Wood Choice for Exterior Column Bases
A woodworker looks for a rot-resistant material for exterior porch column plinths. July 18, 2008
Question
I need to replace some wooden plinths under the columns that support a porch. These sit atop brick pillars, and then support the columns which are round, hollow pine. The columns are okay, but the plinths are rotting. They are 8" by 8" by 2-1/2 " rectangles and I can make them easily enough, but they require jacking up the upper porch, and I would like them to last a long time and resist rot. What to use that would be reasonably available? Treated pine, I suppose?
Forum Responses
(WOODnetWORK Forum)
From contributor L:
Mahogany, Spanish cedar, white oak.
From contributor W:
I would add cypress.
From contributor A:
I have issues with the aforementioned here on the Carolina coast. I pick out tight grained treated pine and give it two sealer coats of acetone thinned West System epoxy and it will not absorb water. I have been using this for over fifteen years and have had no problems. The cypress will rot where it touches the masonry. That was the wood of choice here before treated pine came along. You have to pick the pine carefully, make sure the grain is close and dense.
From the original questioner:
Yes. This is the conclusion I've come to myself, though I was curious what others would say, and no doubt these are all good recommendations. Would you insert a layer of metal or maybe an asphalt shingle between the wood and the masonry?
From contributor L:
Yes, wood and stone should never touch.
From contributor J:
Pressure treated. Composite. Cast cement. Untreated wood is going to fail, sooner rather than later. If you look at old buildings, you can learn a lot about what works and what doesn't. The building codes might have some good ideas, too.
From the original questioner:
This is a pretty old house (1867), though I am dubious that these columns are that old, and these wooden plinths are much younger. Best guess is that the columns were trimmed a couple of inches twenty or thirty years ago and these were inserted to make up the difference and have subsequently failed due to moisture. However, the masonry columns (underneath the porch) are brick about 20" tall. This is probably the third or fourth (well... who knows) time this has been repaired. But I'm thinking that the way to do this is to jack up the upper level, remove the columns, repair the top of the masonry, insert a layer of asphalt and/or flashing and then replace the plinth with treated pine. Anybody else care to weigh in here?
From contributor R:
I have a similar job coming up and I think I am going to use a layer of rubber mat (probably from rubber roof types) between concrete floor and column. Then a pressure treated base for the column. I think the base will be four pie-shaped mortised and tenon glued together and then turned in a lathe for some detail. What do you think of this?
From the original questioner:
These sound a little bigger than mine. I'd just be concerned about the durability of the glue when exposed to the weather. Actually, what I have is a three piece plinth - one piece that is square, about 9 by 9, then a second piece that is about 8 by 8 (each of these about 2 1/2 inches thick), and then a final disk about 7" that has been edge routed - bullnosed. Then the column resting on top of this (about 6"). The columns are hollow and tapered and made of strips of pine, wedge shaped, and (sort of) tongue and groove joined. Presumably, the columns were turned after the strips were formed together. The columns aren't actually joined to the plinth - they just rest on it. This part of the porch is slightly exposed and a good deal of rain puddles on the plinth and at the place where the bottom rectangle penetrates the porch - leading to a good bit of rot. The route of moisture here is generally from the top down and not from the masonry up.
We've repaired the columns by compressing them with ratchet straps and then nailing (long) stainless hose clamps around them and snipping off the screw adjustment after they are nailed. This has worked fairly well, but getting at the plinths requires jacking up the second floor - and this is a repair we'd like to see accomplished for the long term.
Our house is an 1867 farmhouse vernacular - classic in its way - but is a bit of a money pit. Still, it is in astonishing shape for such an old girl, not too screwed up over the years, with 11 foot ceilings upstairs and down, big oak beams in the basement, and a lovely if somewhat shaky walnut banister. That's next. Anybody ever taken a banister apart and gotten it back together again? Looks like a nightmare to me.
From contributor A:
There may be some issues with condensation forming on the inside of hollow columns under some conditions. If this is a genuine concern, then think about how to rid the column of water. I believe the aluminum bases allow for the flow of air which will exit the top as vapor, if the top will allow. Where the aluminum base is not used for whatever reason, a number of small stainless tubes can be inserted near the bottom at an upward angle to prevent water dripping down to and gaining entrance into the wood. I believe I have seen what appears to be a coating of tar on the inside of the column to prevent the interior wood from absorbing the moisture. Since the base is the part of the column which rots, I think it would be prudent to minimize this water issue at the onset. West System claims wood will be completely waterproof with three coats of their product, however there are several factors that will breach the coating, one being compression from the weight load. Most of the time a cement based product is used instead of wood. Also many of the very old columns I have dealt with have been made with heart pine. Many probably have no idea what it is, but an old ship builder advised me to use that which is fat, meaning full of pitch. That wood is native to this area. It is very resistant to insects and rot. You can light a small piece of it with a match and it will begin to bubble and burn and smoke like a piece of rubber. It's hell on belt sanders.
From contributor L:
The West System is not a coating. It will sink in deep, penetrating the wood. Surface scratches and compression will do little to break the barrier.
From contributor V:
Try black locust - it never rots. Drainage and termite shield ideas are also good.
From contributor A:
How deep do you think the epoxy penetrates the wood and what would you add to it to help it penetrate better? I am sure the type of wood is a major factor and also the moisture content. Would washing the wood with acetone to drive out the moisture help?
The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).
Comment from contributor J:
I've had good luck with spar varnish (inside and out) on the two plinths I built for my own front architrave. They've been in service for over 10 years now with no discernable degradation of wood. I "freshened" the finish last year, however my situation allowed for airspace between the bottoms and the deck. The use of an impermeable membrane sounds best in your situation. Spar varnish is a "long oil" which makes it more flexible. If you use it make sure it contains UV inhibitors.
