# Decay resistance of woods (hemlock confusion)



## bob talk (Jan 17, 2012)

Hi all. Does anyone know a good deal about various woods' decay resistance? I have one particular confusion regarding eastern hemlock. I am in the trailwork business and it is common practice (and knowledge I suppose) that hemlock is decent trail wood (bridges and the like) in large part because it is decay resistant (not as good as cedar but better than most others in the Maine forest). 

My confusion stems from other sources which state it is extremely unresistant. See here:http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr190/chapter_14.pdf 
(Table 14-1) a few pages in.

I've also head that pine and fur are junk in the way of outdoor decay resistance, but according to that table they are better than hemlock. Does anyone have any light to shed?

Thanks

Tyler


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## Jim_Rogers (Oct 6, 2011)

I'll do some research on this and get back to you.....

I've just read your link download.
And I have seen that chart before.

I've contacted someone who should be able to post some info to help you. Hopefully he will.


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## GeneWengert-WoodDoc (Mar 12, 2013)

*Decay Resistance--Hemlock*

The basic source of knowledge about wood comes from the WOOD HANDBOOK, published by the US Forest Products Lab. Natural decay resistance of heartwood is listed in Table 14-1, as you state. (Sapwood seldom has any decay resistance.) Hemlock is listed as little or no natural decay resistance. Appreciate that their tests were for severe decay conditions (see discussion below). Further, they were trying to give advice for use as a fence post where failure would be a serious event, so they had to err on the side of safety. (I think a bridge would be a high concern use especially when loaded to its maximum design weights, in which case, a little decay could mean a failure.)

Why do some folks give hemlock a higher rating? It night be that they are exposing the wood in a location that is not likely to decay. That is, in order for wood to decay, the decay or rot fungi needs four essential elements--moisture (very close to 100% RH or even liquid water), warmth (70 to 100 F is the best), oxygen, and food. To control the fungi, we eliminate one of the four.

In an outside exposure, we might use a water repellant and a sloped surface to drain off any rain water before it can wet the wood substantially. We might keep trees and shrubs away from the exposure so the wood, if wet, can dry out quickly. By controlling the water, we can control decay. But it is difficult to control water 100%, so we cannot control decay 100%.

Or the food can be poisoned by Nature or by humans. Humans use various chemical treatments. Often we poison only the outside layers of wood to provide a protective barrier...that works until the wood cracks or someone drills a hole or saws off an end. Hemlock presents a special situation when we try to treat the wood, as the wood is very impermeable. In fact, when treating, we often will cut small slits int he wood (called incising) to form pathways for the chemical to get into the wood more easily.

Some DIY poisons are water soluble, so they will leach out quickly when used on wood exposed to rain.

Hope this lengthy discussion helps with your understanding. Questions?


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## bob talk (Jan 17, 2012)

That is great Info, thank you. I am still unsure as to why there are discrepancies, because in trail applications, the wood is certainly exposed to the elements and many use it knowing and considering this when they select hemlock over pine, fir, most any deciduous tree, and spruce (although I've heard it compared similarly with spruce, which is oddly also not on the list where I thought it'd be)

Perhaps people who recommend hemlock are simply mistaken, but it has been recommended by veterans of the trail work community/career here in Maine, so I am left befuddled. The reason hemlock is resistant (I am told) is due to its tannins, which I assume is distinct from heartwood extractives. Perhaps table 14-1 ignores the tannin effect? Another thought I had was that hemlock may not have extremely resistant heartwood but it has a lot of it? (It does have a lot in ratio to its sapwood, but whether its more than most other trees, I am not sure).


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## Jim_Rogers (Oct 6, 2011)

GeneWengert-WoodDoc said:


> The basic source of knowledge about wood comes from the WOOD HANDBOOK, published by the US Forest Products Lab. Natural decay resistance of heartwood is listed in Table 14-1, as you state. (Sapwood seldom has any decay resistance.) Hemlock is listed as little or no natural decay resistance. Appreciate that their tests were for severe decay conditions (see discussion below). Further, they were trying to give advice for use as a fence post where failure would be a serious event, so they had to err on the side of safety. (I think a bridge would be a high concern use especially when loaded to its maximum design weights, in which case, a little decay could mean a failure.)
> 
> Why do some folks give hemlock a higher rating? It night be that they are exposing the wood in a location that is not likely to decay. That is, in order for wood to decay, the decay or rot fungi needs four essential elements--moisture (very close to 100% RH or even liquid water), warmth (70 to 100 F is the best), oxygen, and food. To control the fungi, we eliminate one of the four.
> 
> ...


Thanks for posting this.

Jim Rogers


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## Brian T. (Dec 19, 2012)

Thanks, Gene. Good read. Does not smell like a desirable choice for outdoor construction.

Hemlock, here, is _Tsuga heterophylla_. Pulp fiber and lumber used in general construction.
Makes crummy firewood = either smolders or melts your wood stove, it burns so hot.

What you have is probably _Tsuga canadensis_, the Eastern Hemlock. Ring shake and star shake make it a poor choice for lumber. Knots hard enough to bend a nail. The bark was a commercial source of tannin, not worth the trouble to bring the stripped logs out of the forest.

Trees In Canada J.L. Farrar

I have a couple of hemlock logs, just 8" x 72", for wood carving. No heartwood visible.


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