Quality Construction Wood Goose Creek SC
Remodeler, Specialty Contractor
Custom Builder, Remodeler
Bonded Builders Warranty Group
Mt. Pleasant, SC
Designer / Architect, Remodeler
American Society of Interior Designers, Builder 20 Club, Charleston Trident Home Builders Association, NAHB Certified Graduate Remodeler, NAHB Certified Green Professional, National Association of Home Builders
2008 Guildmaster with Highest Distinction, 2009 Guildmaster with Distinction, 2010 Guildmaster with Highest Distinction
Mt Pleasant, SC
Custom Builder, Remodeler
Builder 20 Club, Charleston Trident Home Builders Association, Custom Builders USA, Earthcraft House, EnergyStar, I'On Guild: Mount Pleasant, SC, National Association of Home Builders, PRISM Awards, The Leadership in Energy and Environmental Design, U.S. Green Building Council
Goose Creek, SC
North Charleston, SC
Custom Builder, Remodeler
Charleston Trident Home Builders Association, NAHB Certified Green Professional, National Association of Home Builders
2007 CotY Award, 2008 CotY Awards, Better Business Bureau, Charleston Trident Home Builders Association, EPA Lead-Safe Certified Firm, NAHB Certified Green Professional, NAHB Remodelers, National Association of the Remodeling Industry, National Remodelors Council, Remodelign Magazine - Remodeling 550
Mt Pleasant, SC
Custom Builder, Remodeler
Charleston Metro Chamber of Commerce, Charleston Trident Home Builders Association, Custom Builders USA, Daniel Island Associates, NAHB Certified Graduate Builder, National Association of Home Builders
Goose Creek, SC
Goose Creek, SC
The Goods on Wood
Wood is never still. Unlike steel, aluminum, plastic, stone, or brick, wood moves. It is alive, it glows, and, if you care for it, wood will shelter you with maintenance-free strength, wood will warm you, wood will please your eye, and wood will wrap you in its immortal strength when you go to your grave. Nature has provided us with no better material." - John N. Cole, Breaking New Ground I agree. In fact, I think the ambience of wood should be added to the architectural patterns discussed in the previous installment. I have been trying to sort out my feelings about vinyl and aluminum sidings for years.
From purely practical and economic regards, both are superior to wood clapboards or shingles. They cost less to install, align more perfectly and eliminate painting. So why do siding manufacturers try to make them look like real wood? It is, I believe, because an affinity for natural wood is locked into our chromosomes, along with the appreciation of fire. I have nothing against vinyl or aluminum per se. I love plastic plumbing, and I'd rather fry an egg in an aluminum skillet than one crafted from wood. But there is something strange about vinyl siding. I think it's the same thing that bothers me about nondairy coffee "lighteners" and "genuine vinyl leather." It is simply dishonest; it erodes the distinction between the natural and the artificial. It numbs our awareness that we are, after all is said and done, a part of the natural world. For these reasons I believe it is important that the visible parts of a house be constructed of natural materials. In this installment, we will consider the qualities, strengths and weaknesses of nature's primary building material, wood. The Tree Let's begin at the source of all wood - a tree.
The outer bark is a thick layer of dead cells, similar to the outer layers of human skin, that protects the living parts of the tree from insects and fire. A tree is very resistant to insects as long as its bark forms a complete barrier. The inner bark consists of live cells that transmit nutrients, as do the cells of the sapwood. The cambium is a single layer of cells where, remarkably, all tree growth occurs. The cells of the cambium continually divide, first adding a cell to the inner bark outside and then a cell to the sapwood inside. Except for the very first shoot, all growth is, therefore, radial. A limb that first appears at a height of 5 feet will remain at 5 feet forever, although the tree grows taller and the limb longer. The sapwood consists of the most recently formed layers of wood and, as its name implies, carries sap up and down the tree.
The rate at which cells divide is a function of moisture, temperature and length of day. It therefore varies throughout the year, producing growth rings. Wide rings are due to rapid growth in wet summers; narrow rings indicate dry summers. These ring patterns form fingerprints in time that have been used to date historic buildings! Over time sapwood cells die and become heartwood. Chemicals and minerals deposited in and between the inactive cells make heartwood stronger, darker, denser and more resistant to decay than sapwood. The pith, at the very center of the tree, is the remnant of the original shoot. Rays form at right angles to the growth rings. Not defects or cracks in the wood, as they may at first appear, rays are bundles of cells that transport food across the annual rings. Illustration 1 also shows how this tree recovered from damage to its bark (perhaps from fire) by eventually enclosing the unprotected wound with new sapwood. Species Properties Wood is most often lumped into one of two categories: hardwood or softwood. If you have ever driven nails into poplar (hardwood) and Southern pine (softwood), you will agree these two categories are misleading. Better classifications are deciduous (dropping their leaves) and evergreen (retaining their leaves or needles all year). The two types have radically different cell structures, which, coupled with variations in the thickness of the cell walls and the relative proportions of the molecular building blocks, cellulose and lignin, account for most of the differences in their properties. Heavier or denser species are harder, stronger and stiffer, and they hold nails better.
Generalities beyond these are unreliable. Table 2 rates the most common wood species in eight building-related characteristics: Strength in bending is the limiting factor for roof rafters: We care whether a roof collapses under a snow load, but not how much it bounces when walked on. Stiffness is most often the limiting factor for floor joists. Floors joists are usually designed to deflect no more than 1 inch across a span of 360 inches (30 feet) under maximum load. The criterion is the ratio 1-to-360, however, not the size of the deflection. Hardness is important where a lot of traffic is expected. Few woods make a prettier floor than white pine, but white pine is so soft that varnish-like finishes fail when the wood dents under pressure.
At the other extreme is rock maple, a favorite for bowling alleys and gymnasium floors. Nail holding is important in a building's structural frame. Examples are corner braces in a wall, joints and splices in a roof truss and stair treads.
Generally, a wood low in nail-holding ability is also unlikely to split when nailed. Therefore, low nail-holding ability can be overcome by more nails. Warping is unattractive in the exposed rafters of a cathedral ceiling, dangerous in a heavily loaded slender post, and results in uneven floors, walls and ceilings. Decay resistance is important for wood exposed to standing water or constantly moist conditions. Most naturally decay-resistant species are in short supply and are expensive. As a practical matter, pressure-treated Southern pine is the wood to use in moist conditions. Pitch content. Other than an annoying stickiness while building, the problem with pitch is its insistence on bleeding through paint. Minor amounts of pitch can be overcome by sealing with several coats of a white-pigmented shellac sealer/primer.
Paint holding is important only if you plan to paint, and then only if it's to be outside. Wood that ranks low in paint-holding ability can otherwise be treated with a penetrating stain or preservative. Lumber Lumber, a generic term that includes the more specific categories of boards, joists, planks, rafters, posts, etc., is made by sawing tree trunks lengthwise. The most skilled job at the sawmill is that of the sawyer, who turns a log into the optimum mix of lumber. The sawyer must make an instant judgment at each pass of the blade, weighing the defects uncovered by the latest pass against the orders of the mill customers and the relative value of each lumber category. Each time, the sawyer can either repeat the previous pass, change the thickness or rotate the log by 90°. With the exception of specialty items, most lumber is sawn in increments of 1 inch. If you were to measure a just-sawn piece of lumber, it would measure exactly X inches by Y inches. Two further things usually happen to lumber before being cut and nailed into place: It is dried to a stable moisture content, and it is surfaced smooth (planed) to a smaller dimension. Sometimes the order of operations is reversed: surfacing, then drying. In either case, the desired result is the same - lumber of specified dimensions that will neither swell nor shrink appreciably from its installed dimensions. Nothing is lost in the length of the piece, however, since the ends are not planed and wood shrinks negligibly in its long dimension in drying. Thus a 10-foot S4S (surfaced on four sides) 2x8 floor joist actually measures 120 inches by 1 1/2 inches by 7 1/4 inches. Obviously, working with smooth lumber of precise dimensions is easier for the carpenter. It results, in addition, in a more finished appearance. But something is also lost in the process. As a rule of thumb, planing rough lumber decreases its bending strength by one size; i.e., a rough 2x6 has nearly the same strength as a surfaced 2x8. In fact, merely sawing a round log into its maximum square section reduces its bending strength by a full 50 percent! Drying and Shrinkage When a tree is harvested, the wood is referred to as green. The moisture content (MC) of wood is defined as the ratio of the weight of water contained in the wood to the weight of the wood when it is oven-dry, or contains absolutely no water. For example, if a 2x6 weighs 14 pounds before and 10 pounds after drying in an oven, then the original MC was 40 percent. As wood dries, water evaporates first from inside the cells. Only after the cells are empty does the water in the walls of the cells evaporate and the wood begin to shrink. The MC at this transitional fiber-saturation point is about 30 percent. As the MC continues to fall, the wood shrinks by about 1/30 of total shrinkage for each 1 percent change.
The converse is true when dry wood picks up water and swells. Lumber is considered "dry" by the lumber industry when its MC is 19 percent or less, although the final MC of wood in a home typically ranges from 4 to 16 percent, depending on climate and season. There are two problems with wood shrinkage. The most obvious is that the wood gets smaller overall, like a pair of un-Sanforized jeans the first time you wash them. This results in cracks between adjacent pieces, as in a floor, or nail heads popping through drywall, as the framing shrinks away from the nails. The second problem is a distortion of the lumber's cross section due to differential shrinkage. For a reason unknown to me, lumber shrinks about twice as much tangentially to the circular rings as radially. (It shrinks hardly at all in the direction of the grain.) The greater the ratio of tangential to radial shrinkage, the greater the distortion with change in the MC. A board sawed parallel to the rings is termed flat-grained or plain-sawed. A board sawed at an angle greater than 45° to the rings is called edge-grained or quarter-sawed. As Illustration 3 shows, edge-grained boards shrink less in width and, more importantly, cup less. Wood clapboards should always be edge-grained (ideally, radial-sawed). Otherwise they will alternately cup and uncup and eventually pull their nails. The lesson to be learned from Table 3 and Illustration 3 is that lumber should be acclimated (allowed to reach its final MC) in the home before installation. Otherwise, unsightly cracks will appear in wood flooring and finish, and nails will pop from drywall surfaces. Unfortunately, this is a difficult rule to follow in new construction. Framing lumber will have an MC of 19 percent if transported and stored at the lumberyard under cover, but as high as 30 percent if exposed to the elements. Worse, a new concrete foundation will give off thousands of pounds of water during its first year. The best you can do is leave the framing exposed for at least a few weeks before enclosing it in a vapor barrier and leave installation of wood floors to the last. Defects in Lumber Even with the best of care, lumber will still have a variety of defects. Some are cosmetic; others may be serious, depending on the application.
Check is the lumber version of a stretch mark, a cosmetic surface crack caused by the surface of a timber shrinking more rapidly than the interior. This can be dramatic and unsightly in a timber frame that has dried too quickly. Solutions include air drying the timbers a full year before use in building, or treating the surfaced timbers with linseed oil to retard the drying process. Splits, as the name implies, pass clear through the wood and are sometimes due to rough handling. They constitute a serious structural weakness. Timbers with splits should not be used in applications that need to withstand bending (floor joists, roof rafters) or as posts in compression. Shake is the separation of growth rings. Lumber with visible shake should not be used to support bending loads, since the zone of weakness probably extends the length of the piece. Use such wood only in nonbending applications. Cross grain occurs when a board is sawn from a crooked log. Since wood is 10 times stronger along its grain than across, a cross-grain angle of more than one part in 10 seriously weakens the wood in bending. Knots are the high-density "roots" of the limbs. They may actually add to the beauty of a board, but one has to be careful in structural applications. Knots are very tough but not well connected to the surrounding wood.
The rules for use in joists and rafters are:
(1) Tight knots are okay in the top third,
(2) loose or even missing knots are allowed in the middle third, and
(3) no knots at all over 1 inch in diameter are allowed in the bottom third. The solution is sometimes as simple turning the piece to reverse top and bottom edges. Rot results in holes and spongy areas. This is obviously structurally forbidden. However, such boards look just dandy as paneling. Just make sure the bug hotel has been vacated. Warp is cosmetic in a board and can sometimes be overcome by lots of nails. It can be serious in framing, however, if it causes an irregularity in spacing or a bulge in the plane of a floor, wall or roof. Sometimes two warped framing members can be saved by forcing them into alignment and nailing them together. Otherwise, cut the offending piece into shorter pieces so that the warp will be less noticeable.
Wane results from a miscalculation on the part of the sawyer. There simply wasn't enough tree to complete the board. The only real problem, in the case of a joist or rafter, is the lack of a full-width nailing surface on the edge of the board. Placing the bottom side down or making sure no plywood edges or board ends meet at the wane are good salvations. As a last resort, finish the sawyer's job on a table saw. n