lnside story

A trcets insides forctell the performanee of lumber used outside

By Huck DeYenzio Arch Wood Protection

fN THE cross-section of a tree lies a lstory. It tells of the tree's past. but also portends the future of lumber sawn from the tree.

As most every school kid knows, annual rings indicate the age of a tree. The rings also provide evidence for conditions the tree experienced during its life-years of good growth, drought years, fires, competitive stresses.

But those rings are more than a look back in time; they hint at the performance of deck timbers, decking lumber, and fence posts. Within a particular species, cross-sectional differences affect the strength of wood products cut from the tree (denser rings usually mean stronger lumber), their stability (edge-grained lumber tends to shrink less), and the thoroughness of preservative treatment.

The cross-section encompasses three primary zones: heartwood, sapwood, and bark

The heartwood is the center of the tree. It has become inactive, and is unnecessary for the continued life of the tree. Heartwood may be infrlhated with gums and resins, which usually make it darker in color than sapwmd and morc resistant to decay.

Between the heartwood and the bark lies the sapwood, the younger, living part of a tree. It is rnorc porcus than heartwood and more susceptible to decay, but has essentially the same shength.

When lumber is sawn from a log, individual pieces may contain a small amount of heartwood or, in the case of a 4x4 or 6x6 that is usually cut from the center of the log, there could be more than 50% hearnvood. (This will vary by species; southern pine, for example, usually has more sapwood than western hemlock or Douglas fir.)

AVOID SEEING THE SHRINK: Lumber shrinks as it dries out and sweils when it oets wet. When lumber is cut from a tree and again immediately after preservative treatm;nt, the wood is saturated with moisture. This iliustration shows how characteristic shrinkaoe of wood items depends on position in the cross-section. Radial shrinkaqe (across the rings) is about hali as great as tangential (along the rings). Water repelleit inlected into wood and applied on the surface every year or so wiLl minimize shrinkage'and other eflecls of moisture on treated decking and lumber. [Note: most logs are not as large as shown, and posts and timbers come from the center.]

The location of the piece in the cross-section affects the amount of heartwood and the direction of the grain; grain rnay run roughly perpendicular to the width of the lumber (called edge-grained or quartersawn) or it may arc roughly parallel to the width (flat-grained or plainsawn). Or it could be somewhere in between.

Heartwood typically is somewhat more resistant to decay than sapwood, but sapwood accepts pressure-treatment with waterborne preservative more readily than does heartwood. Many pieces of lumber contain sorne heartwood, often surrounded by sapwood. Thus the heartwood, which gets very little preservative penetration in the treating process, is protected by the well-penetrated sapwood around it.

Sometimes heartwood is exposed on a side of lumber; experience has demonstrated that the natural durability of heartwood combined with a thin layer of preservative treatment is nearly always adequate protection for wood in above-ground service.

If you cut a piece of pressuretreated lumber, you may notice that the greenish color of the preservative does not permeate the entire thickness of wood. Industry standards, established by the American Wood-Preservers' Association (AWPA), specify the amount of penetration required for effective treatment. The standards vary for different species of wood and different sizes of lumber. For southern pine, they require penetration of 2.5" or 85Vo of the sapwood. There is no requirement for penetrating heartwood.

Responsible treating companies check after each treatment cycle to see that AWPA standards are met, and they contract with independent inspection agencies to monitor their production. Core borings are taken from processed wood to determine the depth and amount of preservative in the wood.

For an extra measure of assurance, particularly with structural timbers that are sawn during construction, you can coat cut ends with a brush-on preservative to bolster protection of exposed heartwood. (In western species, which are largely heartwood, this step is highly recommended and is required in order to validate most warranties.) Look for topical preservatives containing at least copper.

Well, what about bark side up or bark side down? For years there was discussion over the proper way to orient deck lumber. Most experts advised placing lumber so that the bark side (outward rings) faced upward, but a significant group advocated the opposite. The debate was largely ended in the 1990s when a definitive study was conducted by the Mississippi Forest Products Laboratory of Mississippi State

University with several co-sponsors.

The definitive study, however, did not result in a definitive recommendation. It found that cupping and grain raising were less when decking was installed with its bark side up, but checking was less when decking faced bark side down. The best orientation depends on which tendency you prefer to avoid.

The study was clear on two factors: warping was less in straightgrain all-sapwood decking, and both cupping and checking were reduced when a coating of water repellent was applied annually.

The rings of a tree tell much more than the tree's age; they are a crystal ball revealing the performance and appearance of future decks.

- Huck DeVenzio, who says he has accumulated some annual rings of his own, is manager of marketing communications at Arch Wood Protection, a lead' ing producer of wood treating chemicals and licensor of some of the best known brands of preserved lumber, including Wolmanized wood.