are highly flammable, but you can buy others that are not, so check the label before buying.)

Q: Two copper water lines in my garage under the kitchen branch off to other parts of the house. Both lines seem to have been corroded in spots with some kind of caustic acid. I'm afraid to scrape them clean because they may show porosity and start to leak. Can you tell what caused the discoloration, and how I can clean it off?

A: It is not at all unusual for copper water lines to discolor or develop oxidation. This does not harm and does not indicate a possible defect. It may be caused by something spilled on the pipes, or by exposure to pollution in the air (fumes from your automobile exhaust, for example). I would leave the pipes alone.

Q: I have an oil-fired boiler that supplies hot water for both my heating system and my regular domestic hot water needs. I see that the control box on the boiler has a high and a low setting, and I would like to know which is the most efficient for this control.

A: There is no one "most efficient" setting that will apply to all homes, or under all conditions. Generally, this is something you should talk over with your heating contractor. But as a rule of thumb you can set the lower limit to about 160 F for the winter and about 140 F for the summer (assuming you have a dishwasher. If you have no dishwasher, the summer setting can be about 20 degrees lower). In each case the upper limit (which is actually a safety) should be about 20 degrees higher.

Q: During a recent rainstorm I had a few inches of water in my basement, which has unfinished plywood panels on the walls. The water left stains along the playwood walls, up to six inches off the floor. Will varnish stain eliminate these stains, or can you recommend some other method to get rid of them?

A: If you pick one that is dark enough, varnish stain may cover the stains or hide them, but it will not get rid of them. This assumes, of course, that you intend to coat the entire wall with the same varnish stain. It may require a second coat on the lighter parts to make them as dark as the darker areas. To really get rid of the water stains (assuming they are darker) you will have to use wood bleach, then stain all the plywood to a uniform shade.