Tsuga canadensis part 1: the wood

Hemlock isn’t a glamorous wood. It is coarse, brittle, and full of splinters. It has no value as a furniture wood,  it checks and warps significantly as it dries, often randomly splits apart where its growth rings meet, and its rock hard knots can dull or even chip the steel of sawblades or chisels—and will bend near any nail you attempt to drive into one (not to even mention what it takes to pull out a half-embedded bent nail from those knots).  It would make a terrible firewood, as it has minimal Btu value, and throws off sparks that could start an unwanted fire elsewhere. Yet with all that being said, hemlock does have its own significant and unique values, both from utilitarian, and especially ecological standpoints.

Ironically, one of the earliest and most extensive uses of Eastern Hemlock (Tsuga canadensis) didn’t involve the wood at all. Hemlock bark is extremely rich in tannic acid (10-13%), which back in the day was essential in the tanning of leather. Entire forests of hemlock were felled in the pursuit of the bark, which was peeled and hauled to local tanneries, with the stripped logs most often left behind to rot. It is estimated that over 70 million hemlocks were harvested and peeled in the Catskill Mountains of New York in the 19th century, before synthetic tanning agents were developed.

While hemlock was surely always used to some degree by early Americans as coarse construction lumber (I’ve seen 24″ wide hemlock floorboards in 19th century farmhouses here in New York), it was historically a distant second softwood option, behind Eastern White Pine (Pinus strobus), which is a fine and stable cabinet/furniture wood, as well as an easily worked and high quality construction timber. These qualities led to White pine being drastically overharvested throughout the northeast during the colonial era; this, combined with the advent of a few devastating insect and disease problems, and the transition of many former pine forests to a mix of more shade tolerant hardwood species, have kept the pines from their former abundance and majesty. While late 18th century records speak of entire groves of pines at over 250 feet tall and 5 feet in diameter in my town, the tallest pines in all of the northeast U.S. no longer reach 160 feet in height. So, largely of necessity, hemlock has seen a resurgence in more recent times, as rough construction lumber, and particularly, as a barn wood.

hemlock post

In the picture above, which is a 6×6 post, in addition to the residual marks left from the circular blade that was used in milling the wood, you can clearly see the coarse texture of hemlock’s grain, with the light colored early wood and plainly darker summer grown wood of each annual growth ring clearly distinct from each other. Hemlock does not have any distinct color variation between heartwood and sapwood, as do pines, as well as most hardwoods. When used as framing lumber it is rarely kiln-dried before use, so arrives fresh from the mill in a very wet, very heavy state. It tends to smell a little sour and rank when green–unlike the fresh resinous scent of the pines–but becomes pretty well odorless once dry, at which point it becomes very light in weight, at just under 27 pounds per cubic foot (compare to air-dried White oak at 48 lbs./cu. ft.).

The two photos below show one of the often fatal flaws of hemlock: ring shake, which is (obviously from the images) a separation of the wood between growth rings, which at times will not reveal itself until a wet board or timber is already in service.

hemlock 2x4

hemlock ring shake

In addition to lower cost, and in many regions of the northeast more availability, two significant advantages of hemlock over pine for barns and other outdoor structures are that: 1) it tends to hold nails more securely than pine; and 2) hemlock, while not truly rot-resistant, holds up much better over time than does pine when exposed to the elements, such as in board and batten siding.

board and batten siding

By the way, the picture above shows the proper way to install board and batten. Behind the battens there are 1/2″-3/4″ spaces between the boards (which are 12″ wide). The boards are fastened with two nails near the middle of the board only (here shown at about 3″ apart; it could be one nail in the middle for boards 10″ or less wide), with no nailing at the outer edges. The long nails that hold the 3″ wide battens in place run through the gaps between the boards before reaching the wall purlins, thus allowing the wide boards to expand and contract freely behind the battens with changes in humidity. Nailing these boards down tight (especially when still green or wet) would pretty well guarantee major cracks and splits where those nails restricted the inevitable wood shrinkage.

Another rule of thumb when installing hemlock siding is to orient the boards whenever possible with the heart of the tree facing out—the bark side in—in order to minimize the warping of the boards from pushing the battens away from the building. The reason this works is that while a board will always cup toward whichever side is driest (which is why if you leave a board on the lawn on a sunny summer day it will quickly curl toward the sun), all things being equal a board will most often cup away from the heart as it shrinks, which in turn is because wood shrinks more along its growth rings than across them. Put another way, in most all wood species, tangential shrinkage will be far greater than radial shrinkage, which is why quartersawn lumber is so much more dimensionally stable than flatsawn, but that’s a story for another day. You can see some of this in action in the photo below: heart is on the bottom, the board is warping upward, away from the heart. Install this board with the heart out, and it won’t want to push the battens away from the wall.

hemlock end grain

That’s it for the wood. Next time I’ll delve into the character and nature of the tree itself.

 

3 Comments on “Tsuga canadensis part 1: the wood

    • Me too. I currently use a chainsaw mill for milling hardwood logs into lumber, but for a project of this size a small band mill (such as a Wood Mizer) would be just the thing, and would pay for itself over time.

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