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Decay resistance of Eastern Hemlock

Started by Spruce_Goose, March 12, 2013, 07:10:35 PM

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Spruce_Goose

Hi all. I am new to this forum. Hopefully this post is in the ideal category...

I am a trailworker in Maine, which means bridge building and the like. The wood is obviously exposed to the elements, so decay resistance is of utmost concern.
I have been informed by a variety of trailwork veterans that eastern Hemlock is a good wood choice for for trail structures. Cedar being the top dog, tamarack being way up there as well, but Hemlock supposedly just following (keeping in mind availability in Maine).

I am confused because some sources I have encountered list Hemlock as being very unresistant to decay, and additionally, list species such as pine and fir as moderately resistance, which I've been told are quite poor choices. Here is the major source I reference: http://www.fpl.fs.fed.us/documnts/fp...chapter_14.pdf
(Table 14-1) a few pages in.

One thought I have is that this table refers only to resistance due to heartwood extractives and Hemlock is resistant for other reasons (Such as high tannin content, low permeability, etc) (it is both of these things I understand).

Does anyone have any light to shed and/or experience with hemlock durability in comparison to such trees as fir, spruce, pine, red oak, maple (common Maine trees). keep in mind these are for trail structures and so often make ground contact and are generally exposed to 'the elements'.

thanks,
Tyler

woodmills1

none of the species you list, except for cedar are noted for either decay or insect resistance.  I think hemlocks value is in it high strength to dry weight vs pine and the rest.   Back when I cut much scaffold wood the masons wanted hemlock, it sheds water well and is tough, as in not breaking easily at knots.
James Mills,Lovely wife,collect old tools,vacuuming fool,36 bdft/hr,oak paper cutter,ebonic yooper rapper nauga seller, Blue Ox? its not fast, 2 cat family, LT70,edger, 375 bd ft/hr, we like Bob,free heat,no oil 12 years,big splitter, baked stuffed lobster, still cuttin the logs dere IAM

Jay C. White Cloud

Hi Tyler,

Welcome and thanks for joining in the FF.  Your first statement is very accurate.  Cedars, Tamaracks then Hemlocks, in that order of decay resistance.  Hemlocks are also very variable depending of the area of the country, and (biome) they grow in from my experience.  It also has a lot to do with the construction method you use with the wood.  Some promotes rot others tend to drain water away, and allows the wood to dry out, breath.

Regards,

jay

Sorry Woodmills 1, I must have been writing when you were.  In some areas Hemlocks are considered a close second to Cedar for rot resistance, even over tamarack.  It appears to be regional and biome specific.  Some varieties of Hemlock have a very heavy concentration of tannin and oils in their tissue, often from boggy areas.
"To posses an open mind, is to hold a key to many doors, and the ability to created doors where there were none before."

"When it is all said and done, they will have said they did it themselves."-teams response under a good leader.

GeneWengert-WoodDoc

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.  A decorative fence post would not be a high risk situation, but if there were cattle behind the fence and the fence posts failed, then it is a different story.)

Why do some folks give hemlock a higher rating? It might be because wood from certain areas has some natural decay resistance that the WOOD HANDBOOK writers were not aware of.  Also, the WOOD HANDBOOK must consider the characteristics throughout the range.  It also could 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. This information is in addition to previous postings that also had valid info.Questions?
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

thecfarm

Tyler Keniston,welcome to the forum. Been working on trails long? I worked for the YCC in Acadia for 2 summers. I was in high school at the time. Just a drop in the bucket with what you do,but I learned alot and hard a great 2 summers.
Model 6020-20hp Manual Thomas bandsaw,TC40A 4wd 40 hp New Holland tractor, 450 Norse Winch, Heatmor 400 OWB,YCC 1978-79

Jay C. White Cloud

Hi Gene,

That was a great addition to this, and I might be able to shed some additional light on it as well.  I believe, besides possible biome influence, any of the old text that would describe it as rot resistance may have been using it in a fashion that would allow the wood to dry each time.  In vintage barns, that had chosen to use hemlock as the framing framing material, have been some of my best examples of the varying degrees of decay resistance.  However, where ever you combined heavy moisture inundation and other organics, (manure) the wood decayed rapidly.  It would seem that, unlike Cedars, Hemlock can get wet, but must dry out afterwards.  So even as an exposed timber structure, as long as there isn't soil content (some) Hemlocks present as decay resistant  Joints should drain water, not hold it and be able to breath out the moisture it takes on.  Also, any additional treatment you can do will extend life spans considerably.
"To posses an open mind, is to hold a key to many doors, and the ability to created doors where there were none before."

"When it is all said and done, they will have said they did it themselves."-teams response under a good leader.

Spruce_Goose

Thanks all for the information and welcome. To answer thecfarm, I've been in trails for about 3 years now. I love the work.
So it seems that hemlock might have some decay resistance, but should definitely be allowed to dry as much as possible. This, coupled with its strength to weight ratio, probably makes it a good choice for things like bridge stringers. In situations where it is contacting the ground (sills) I wonder if there are better alternatives? (Besides cedar and PT, which are clearly better) I think a common solution is to simply replace sills by jacking up the bridge, because chances are the ground contact will do them in if they are anything besides PT or cedar...

Tyler

Jay C. White Cloud

Hi Tyler,

When we did it at Seneca Rocks in WV and the AT in PA, we would scribe fit the Hemlock to rounded stone.  It gets it off the ground and drains water well.
"To posses an open mind, is to hold a key to many doors, and the ability to created doors where there were none before."

"When it is all said and done, they will have said they did it themselves."-teams response under a good leader.

Spruce_Goose

Quote from: Jay C. White Cloud on March 13, 2013, 07:50:17 PM
Hi Tyler,

When we did it at Seneca Rocks in WV and the AT in PA, we would scribe fit the Hemlock to rounded stone.  It gets it of the ground and drains water well.

Hmm interesting. Would this be for small (in width) bridges typically? Would you use any sort of fastener to attach it to the rock?

John Mc

What about using black locust for the sills (if you have it in your area)? It makes for some long-lasting fence posts... might be good for keeping your hemlock stringers off the ground.
If the only tool you have is a hammer, you tend to see every problem as a nail.   - Abraham Maslow

Jay C. White Cloud

Hi John Mc,

Usually you have to use what is close by that can be shaped by hand, but not always.  We have used Locust on some trails.

Hey Tyler,

Big, small and just about everything up to shelter size.  I miss the work.  When my son gets older, I was hoping he would join a crew.  Then I could go back to it part time.  I facilitated workshops on timber framing for trail building; which included rigging methods, stone splitting shaping and the related stuff.  If a crew has the skill sets, and good tool-material support, they can do amazing stuff.

Regards,

Jay
"To posses an open mind, is to hold a key to many doors, and the ability to created doors where there were none before."

"When it is all said and done, they will have said they did it themselves."-teams response under a good leader.

GeneWengert-WoodDoc

I will give you my theory on why some pieces of hemlock seem to resist decay better than other pieces.  As a hemlock tree approaches 75 years, it is almost certain that the stem will be significantly affected by an anaerobic bacteria.  This bacteria is known to weaken the wood, resulting in wind shake, and also creating wet pockets or wetwood when drying.  What is seldom mentioned is that such infected wood also has a major change in pH.  We also know that the decay fungi are very sensitive to pH, so, I believe that when the pH changes (more acid) that the decay fungi find the wood "distasteful." 

Regarding the US FPL tests and ratings, part of their testing involved two test fences where they had hundreds of fence posts in the ground, among other tests.  The fences were in Madison, WI and near New Orleans.  Every few years or so, a person would go to the fence and give each post a push.  If the post failed, they noted that.  The data exists, but I do not know how many failures with a species before it was given a poor rating, and, if the LA site had failures but WI did not, if that made any difference.  This was an attempt to simulate real world conditions.  All we see in the WOOD HANDBOOK is a summary.

They also had a test where the put a small specimen in a jar that had decay fungi in the jar also.  After a while, they examined the jar and sometimes weighed the sample to see how much wood had been eaten by the fungi.
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

Jay C. White Cloud

Thank's Gene,

I hand always speculated that PH was a contributing factor to decay resistance.  You have helped confirm that theory.  It may also explain why some species with more tannic acid (higher PH) and are also more decay resistant, while others do not.

Regards,

jay
"To posses an open mind, is to hold a key to many doors, and the ability to created doors where there were none before."

"When it is all said and done, they will have said they did it themselves."-teams response under a good leader.

woodmills1

James Mills,Lovely wife,collect old tools,vacuuming fool,36 bdft/hr,oak paper cutter,ebonic yooper rapper nauga seller, Blue Ox? its not fast, 2 cat family, LT70,edger, 375 bd ft/hr, we like Bob,free heat,no oil 12 years,big splitter, baked stuffed lobster, still cuttin the logs dere IAM

Old Wood Whacker

We used to build sheds out of green hemlock for sale back in the mid 1980's, and every one of them is rock solid today. I have one of them right here, and it's in great shape. I build a deck with green hemlock for my home back in 1980. I stained it every couple of years and included a mix of Woodlife to the stain, but that deck was like new for at least 20 years until the people who bought the house put an addition on to it. We have hemlock fletch siding on our home and it looks great after 10 years.

Like pine or anything else, try use bigger logs, because the sapwood doesn't last nearly as well.

I don't know what the reason was, but when I had my old mill, we sold probably a half million feet of hemlock boards to the mushroom farms down at Kennett Square. They used it for their beds.

Jay C. White Cloud

 8) Welcome "Old Wood Whacker," looks like you and I are in the green woodworking end of things.  I have the same findings about most Hemlock, and the old fellow that I learned milling from always said that, "if Cedar was a 10, then good Hemlock was an 8 or 9."

Regards,

jay
"To posses an open mind, is to hold a key to many doors, and the ability to created doors where there were none before."

"When it is all said and done, they will have said they did it themselves."-teams response under a good leader.

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