fir exposed to the elements

[Lee44]

Hello, 

I know that fir is a good lumber choice for timber framing, but what about in situations where the wood will be exposed to the elements? Does fir hold up well if it is treated properly? If so, what would you say is the best finish?

Thanks for any thoughts. I really appreciate them. 

[Southside]

Doug or Balsam Fir?

[D L Bahler]

 

 

Looks fine to me, after a couple hundred years or so... 
That's Alpine fir, which has very little if any rot resistance at all.

The thing is exposed timber really has a lot of variables involved. Even species with no rot resistance at all can last exposed if the structure is built with this in mind. Keep it isolated from the ground, make sure water can run off and gets away from the wood and out of the joints and it will be fine. Just be sure the wood stays pretty dry, and if it does get wet be sure the water won't stick around at it will last. In Europe they even make shingles out of fir. They do fine, because the water can get off of them. 

Like this solid fir house from the mid 17th century:


 

[Don P]

Or, it'll last 5 years if you get it wrong. Don't expose structural members lightly. Cycles of wetting and redrying is what makes checks dive deep and admit water into places in the wood that do not redry readily.

[Lee44]

Thank you guys for the input. I really admire those old buildings. I have a book about old Norwegian stave churches that is very interesting. I knew that those were made from fir, but I also understood that the specific variety of fir that they used is no longer available. I wasn't sure how fir that we have available now would hold up. 

To answer your question, Southside Logger, it is Douglas Fir.

This structure is going to be a small porch added to the front of my house. Something sort of like this:




 

Or this:


 

What finish would you guys recommend for douglas fir in this situation? I'm sure there is no absolute consensus, but what are some products you guys like? Also, are there specific strategies you would suggest to facilitate the joints drying when they do become wet?

Thanks again. 

[Don P]

I would borate and not mortise the braces into the posts, or omit them, that was a 10 grand repair on a porch nearby not long ago.

[D L Bahler]

If you have mortises on exposed timbers, make sure water will not stay trapped inside of them. For example, cut the bottom of a brace mortise sloped to the outside, and make sure it is very clean. Also ensure they are tight to prevent bugs going inside and making nests. If you have a mortise on top of a timber that may catch water, drill a weep hole through to the bottom or sloped downward to an obscured face of the timber. Also make sure the joints are clean and cut very smooth. Any frayed bits of wood will only hold on to moisture. Round of the edges of your tenons. Water will more readily shed of of rounded surfaces. Surface tension can cause it to stick to sharp edges. (It will form nucleation points on an edge, and form a "drip". That means the water is sticking around longer than you want, and has more opportunity to be absorbed into the wood) If you're using rough-sawn timber, make sure it is planed or shaved smooth at all joints. Again, this is because the frayed edges will hold on to moisture and wick it into the wood. 

End grain is particularly susceptible to absorbing water. Keep care that any end grain that may be exposed to water is clean and smooth so the water has less opportunity to be absorbed into it.

Any finish can potentially be a double-edged sword. They offer great short-term protection, but can potentially become an Achilles heel (I'm trying to see how many idioms I can cram into one paragraph here apparently). When they degrade, they can often just hold water to the wood instead of repelling it. This is fine for surface stains and oils, but I would stay away from treating the insides of joints. Again, it will work great in the short term but in the end may only cause problems. Just remember that anything that keeps water OUT of wood also keeps water IN the wood. 

[Don P]

I'm on the third big dumpster of rot removal at work, I don't see much point in pushing that envelope. If you cut a pocket in wood and invite water in, again be careful. No matter how you slick it up initially you are holding moisture. Then add some time. The local environment means an awful lot. There are a couple of general maps in the building code that give some guidance on where your chances are good with respect to pushing the envelope and where the chances are less good. You can get away with more where the snow is on the ground half the year or in an arid environment than in the east.

For naturally decay resistant species, as a species they are decay resistant. Each individual varies in how decay resistant though.



 



Interesting, I need to dig up one of my old termite maps.

[Lee44]

Wow, great information. Thank you guys very much. 

I live in Ohio, and we get our share of snow and rain.

Do you guys think a species such as cedar would be a better choice than douglas fir for this sort of project? I have ready access to both. 

If you did not mortise in the braces, what method would you use to fix them to the posts? 

Yes, finishes can be tricky for sure. So would you leave the wood unfinished? I had been thinking some sort of oil finish; not a film finish; but I'm sure that even oil can slow water exiting the wood, just as it slows water getting into the wood. 

[Don P]

In the second picture you posted the posts are on raised piers. I like that, get them above the splash as much as you can. Heart cedar would be better than fir for the posts if the bearing capacity is up to it. When I can I soak those parts in a borate bath, and I've roughed the brace mortise and tenons and soaked them then let them dry overnight and fit. That isn't something from the mountain just more rot thoughts. I've been rethinking what most bracing is doing and especially so on exteriors. The bracing for that entry is really the braced wall behind it connected to the roof diaphragm of the addition. The braces are just for pretty and they are a potential problem. Do we mortise them in because that's the way its always been done. If we're working green that brace isn't doing much useful work next year anyway. Do we table it in, that gives bearing and then ledgerlock it. Or do we just ledgerlock it, that drains best, braces least, or do we omit the brace altogether. I lean towards that. Its honest, the brace is unnecessary, even a detriment. The roof plane must be rigid and well connected. Depending on overhang and local climate that might be over the top. s framing gets higher and better protected you can do more and use more.  When its obvious I turn the tree upside down on exterior posts so that water flows down and out of knot checks instead of down and in if the tree were upright.

A water repellant preservative finish has been the go to finish. There are many makes and opinions but basically you want to repel as much bulk water as possible while letting water vapor back out. Most don't last long and repeated applications can form a film that becomes a composting bag. Plan on stripping back to wood if that happens, then refinish.

[Lee44]

Yes, I like that detail too, getting the posts up off the ground. I had been thinking to do that as well. 

I hadn't been planning to eliminate the braces, but the more I think about it, you are exactly right. I really ought to let function dictate something like that. There will be opportunity enough to make the porch attractive without including something superfluous. 

Durability is a foremost concern for me with this project. I never cease to be amazed by how insidious water can be. I am playing around with the design at this point to see how much of the joinery I can get under cover without attracting attention to having done so. I'm thinking to perhaps use cedar, at least for the posts. 

Thanks again. This has been extremely helpful.