Knock on Wood

Get the nitty-gritty on wood species, their char­ac­ter­is­tics and how trees re­act to be­com­ing part of your log home.

Log Home Living - - 2018 AN­NUAL BUYER'S GUIDE -

More than two dozen species of wood are com­monly used to build log homes in North Amer­ica. Al­most all are soft­woods: ev­er­greens such as pine, cedar, fir, cy­press and spruce, though some hard­woods, like oak, are also used. Though each log home pro­ducer fa­vors cer­tain species, the suc­cess­ful use of so many va­ri­eties is a clear in­di­ca­tion that there’s not one type of tree that makes a bet­ter log home than an­other. In­stead, the choice rests in the type of wood your log home com­pany and you pre­fer. De­spite their dif­fer­ences, these var­i­ous woods share cer­tain char­ac­ter­is­tics.

WOOD SCI­ENCE

Tech­ni­cally, wood is the hard, fi­brous sub­stance be­neath the bark of a tree. It owes its char­ac­ter to the hol­low, spin­dle-shaped cells that con­sti­tute it. These cells are ar­ranged par­al­lel to each other along the tree trunk, and this ar­range­ment af­fects some prop­er­ties of the wood, no­tably strength and shrink­age. The wood’s fi­brous na­ture in­flu­ences how it is used.

Trees grow by adding new wood. Wood that has al­ready been formed does not con­tinue to grow, but each year, a new layer of wood, called an an­nual growth ring, is added. The por­tion of the ring formed in the spring is light in color and is called ear­ly­wood. The por­tion formed later in the grow­ing sea­son is darker and is called late­wood. Late­wood is gen­er­ally denser and stronger than ear­ly­wood.

The wood formed just in­side the bark is known as sap­wood. De­pend­ing on the size and species of the tree, sap­wood can mea­sure one to three inches be­neath the bark. As a rule, the more vig­or­ously grow­ing tree species have wider sap­wood lay­ers. Sec­ond-growth trees of mar­ketable size con­sist mainly of sap­wood.

Sap­wood con­tains mostly liv­ing cells that carry sap, the tree’s food, from the roots to the leaves. It’s

not durable, and if ex­posed to mois­ture and other fac­tors, it can de­cay. But in terms of log home con­struc­tion, sap­wood usu­ally ab­sorbs preser­va­tives read­ily, so when wood is im­preg­nated with a good wood preser­va­tive, the pres­ence of sap­wood can be an ad­van­tage. If thor­oughly treated, sap­wood will usu­ally be at least as de­cay re­sis­tant as the treated heart­wood, maybe even more so.

In­side the sap­wood is the heart­wood. Heart­wood con­sists of in­ac­tive wood cells that have been changed slightly, both chem­i­cally and phys­i­cally, so that they no longer con­duct sap. Heart­wood is usu­ally more de­cay re­sis­tant than sap­wood.

PHYS­I­CAL TRAITS

Wood has sev­eral dis­tin­guish­ing qual­i­ties that af­fect its use for log home con­struc­tion.

Grain usu­ally refers to the log’s an­nual growth rings or to the ar­range­ment of the wood fibers. An­nual rings are said to have ei­ther a fine or a coarse grain. Close-grained wood, such as pon­derosa pine, has nar­row, in­con­spic­u­ous an­nual growth rings and closely spaced pores. In con­trast, coarsegrained wood, such as south­ern yel­low pine, has wide, con­spic­u­ous an­nual growth rings.

Tex­ture, used syn­ony­mously with grain, usu­ally refers to the size, ap­pear­ance and qual­ity of fibers in the wood. The ear­ly­wood (cells grown in spring and sum­mer) in coarse-grained wood is light in color and soft in tex­ture. Late­wood (cells grown in fall and win­ter) is much darker and harder.

The ar­range­ment of a log’s fibers, re­ferred to as ei­ther straight or spi­ral grain, can af­fect the log’s structural prop­er­ties. Some ar­range­ments are con­sid­ered more de­sir­able for build­ing than oth­ers. A straight grain, where the fibers run par­al­lel to the length of the wood, can be highly de­sir­able. This wood tends to re­main straight while it is dry­ing. A spi­ral grain, where fibers swirl around the tree trunk, is less de­sir­able be­cause cut tim­ber tends to twist as it dries. A tree may form a spi­ral grain as it grows and seeks sun­light.

Wood’s strength de­rives from its cel­lu­lar makeup. Its cells ( hol­low, cel­lu­lose blocks) are bonded by a sub­stance called lignin. The re­sult­ing ma­te­rial is stronger, pound for pound, than steel. Lignin also tends to be an elas­tic ma­te­rial that adds to wood’s re­siliency.

For some wood used in log homes — such as rafters, beams and posts — strength, re­siliency, hard­ness and shock re­sis­tance are im­por­tant con­sid­er­a­tions. But for logs stacked in the walls, strength and re­siliency aren’t as crit­i­cal, as one sin­gle log won’t carry the load alone. Log home pro­duc­ers re­fer to en­gi­neer­ing cal­cu­la­tions to de­ter­mine the proper size or species of wood for each job.

Wood’s weight varies greatly from the time it’s first cut and sat­u­rated with wa­ter un­til it is dry. Wood’s weight is an im­por­tant con­sid­er­a­tion in de­ter­min­ing the mass of in­di­vid­ual logs as they’re lifted into po­si­tion, and, more im­por­tantly, the load of the en­tire house as it rests on its foun­da­tion.

THE BEN­E­FITS OF THER­MAL MASS

Wood has sev­eral ther­mal prop­er­ties that af­fect its en­ergy ef­fi­ciency.

In the sim­plest, most easy-to-un­der­stand terms, wood re­acts to heat re­ten­tion and re­lease in much the same way as stone. Take a good sized rock and set it out­side in the sum­mer sun all day. Then as dusk falls and tem­per­a­tures cool, bring it in­side. De­pend­ing on its size, that stone will con­tinue to emit the heat it ab­sorbed for hours. Logs work the same way. This is ther­mal mass.

Con­duc­tiv­ity is an in­verse mea­sure of the in­su­lat­ing value or re­sis­tance to heat flow of the ma­te­rial. The lower the con­duc­tiv­ity, the higher its in­su­lat­ing value.

Wood con­ducts heat more slowly than other build­ing ma­te­ri­als. Structural lum­ber, such as south­ern yel­low pine, has a con­duc­tiv­ity of only 0.8 Btu per inch, per hour, per square foot, per de­gree of Fahren­heit. By way of com­par­i­son, the rate for steel (a poor in­su­la­tor with high con­duc­tiv­ity) is 320 Btu.

Ther­mal re­sis­tance is the in­su­lat­ing value of a ma­te­rial. This re­sis­tance to heat flow, usu­ally ex­pressed as R-value, varies among wood species and de­pends on the wood’s den­sity and other qual­i­ties. While R-val­ues alone aren’t a proper mea­sure of the en­ergy ef­fi­ciency of a log wall, they are widely used. This is why look­ing at the full pic­ture of a log’s ther­mal prop­er­ties — not sim­ply R-value — is vi­tal to de­ter­min­ing a log home’s en­ergy ef­fi­ciency.

MOIS­TURE CON­TENT

The mois­ture con­tent of tim­bers used in log home con­struc­tion is cru­cial to pre­dict­ing the amount of set­tling that will oc­cur in your wall sys­tem. Mois­ture con­tent is the amount of wa­ter con­tained in wood, ex­pressed as a per­cent­age of the weight of wa­ter rel­a­tive to the dried weight of the wood. Wood may con­tain “free wa­ter,” be­tween wood cells,

and “bound wa­ter,” found within the cell walls. Just as hu­man bod­ies are an av­er­age of 60 per­cent liq­uid, liv­ing trees typ­i­cally con­tain more mois­ture than wood.

The mo­ment a tree is cut, wa­ter be­gins to evap­o­rate from the wood as it seeks equi­lib­rium with the rel­a­tive hu­mid­ity of its sur­round­ings. Wood doesn’t be­gin to shrink un­til all the free wa­ter has evap­o­rated. The point at which no free wa­ter re­mains and shrink­age be­gins is known as the fiber sat­u­ra­tion point. De­ter­min­ing mois­ture con­tent lets pro­duc­ers pre­dict how much log shrink­age they must accommodate. The lower the mois­ture con­tent, the more sta­ble the wood.

Green wood has 30 per­cent or greater mois­ture con­tent. This is the con­di­tion of newly har­vested, healthy trees. The wood fibers are to­tally sat­u­rated with wa­ter.

Sur­face-dry wood has 25 per­cent or less mois­ture con­tent. This is the con­di­tion of wood two to four weeks af­ter cut­ting and de­bark­ing. The outer 1/8-inch of the sur­face

feels dry to the touch. Many wall logs are sold in this con­di­tion.

Air-dried wood has 19 per­cent or less mois­ture con­tent. It nor­mally takes at least a year to air-dry wood to this per­cent­age.

Kiln-dried wood has 15 per­cent or less mois­ture con­tent. Be­cause of their thick­ness, the logs used in homes re­quire two to three weeks in a kiln to reach this bench­mark. If the cen­ter of a log is re­duced to this level, very lit­tle ad­di­tional shrink­age will oc­cur.

SHRINK­AGE

As wood ap­proaches its mois­ture equi­lib­rium, it changes di­men­sion­ally. Shrink­age is a byprod­uct that can af­fect log home con­struc­tion. Why? Wood shrinks un­evenly. Most shrink­age oc­curs tan­gen­tially, (i.e., in the di­rec­tion of the growth rings). Tan­gen­tial shrink­age causes logs to check, or crack, and the com­bi­na­tion of tan­gen­tial and ra­dial shrink­age can cause an 8-foot-high log wall to shrink as much as one to two inches when green (moist) wood is used.

Most shrink­age will oc­cur be­tween 25 per­cent and 15 per­cent mois­ture con­tent lev­els. Even kiln- dried logs, which have had most of their mois­ture re­moved, and thus al­ready shrunk, still will shrink slightly.

Log home pro­duc­ers, de­sign­ers and builders take wood shrink­age into ac­count. (It’s par­tic­u­larly vi­tal to hand­crafters who of­ten work with large-di­am­e­ter logs that take a con­sid­er­able amount of time to air dry.) They can cal­cu­late the amount of shrink­age logs will un­dergo and use con­struc­tion tech­niques that ac­count for it. If your home is prop­erly built ac­cord­ing to pro­fes­sion­ally de­signed plans, shrink­age shouldn’t pose a prob­lem.

Most log set­tle­ment and shrink­age hap­pens within five years of con­struc­tion. Af­ter that, a log home finds its equi­lib­rium within its en­vi­ron­ment.

Log home pro­duc­ers can cal­cu­late shrink­age and use con­struc­tion tech­niques that ac­count for it.

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