Hitting a rough patch with plaster walls

Q. I took off the wallpaper in my turn-of-the-century house to reveal real plaster, not horsehair or any other kind. It has never been painted, but it is a little rough. I plan to paint the walls. When we filled several holes with joint compound and sand smooth, the patch finish was smoother than the rough plaster, and stuck out glaringly, even when it was painted. How can we get the patched material to match the rough walls?

PAM GOUGIAN, Beverly

A. As a start, check to see why that plaster is rough. It may have a residue of glue size, which has a rough feel. It should be washed off before painting. If it smoothes off with washing, the patches will match the original plaster, and you can paint.

If there is no glue size, then the plaster might have its own roughness, and when you patch, you can add sand to the patching material to try to make a match. Or, as the patch begins to set, you can float it, meaning you can rub it with a wood float. This gives a natural rough surface that might match the rough plaster. A wood float is a trowel made of wood or hard foam plastic.

If none of this works, you can apply a wall covering that is like a white, light canvas with a barely rough finish that will cover everything. If you plan to paint the walls white, you may not have to because the white wall covering is a finish all by itself. You can paint it in the future if you want to change colors.

Q. I enclosed the breezeway on my 1950 Cape-style house and put sheet vinyl on the plywood floor. Can I put hardwood on the floor, which has a very small crawl space under it? The flooring man said I do not have to put a half-inch expansion joint on the sides because the area (10 by 11 feet ) is small enough not to need one. There is barn-board wainscoting on the walls with no baseboards.

LORETTA FISH, Hingham

A. You certainly can, and it is essential to put a half-inch expansion joint on the borders where the boards are parallel to the walls, to allow expanding board to have a space to expand into. You can put the hardwood right over the vinyl, which is a vapor barrier to keep out moisture from that crawl space, an all-around good thing. And you can easily cover the expansion joint with an oak shoe mold, or a simple quarter round molding. Nail the molding to the wall, not the floor.

Q. I changed my roof four years ago and also put a new roof on a shed. They are fine, clear as anything. But a small pool-pump shed roof is full of light green stuff, some in the shape of silver dollars. How can I get rid of it?

BOB DOLAN, Canton

A. To answer your question, the light green stuff is lichen, and you cannot scrape it off asphalt shingles without scraping off the gravel on the shingles, so don't try. With such a small roof, you could replace the shingles easily and inexpensively. They are probably in shade, which is the best environment for lichen to grow on.

Q. Water is pouring over a retaining wall I built to hold back my neighbor's earth. It is coming from my neighbor's downspouts, and it is coming in torrents. She refuses to divert the downspouts or do anything else to alleviate this flooding. What can I do? Would a trench at the bottom of the wall, where the water ends up, be sufficient to divert the water downhill and to the street or elsewhere safe?

JIM SMITH, Wellesley

A. I think a trench, or even a little wall a foot or so away from the wall that would form a sort of trench, will divert the water. Or (heh, heh!) build a stout brick wall on top of your retaining wall, to let the water build up on the neighbor's side. That is tricky because it might not work, or your neighbor won't care.

Q. We put in a new bath in 1987 in our 1840s Greek Revival house. Recently we discovered bubbles in the paint on the ceiling of a first-floor room, right under where the upstairs toilet is. A plumber said he would have to cut into the ceiling to determine where a leak is. Is there anything we can do to avoid that, such as lifting the toilet and checking the drains from above?

HANDYMAN'S FORMER STUDENT

A. Lifting the toilet might reveal a possible drain leak, especially if a searching tool can be inserted. But I suggest you wait a while. Drain leaks occur only when water drains down the pipes, not constantly like a water leak. And if the ceiling does not show a water mark (with brownish edges), the leak may be very minor, and may not get any bigger.

I suggest you sand the bubbling paint and repaint the ceiling with one or two thin coats of a latex ceiling paint. If the bubbling does not reappear, you can relax.

And here is a funny idea as to why the bubbling occurred: The bathroom drain pipe is close to the ceiling, and is rather cold; this cold was transferred to the ceiling where the cold spot caused water vapor in the room below to condense, creating the bubbling. Or, water vapor condensed on the cold pipe and water dripped through the ceiling, causing the bubbling.

Preposterous? Maybe, but in old houses, new houses, handyman's specials, jury-rigged plumbing, and with hundreds of weird things called into the Handyman, I think anything is possible.

Q. My bathroom on a concrete slab has a sunken bathtub. I am redoing the bathroom, replacing the sunken tub with a large tub on the slab. What can I fill in the tub hole with?

FILLER STUFF

A. After rearranging the rough plumbing so that it can handle an above-grade tub, fill in the hole with sand or crushed stone. Fill halfway and tamp thoroughly. If you use sand, wet it to help the tamping process.

Add sand or stone to within 6 inches of the top of the existing slab. Pour in 6 inches of concrete; it will not hurt to put in a few rebars - steel reinforcing rods. You could cut grooves on the edges of the original slab, deep enough to drop in the rebars to span the opening. then when you pour the concrete, the rebars will help tie in the new slab with the old.

Q. You mentioned recently never to put a gas furnace in the attic, and I agree that it is not a good idea. But would a gas space heater be OK in the loft of a condo?

ELLIOTT MYERS, Sharon

A.Yes, because it is in the living space. But make sure it is vented to a chimney or is power vented through a wall. The venting is necessary for two reasons: One, obviously is to keep burning fumes out of the living space. The other is to keep water vapor created by the burning gas out of the living space, venting it through the power vent or up the chimney.

And one more caution: Even a small space heater can warm that loft very quickly and thoroughly, so think twice about it and see if you can stand the cool in the loft.

Another caution: Do not use a ventless gas heater. That is like building a fire in your house. These units, which are actually approved in Massachusetts and are designed for supplemental, not primary heat, have a fail safe oxygen detector that turns the unit off if oxygen levels fall too low. Too low for what: life?

Globe Handyman on Call Peter Hotton is also in the Styles Section on Thursdays. He is available 1-6 p.m. Tuesdays to answer questions on house repair. Call 617-929-2930. Hotton also chats online about house matters 2-3 p.m. Thursdays. To participate, go to Boston.com Hotton's e-mail is photton@globe.com.