Framing Lumber for Planting Boxes
A discussion of treated wood, naturally rot-resistant lumber, outdoor exposures, and pests. April 14, 2010
Question
I have a production line for an outdoor planting cabinet that will get wrapped in cedar. I don’t want to deal with pressure treated dust and etc in my shop. Do I really need to use pressure treated wood for the framing if it will not have any contact with the ground? It will be exposed to moisture from the air. They will not be finished.
Forum Responses
(Cabinetmaking Forum)
From contributor J:
If it’s an outside project and exposed to moisture, why not just use cedar 2x4's also? Keep the whole project cedar.
From the original questioner:
Structural and cost reasons.
From contributor R:
You are better off with the pt as wet wood can attract bugs even though it won’t have contact with the ground. I agree with you about the pressure treated dust. I can’t stand it either. One thought though - have you considered getting a quote from your supplier to send you the pt parts cut to length? Then all you need to do is bolt them together and you can focus on the cedar parts. It might be cost effective if you are building enough of them.
From Gene Wengert, forum technical advisor:
It is indeed very smart to avoid sawing in a shop or confined area and also to mix PT sawdust with other dust. It is allowable to dispose of PT dust in a landfill if you cut outside, collect the dust (drop cloth; no wind) and then dispose of properly? If dust were to get into the ground at your site, it would actually contaminate the ground for many years. In fact, when PT wood is used for a playground and the builders just let the dust accumulate in the soil, someone will come in later and find high and dangerous concentrations of chemicals from the dust and not from leaching.
From contributor A:
Regular pine used in outside conditions just will not last. As the one person suggested, cedar 2x4's would be a minimum to use.
From the original questioner:
Thanks for all the responses. Cedar won’t work due to the span and design plus cost. Precut would be great but it would have to stay fairly true I usually end up milling them to true. Even a little milling creates the same dust and waste. I would also have a check problem I'm sure. Note the structural frame would note see water exposure but bugs would still be an issue. Maybe I can make the frames out of pine and spray a bug treatment/sealer on them and waterproof sealer?
From contributor R:
Do you have a source available that you can buy a common grade of white cedar in the rough? If appearance isn’t an issue you may want to check that out.
From contributor R:
If they won't get wet then I don't see and issue with pine. There are a lot of buildings out there framed with pine. Another wood would be cypress. How many truckloads do you want?
From Gene Wengert, forum technical advisor:
You cannot safely apply a chemical to develop insect and/or decay resistance for any length of time. But, you can apply a water repellent that will help keep the wood dry enough to avoid decay and many insects. How big is this cabinet? I cannot image that cedar (not western red as it is expensive) would be that expensive. What strength do you need? You are correct that cedar (all of them) is not strong.
From the original questioner:
The cabinet is unique with a 6' span x 30" deep. It’s only allowed four corner legs and a concentrated load of 300lbs. placed in the center of the unit. Over time it will sag so my engineering is important. I call it spider web framing.
Cypress has interesting mechanical properties; however, I don’t fully subscribe to them. My frames are only 1.25"x2" so any splitting while screwing leads to starting over, since they are glued the entire structure gets chopped up for scraps. Cedar is weak but more importantly it is lobster tail to carpenter bees, huge issue in some parts of the country. I will research rough cypress and test the sag with a dead load of 500lbs. Not to say the dust from cypress is much better. I do get a lot of waste from the PT since the load will not allow for knots obviously. White oak works great but it is gnarly! The only reason I was asking is I read an comment by a respectable woodworker who stated there is no advantage to PT if it is not exposed to direct exterior elements.
From Gene Wengert, forum technical advisor:
Have you looked at syp PT 5/4 radius edge decking, which is both a strength and appearance product? The small knots will not overly affect strength.
From contributor W:
Gueydan Lumber in New Orleans sells pressure treated, kiln dried cypress. They came up with it for making outdoor millwork. The lumber is kiln dried then pressure treated then re-kiln dried. It works great for shutters, planter boxes and outdoor furniture.
From Gene Wengert, forum technical advisor:
Wood used outdoors but protected from rain, water sprinklers, and other sources of liquid water is almost always not wet enough to decay or rot. However, it may be wet enough for insects to make it their dinner or their home or both. Hence, even though the wood is protected from the elements, there can be an advantage (small in your case it seems) in using PT wood for exterior locations. If I understand correctly, you will also be near or in contact with soil, which would suggest PT wood, plus you might not be able to assure that the owner would not get the wood wet occasionally.
Note that old growth cypress, SYP and several other old growth species have natural decay resistance. However, today's wood does not have such high (or even medium) protection, so the wood must be PT. Comparing PT cypress to PT SYP, you will get more strength from SYP.
Pre-drilling holes will eliminate splitting up to 90% or the root diameter of a screw, etc. From all the discussion and info you presented, it might be most prudent to use PT SYP but contract with someone else to cut the pieces you need (assuming they do the cutting safely and do not damage the environment).
From the original questioner:
I will look at both PT cypress and 5/4. I have been contemplating the 5/4 for a while. It has some very appealing features, but I do have to remove the radius. Pre-drill with self-tapping screws works well - still some risk. High quality is the only way to go in today’s market. I started out low quality and was copied instantly. I have a low quality design now that I could make hear and sell $200k worth next year. So now I am trying to make a top end unit proudly made in the USA and hope it will sell at a US price. I am using the glue and screw construction and last forever as a main selling point.
From contributor R:
My comment was not about a quality issue, per se, it was to point out that, it is an outside product which I doubt most anyone will expect to last 100 years. Comparing yesterdays SYP which was almost 100% longleaf and todays, which is 4" top whatever hybrid grows the fastest, without clarifying the difference is irresponsible. Further, the statement that second generation longleaf pine and cypress doesn't have the resistance that old growth did is, when comparing apples to apples I believe, tainted by emotion and is based on nothing more than subversive opinion. If there is research to prove me wrong I would love to see it. I see PT as a trade off, you can use fast growing pines, and pressure treat with toxins that will contaminate the environment for 100 plus years or you can use a quality log that will regenerate in 100 years.
From the original questioner:
I will say from experience that I have seen carpenter bees eat fresh PT or cedar but they have never touched my cypress. I will seek out a natural board alternative and study the cost factors.
From Gene Wengert, forum technical advisor:
Carpenter bees like redwood, cypress, cedar and pine. They do not eat the wood, but remove the wood without digesting it to make galleries to live in. Hence, some preservatives for the wood are not especially effective. Also, the decrease of decay resistance of second growth timber of many species, including heartwood, is well documented.
From Gene Wengert, forum technical advisor:
There is no question that in the southern states, longleaf pine forests were extensive and were heavily cut prior to the 1920s, with the peak harvest occurring about 1909 or earlier. (Forest harvesting moved from New England to the Upper Midwest to the South as each area was heavily cut over.) As we are now harvesting the South's fourth (or fifth) harvest of SYP (and longleaf has not been regenerated to any significant degree), the second, third and fourth forests were not longleaf to any extent. Hence, it is not correct to state that yesterday's pine was 100% longleaf. It is probably true that lumber made prior to 1920 in the South was heavily longleaf, but even then it was not even close to 100%. Regarding decay resistance, even the US Forest Products Lab lists the decay resistance of old growth longleaf being more than current longleaf (Table 3-10).
From contributor R:
Longleaf takes 100-150 years to mature so we are just coming into mature stands again. When referring to a resistant pine normally, of the major species, longleaf is the pine that is being referenced, whether known or not. I didn't state 100%, I stated almost and slash is another that is resistant but not as well referenced.
As to these hybrids they are harvesting today vs. the longleaf or even slash they harvested yesterday (in terms of longleaf harvest dating) I agree they aren't very if at all resistant. Believe it or not there is still high quality heartpine and cypress out there you just aren’t going to find it at the big box stores or from the big mills cutting 4" tops.
From Gene Wengert, forum technical advisor:
Mature longleaf is the only nesting habitat for the endangered red-cockaded wood pecker, so many of the maturing sites (public or private ownership) cannot be harvested due to the Endangered Species Act. This bird only likes trees that are at least 75 years old. There are lawsuits about this and in fact there are some longleaf sites that are not 75 years old that have been restricted from harvest because they will be nesting sites in the future. (I am not saying that I agree, but it is the way it is now.)
Longleaf pine is being planted in plantations (hybrids too) but most landowners cannot afford to hold the timber for 150 years without income; they do have to pay taxes and have the risk of insect or fire damage causing a financial, so such an investment is risky indeed. Also, investing money without substantial return for 150 years is just not the way we do business. Planting loblolly and getting thinning income and also a harvest income in 25 to 30 years makes more financial sense. Of course, selling the land for development can provide even more income than growing trees.
The heartwood of cypress that has less decay resistance now than the heartwood of old growth will not change. Once heartwood is formed, it is dead and constant in composition. So today's heartwood in young trees will be the same in 50 years or more. This loss of decay resistance in second growth is well documented and the growth difference or explanation can be found in several papers, but I do not have the time to look them up. Note that the higher heartwood decay resistance of old growth cypress is also mentioned in Table 3-10 in the Wood Handbook. I believe that your statement "tainted by emotion and is based on nothing more than subversive opinion" needs to be withdrawn as the scientific evidence states otherwise.
Regarding the decay resistance of slash pine being high, this is in contrast to the US Forest Products Lab, as they do not have that info in their discussions or tables. Although a pine mill today may bring in tree length logs to a 4" top, most of them cut the logs into various lengths (mechandising is the name of this process) and will put the smallest diameter stock into chip or pulp mills rather than into the sawmill. It is not economical to process such small logs in a modern mill.
Incidentally, you did in fact say 100%; I quote you: "...yesterdays SYP which was almost 100% longleaf ..." This statement is incorrect even if you define yesterday meaning 100 years ago, and not 75, 50, or 25 years ago. There was more longleaf in the deep south, but that ignores VA and NC and a few other southern states. A good book is Utilization Of Southern Pine by Peter Koch. It is a USDA Handbook.
