Carpentry Q&A Archives

2006 CARPENTRY Q&A ARCHIVES

Here are some helpful tips from Mark Feirer, the author of Carpentry & Building Construction.

Q What causes woodpeckers to create holes in wood siding?

A They may be looking for insects hiding underneath the siding, trying to establish territories and attract mates, or actively excavating to make a nesting cavity. Some ornithologists also speculate that the buzz made by electric wires or even the vibrations of a washer and dryer may fool birds into thinking that insects are inside house walls. In most areas, woodpeckers are considered a protected species of bird, so methods for their control should be locally appropriate. For more information on wood siding, see Chapter 32 of Glencoe’s Carpentry & Building Construction ©2005, by Mark Feirer.

Q What types of venting pipe are recommended for the vent fan over a stove?

A The best vent pipe is a smooth-walled metal duct pipe. The use of flexible plastic duct pipe is not recommended by vent-fan manufacturers and may be restricted by code. Plastic duct pipes increase turbulence, reduce airflow, and can be a safety hazard if a cooking fire occurs while the fan is on. A vent pipe should take the most direct route to the outdoors. Each elbow in the pipe adds an amount of airflow resistance equivalent to five to ten feet of straight pipe.

Q I have seen electrical receptacles installed with the grounding-hole above the slots, while others are installed with the grounding hole below the slots. Which installation is correct?

A Both are correct. Electrical codes do not specify the position of the receptacle, and in fact it could even be installed sideways. However, this is something that may change. Various proposals to require placing the ground prong up have been introduced over the years to code-making agencies. None have been approved yet. However, always check with local code officials regarding code questions. The National Electrical Code has been adopted nearly everywhere in the US, but local code officials can amend its provisions. It is therefore possible that some local codes do indeed require a "ground prong up" orientation. For more information on electrical systems, see Section 38.2 of Glencoe’s Carpentry & Building Construction ©2005, by Mark Feirer.

Q What can be done to prevent mildew and algae infestations from taking hold on an existing roof?

A The best prevention is to roof the house with algae-resistant shingles. To reduce infestations on an existing roof, slip a strip of zinc or copper flashing partway under the ridge cap along its whole length. Rainwater will wash zinc or copper ions off the strip of flashing and carry them down the roof, inhibiting further infestations of algae. There are several companies that market products made specifically for this purpose.

Q What is the best way to remove mildew and algae stains on a shingled roof?

A In order to remove the stains, it is necessary to kill the organisms that create them. This can be done using 1 ounce of ammonia-free detergent and 1 quart of household bleach diluted with 3 quarts of water. Do not mix ammonia and bleach! (The combination can produce dangerous reactions; for example, chlorine gas.) Spray the solution over the stained areas, let it sit for a few minutes, and then rinse the area with plenty of clean water. Bleach is the killer—the detergent just helps it penetrate. You may also have to scrub the roof gently to work the solution in, particularly if the staining is extensive. To avoid harming the shingles, use a telescoping window-washing brush with soft nylon bristles. To ensure safety, work from a ladder instead of crawling out on the wet, slippery roof.

Q I’ve heard of an air-conditioning system called a ductless mini-split system. How does it work?

A With the mini-split system, a small condenser outside the house is connected to an air handler/blower that fits entirely within a wall. Refrigerant lines connect the two and can be snaked through walls. The units are quieter and have greater cooling capacity than window-mounted air conditioners, and they can even be put on an interior wall.

Q Is it possible to level an uneven concrete floor?

A Yes. Concrete can be topped with a gypsum-based leveling compound. Mixed to the consistency of pancake batter and poured over a floor, the self-leveling compound hardens to a surface smooth enough to accept finish flooring.

Q I notice that even floors framed to current building codes sometimes vibrate a little when someone walks over them. Why does this happen?

A The span tables used to size floor joists are based on certain requirements for stiffness and strength. This applies to solid lumber joists as well as engineered joists. It's a lack of stiffness that causes a floor to vibrate, or flex, in response to a "live load" such as someone walking on the floor. Floor framing is not sized to completely eliminate flexing because that would be too expensive. Instead, building codes allow a floor to flex within a certain range that should be barely noticeable, if at all. However, it's important to understand that the stiffness of a floor is based on more than just the joists. The subfloor and sometimes even the finish floor also play a part. In addition, other live loads--the weight of people, furnishings, a pile of books, or anything else that's not part of the structure itself--can cause flexing, particularly if the load is concentrated near the center of the floor's span.

To stiffen floors, builders have several options. The joist spacing can be reduced or the joist depth can be increased. But usually the most effective way to stiffen the floor is to reduce the span of the joists.

Q What are the most common locations for roof leaks?

A Tracking down the source of a leak is always difficult because water often takes an indirect path between its entry point and the point where you actually notice it. The most common sources of trouble are around flashing, including the flashing collar around plumbing vents and the flashing around a chimney. The right combination of wind and water can get past flashing that's otherwise watertight. Other areas prone to leakage are the edges of any gable or dormer.

The difficulty of tracking the source of a roof leak is why some roofing contractors never reroof over existing shingles, even though building codes allow the practice. By stripping off the existing shingles, a contractor can inspect flashing and sheathing and replace it if necessary.

Q Is there an organization that rates the gas boilers for hot water heating systems? I would like to compare the energy efficiency of various boilers.

A The efficiency of any boiler is based on how well it converts fuel into heat. A system that is 60 percent efficient means it wastes at least 40 cents of every dollar you spend on fuel. The key efficiency figure to look for when shopping for a boiler is the AFUE rating. This is Annual Fuel Utilization Efficiency, and it's based on a standardized test developed by the American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (ASHRAE). It isn't hard to find boilers with an AFUE rating of 85 percent, and some condensing-type units reach AFUE ratings of over 90 percent. The Gas Appliance Manufacturers Association (GAMA) certifies the efficiency ratings of gas appliances, including boilers. Look for a system with pilot-less ignition, which eliminates the need for a constantly running pilot light--pilot lights burn up a surprising amount of fuel.

Q What causes gaps to open up in solid wood paneling after installation?

A Wood expands and contracts with changes in humidity. It's a particularly common phenomenon in new houses, because it can take as much as two years for a house to reach a sort of moisture equilibrium. The wider the board, the more it will move. People notice the movement most in the winter, when heating systems dry out the wood and cause it to shrink. But the reverse happens in the summer, when humidity causes the wood to expand. Painting the wood after installation, especially with a dark color, makes the problem more apparent. To minimize the movement of solid wood paneling, prime both faces and all edges before installation.

Q What is the best way to repair a nail-pop in drywall?

A Push on the drywall to hold it against the stud and then drive a drywall screw 1 1/2" above and another below the popped nail. Pull the nail out if the head is accessible, otherwise just drive back into place so that the head is below the surface of the drywall. Use joint compound to fill the depressions created by the screws as well as the damage created by the nail pop. Generally two coats are required. Allow the first to dry completely before applying the second. Sand the repaired area smooth with carbide-grit sandpaper or a carbide-grit sanding screen. The open weave of this material eliminates clogging.

Q Solid oak flooring boards often have grooves on the back side. Do these grooves have a purpose?

A The grooves beneath an oak flooring board are called a "hollow back" and they play several roles. Grooves reduce shipping weight, for one, and though it might not seem important when you consider one board, think of the weight saved in a truckload of flooring. The grooves also encourage air circulation beneath the floor that cuts down on trapped moisture. But perhaps the most practical role of grooves is the simplest: They show you which side goes down during installation. Each floor board is thicker above the tongue than below, so a floor that was accidentally installed upside down would wear through to the tongue faster than one installed properly. Two grooves is the standard configuration, but some boards might have three and some only one.

Q What type of house insulation is the most effective at blocking sound?

A The type of insulation called low-density polyurethane spray foam is best at blocking sound. This is partially due to the installation method. The material is sprayed into wall and ceiling cavities. This enables it to fill crevices that might turn into “flanking paths.” A flanking path is an air space between building materials that allows sound waves to enter the house unobstructed. Once sprayed onto a surface, polyurethane foam insulation expands dramatically and fills cracks and crevices better than other types of insulation. It cures into a dense blanket that soaks up sound.

Q The floor framing around a masonry chimney is often doubled up and spaced away from the masonry. What is the purpose of this technique?

A Maintaining a separation between wood floor framing and a masonry chimney reduces fire hazards. But there is another reason that is less obvious. In many old houses, floor framing was partly supported by the chimney. In fact, the end of a floor beam was sometimes housed in a pocket created in the side of the chimney. But when the chimney settled, as most do, it took the floor framing with it, causing the floors to sag towards the chimney. Modern building codes have eliminated this framing practice. A modern chimney is entirely separate from the rest of the house, and floor framing must be kept at least 2" away from it.

Q I’ve heard that some painters apply a wood preservative to bare wood siding before painting. Is this a good idea?

A Testing has shown that coating bare wood with a water-repellent preservative reduces the wood's tendency to shrink and swell. The more stable a surface, the better it holds paint. There are water-based and solvent-based repellents that are equally effective. The important thing is to make sure the repellent is labeled as "paintable." If you have any doubts about the product, don't use it. Just remember that a preservative isn't an alternative to a primer. As soon as possible after the preservative is dry, prime all edges and surfaces (ends, front and back) with a high quality all-purpose primer.

Q Does a ceiling fan help to keep people cool in humid weather?

A Yes, but it doesn’t actually change the humidity in a house. Its job is to move air over your skin--that's what makes you feel more comfortable when the weather is humid.

Q What is the proper pitch for rain gutters?

A Gutters should pitch at least 1" every 16' of length. One way to determine this is to use a 4' level for the job. Establish a level line 4' long on the fascia, then draw a mark ¼" below it at one end. This is the line your gutters should follow.

Q What causes drafts near windows during cold weather?

A Windows are a common source of drafts. Drafts are often caused by cold air patterns started by cold window surfaces. Air next to the window is cooled and drops to the floor. It is then replaced by warmer air from the ceiling, which in turn is cooled. This sets up an air movement pattern that feels drafty. Double- or triple-glazed windows reduce the problem by increasing interior glass surface temperatures. Drafts can also be caused by leaky windows due to faulty or damaged weather stripping.

Q What are the advantages and disadvantages of including a humidification system when installing a forced-air furnace?

A Humidity--the amount of moisture in the air--has a huge impact on comfort. When you heat and cool the air inside a house, you change humidity levels as well as temperature, and if the changes are extreme, you'll feel uncomfortable. In a forced-air furnace, cool air is heated as it passes through a heat-exchanger where temperatures can reach 500°F. After the furnace has been operating for a few weeks, it's no wonder that the air feels bone dry. When air is unusually dry, people often complain about dry skin or an increase in respiratory or sinus problems. The solution is to replace some of the moisture baked out by the furnace. Some moisture comes from cooking vapors and shower steam but usually isn’t enough. That's when a humidifier comes in handy. Portable units are available to humidify individual rooms, but if the whole house needs humidification you'll have to attach a humidifier to the hot-air supply duct where it exits the furnace. The water supply comes from a flexible copper water tube tapped into the nearest cold-water pipe. Furnace-based humidifiers work well, but they can turn into a maintenance headache if the water supply contains lots of minerals. When the water vaporizes, minerals are left behind, and the unit eventually cakes up and stops working.

Q How can wallpaper be removed from plaster walls?

A If the walls are in good shape, here’s how to proceed. To remove wallpaper that can't be dry-stripped, apply a diluted wallpaper stripper solution to the surfaces using a pump spray tank. While the surfaces are still wet, scrape off the paper using a 3" or 4" wallpaper shaver blade, removing as much paste residue as possible. When the paper is completely removed, spray the walls again with the stripper solution, then scrape the surfaces with a broad knife and immediately wipe down the walls with a large sponge. Rinse the sponge frequently in clean water and keep wiping until there's no glistening residue of the paste. After completing all this, wait at least 24 to 48 hours for the walls to dry before painting.

Q What causes nails to “pop” in drywall surfaces?

A Nails often pop due to lumber shrinkage. This creates a space between the drywall and framing, and allows nails to push out through the surface of the drywall, or “pop,” if any pressure is put on the drywall. Ring-shank drywall nails hold better than standard drywall nails, but drywall screws have the best holding power.

Q What causes the surface of brick to flake off in small chips?

A This problem is called “spalling” and it is often caused by moisture that is absorbed into porous brick. When the moisture freezes, it expands and can pop off portions of the brick surface, leaving little craters behind. A liquid sealant is sometimes applied to the brick to block moisture, but it doesn’t always solve the problem. Also, it may have to be reapplied every few years.

Q What is a saltbox style house?

A The saltbox style was popular in colonial New England. The front of a saltbox house is two stories tall, while the back is only one story. The style evolved from Colonial architecture. When adding on to a standard two-story Colonial house, it was common to add a single-story addition to the back, sheltered by an extension of the main roof. This long, sloping roof that covers the back of the house is the most noticeable feature of the style. The classic saltbox features a large, central chimney.

Q Codes are beginning to recognize the importance of supplying outside combustion air to fireplaces, and it may be required when building new fireplaces. Can the ash dump on the floor of a fireplace be used as part of the system to supply outside combustion air?

A Probably not. Codes typically require an ash dump and ash cleanout on fireplace construction. If you use the ash dump for supplying combustion air, it's not an ash dump anymore and that puts you at odds with the code. In other words, in the eyes of the code, an ash dump can't serve two purposes. However, codes are continually changing, so check with your local building official before a mason starts to build the fireplace and chimney.

Q There is a large white pine tree in the middle of the lot we wish to build on. Is there enough value in old trees for a mill to want them? Selling the wood might help to offset the cost of clearing the site.

A Eastern white pine (Pinus strobus) is dimensionally stable, straight grained, and easily worked. A local lumber mill might remove the tree in return for getting the wood. Another option is to find a local tree service with a portable mill that would mill the tree into boards on-site. However, the boards would have to be stacked and stickered for quite a while before they could be used. The rule of thumb for air drying lumber is to season it a year for every inch of thickness. If a tree does not have any value as lumber, it might at least be suitable as firewood.

Q Mortise locksets must be slipped into a deep, long slot in the edge of a door. How do finish carpenters make this slot?

A The traditional method is to use a hand brace and an auger bit, but modern carpenters sometimes use an electric drill with an auger bit. The technique calls for setting the drill to a low speed and then drilling several adjacent holes in the edge of the door. The remaining wood must be chiseled out to create a large, rectangular slot (the mortise). The case of the lockset fits into the mortise. This is time consuming work that calls for patience and care. The most difficult part of the job is making sure the holes are exactly parallel to the face of the door.

The same job can be done much more quickly and accurately using a portable tool called a lock mortiser. This is an adjustable metal jig that holds a router on the edge of the door. As the router is gradually lowered, an extra-long carbide-tipped bit cuts the mortise. Some cleanup with a sharp chisel is still required, but not much. The work goes quickly and the mortise will be perfectly centered. This tool is sometimes available at tool rental yards. Porter-Cable is one company that manufactures lock mortisers. Their tool cuts mortises up to 7" long, 1-1/4 " wide, and 4-3/4" deep.

Q The grout on a recently completed ceramic tile countertop has high spots. Does the countertop have to be retiled to correct this?

A The countertop does not require removal. The grout on countertops is usually unsanded grout, a finer-grained material than the sanded grout that's used on floors. You may be able to remove the high spots using a moistened nylon scouring pad. If that doesn’t work, try very fine steel wool. It will take down the grout without scratching the glaze on surrounding tile. Wear gloves to protect your hands from the steel wool.

Q What is the difference between a brick veneer wall and a brick cavity wall?

A In a brick veneer wall, the brick is like siding: It isn't load bearing, and there's a standard wood-framed wall behind it. After the framed wall is sheathed, bricks are laid in a single layer outside. Metal ties embedded in the mortar and nailed to the sheathing connect the two walls, leaving a 1" gap between them for ventilation. The framed wall is easily insulated using standard methods.

On the other hand, a brick cavity wall is actually two parallel walls of brick, called wythes, that are separated by an air space anywhere from 2" to 4-1/2" wide. Sometimes the wall is insulated during construction, sometimes it's not. The indoor face of a cavity wall is often plastered.

Q In order to reduce sound transmission between rooms, why aren’t interior walls insulated?

A This involves extra expense and complicates the construction process. The most effective insulation for residential sound reduction is called low-density polyurethane spray foam. It is sprayed into wall cavities, where it expands. Spray foam insulation dramatically reduces sound transmission. However, installation on interior walls must be coordinated with the drywall contractor. This is because the drywall on one side of a wall must be in place before the insulation can be installed. Then the excess insulation must be trimmed off before the other side of the wall can be drywalled. The electrical contractor and plumbing contractor should also be consulted early in the planning process.

Q What are the benefits and disadvantages of installing an indoor vent on a clothes dryer?

A Dryers create a lot of heat, and it might seem wasteful to dump it outdoors when heating costs are so high. But remember what else the dryer exhausts: a great deal of moisture. Older houses can tolerate a bit of this excess moisture because they weren't sealed very well. However, modern houses are sealed tightly to reduce energy costs. The amount of moisture kicked out by drying loads of laundry can easily overwhelm the ability of a house to handle it. This is why building codes typically require that dryers be vented outdoors in new construction.

For more information on ventilation, please see Chapter 39 of Glencoe’s Carpentry and Building Construction © 2005 by Mark Feirer.

Q How can I make curved hand railings?

A If a curved a piece cannot be cut from solid wood, the two main methods of creating curved parts are steam bending and laminating. Steam bending is time consuming to set up and the process can be difficult to control. This is why laminating is generally the preferred method.

Laminating wood is a matter of layering thin, pliable wood strips around a curved form. As each layer is glued to the previous layer, it is clamped it to the form. It is common for laminations to have eight or more layers. When the glue cures, the clamps can be removed. The resulting piece will maintain the same curve as the form. Once the excess glue has been scraped off, the piece can be machined to its final shape. Cedar bends more easily than many other woods and is easy to cut into strips for lamination. The strips should be no thinner than necessary for bending around the form. For example, a large-diameter curve might be assembled using 1/4" thick layers, but a smaller-diameter curve might require layers that are only 1/8" thick.

For more information on railings, please see Chapter 45 of Glencoe’s Carpentry and Building Construction © 2005 by Mark Feirer.

Q What is whitewash?

A Whitewash is a low-build (thin) finish designed more for making a surface look clean than for protecting it. It is easy to put on but requires frequent reapplication. Traditional whitewash has a calcium carbonate (ground limestone) base. Through a series of processes, it is transformed into a paste of hydrated (or slaked) lime. When the lime is mixed with water and brushed on a surface, it combines with carbon dioxide in the atmosphere and crystallizes into small interlocking particles similar to the original ground limestone. So in a sense, whitewash is essentially a thinned masonry material. It's not very durable, but it is very inexpensive. Whitewash was once quite popular for painting fences and farm structures. However, its ingredients are less widely available than they once were, so whitewash has largely been replaced by paint.

For more information on finishes, please see Chapter 41 of Glencoe’s Carpentry and Building Construction © 2005 by Mark Feirer.

Q What is the advantage of a technique called “stack” framing?

A This technique is energy and structurally efficient. Here’s how it works. If you vertically align the load-bearing parts of a house frame (rafters, joists, and studs), weight will be transferred in a straight line from roof to foundation. This contrasts with the somewhat indirect path typical of standard frame construction. As a result, an exterior wall with stack framing doesn’t need as many studs, and the stud spacing can be increased from 16" on center to 24" on center (though the studs should be 2x6 instead of 2x4). Also, the walls can be topped with a single plate instead of a double plate. Building codes generally allow stack framing, though you should check to make sure it's approved in your area.

There are also some disadvantages to the technique. You may have to increase the thickness of the wall sheathing because of the greater distance between studs. Indoors, you might notice that the wall surface doesn't seem quite as solid, even though building codes may allow standard ½" drywall to be installed over studs spaced 24" on center. The use of 5/8" drywall is one solution to this problem.

For more information on framing, please see Chapter 19 of Glencoe’s Carpentry and Building Construction © 2005 by Mark Feirer.

Q What is the best roofing material for a flat roof?

A Two types of products are typically used to cover a flat roof. A tar-and-gravel roof, called a built-up roof (BUR) is the traditional option. It consists of layers of roofing felt stuck to each other with hot-mopped layers of melted asphalt. After the last layer of asphalt is mopped on, ballast (pea gravel) is spread over the roof to protect the asphalt from deterioration caused by UV radiation.

A newer way to cover flat roofs is called single-ply roofing or membrane roofing. It consists of a single layer of EPDM (ethylene propylene diene monomer) or CSPE (chlorosulphonated polyethylene). Single-ply roofs are common in commercial construction, where flat roofs are often used on large, low-rise buildings. The material comes in thicknesses of 45 mil (1.1mm), 60 mil (1.5mm), and 90 mil (2.3mm), and in rolls at least 10' wide. It can be fastened to the roof sheathing in various ways, but in residential work it's often fully adhered to the sheathing with a type of contact cement. Installing EPDM calls for meticulous craftsmanship in order to create a continuous, flexible membrane that won't leak.

For further information on built-up roofing, please see Chapter 30, page 598, of Glencoe’s Carpentry and Building Construction © 2005 by Mark Feirer.

For more information:
NRCA Roofing and Waterproofing Manual, National Roofing Contractors Association

Single-Ply Roofing Institute

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