I have read most of the threads here and elswhere regarding decay resistance of wood. Woods of choice seem to be white oak, black locust, sassafras, ipe, walnut, etc. I am planning on building an outdoor play structure for my kids and want to choose an appropriate wood. At the same time, I want to be able to balance availability and most especially, cost. Most of the preferred woods typically are on the higher scale of cost.
My question is, how much resistance do I need? Of the usual woods mentioned, is the resistance to decay noted for wood lying on the ground or not? For a play structure, if I limited wood in contact with the ground to a highly resistant species, can I use something like regular contruction timber for the remainder? Sure it will get rained on occassionally and sit through the winter covered in snow at times, but the exposure would not be constant as with on the ground. I live in upstate NY and I only expect that the structure will be in use for 10 years.
Replies
Mike,
Regular construction lumber won't last more than a year or two under those conditions. And for playground equipment you should avoid treated lumber. Consider cypress. Around here it is priced fairly reasonably.
BJ
Here chestnut is the wood of choice for decay resistance in outdoor applications. In vinyards and in farm fences it is always used and farmers swear by it. Gardeners also swear by it for poles to support transplanted trees. Here it is relativley cheap and it works. If you want something cheap that works in your area try asking a couple of local farmers for advice.
Philip
Decay resistance is a measurement of durability of wood that has ground contact. It is generally determined with stake tests where stakes of the heartwood of various species are set in the ground and then monitored for a number of years.
Non-decay resistant species will generally fail with less than 4 years. Slightly decay resistant material will generally fail within 7 years at most. Decay resistant material will tend to last more than 15 or 20 years but this is considerably variable.
The variables that affect a pieces in situ service life will depend on the soil itself (eg acid or akalai), drainage (as with gravel in the bottom of the hole) and how the post/stake is actually placed in the ground. In regard to this latter aspect, setting a post in concrete will generally shorten its in service life because of the associated pH of the concrete. Depth of penetration is also a factor because if you get below the aerobic layer of the soil, decay will not occur because these fungal organisms require oxygen (this explains why deep pilings into rivers and for foundations can last for literally hundreds of years. The aerobic zone is generally the first 6 to 8 feet. Weather and the soil temperature are also a factor -- if the ground remains frozen for long periods of time during the year, the wood will decay less rapidly than if were located in a more temperate climate. Soil moisture contact is also a very big factor (for those living in arid environments).
Additional variables, relative to the wood itself include, the amount of extractives that are present. There is a distribution of wood extractives in the tree with greater concentrations in the butt. Oftentimes the very center of the log/tree is not the most decay resistant which is why, even among decay resistant species heart rot can occur. There is also variability within trees to the amount of extractives that are present based on probably some genetic predisposition and also age. For example, the wood of old growth decay resistant species like Redwood and Western red cedar are much more decay resistant than second growth trees of the same species. But this is not entirely consistent because very old trees that are either unhealthy or at the end of their lives can be somewhat less decay resistant than is normally expected. Generally however there are other indicators (such as lumber grade degration that make this apparent).
The size of the member is also a factor. Larger pieces tend to last longer than smaller pieces. The presence of bore holes and metal fasteners also can significantly decrease in situ service life; the bore holes open up the interior structure of the wood to colonizing organisms and the presence of metal fasteners contributes to electrolysis or chemical reactions that may effect extractive chemistry.
I would normally recommend that for ground contact woods especially structural members (such as primary posts) that individuals used pressure treated material that has the highest available ground contact number (which indicates a level of wood preservative retention). However with the changes in wood preservative chemistry (primarily the removal of arsenic from formulations) will likely mean that the treated wood may be less decay resistant than before. I am not certain if this is a good idea or a bad idea. Yes arsenic is a potentially dangerous substance but in most instances it is already present at some level of concentration in the soil and if reasonal cautions are exercised (for example hand washing of children who have played on treated wood structures before eating), human exposure is generally very limited.
And contrary to the advertising message of companies that produce products like Thompson's Water Seal, such treatments, unless they contain a fungicide in the formulation are not a good idea. If you apply an oil without a fungicide, the oil becomes another available food source for the organisms. Application of a wax however is a good idea because it helps the wood shed water.
Niemiec1,
Wow, thanks for the great info. I appreciate the time you took to respond like that. Given your explanation for how decay resistance is measured, if a member is not in contact with the ground, is there a generally accepted percent increase in life you can expect? In addition, it would seem to me that for non ground contact, I could use a wood that does not offer superior decay resistance and that might be a little more gentle on the wallet. Sound reasonable or no? As I stated earlier, I don't need the thing to be the next coming of the pyramids.
With regard to pressure treated lumber (as for example CCA) there are generally two levels of preservative retention -- .25 and .40 pounds per cubic foot (I think). The higher level of retention is definitely required for ground contact but you still need to have preservative tendencies (ie decay resistance) in areas of overlap and joints. These areas remain wet and as such are prone to fungal degradation.
So with this as an analogy, using timber with a lesser decay resistance (as would be appropriate with the use of the .25 level of retention of CCA) is OK for above ground applications.
The basic rule of thumb is that if the wood is wet, does not have good air circulation around it and has nooks and crannies for the fungi to hide, you will get decay.
Wood above ground may not decay but it can degrade from repeated wetting and drying. The wood can warp and check and split so decay resistance should not become the only criteria in selecting wood. The problem with woods that check excessively is that these fissures allow the fungal spores to penetrate deeply into the wood where moisture contents are generally higher and potentially high enough to facilitate organism growth (ca > 25%MC).
As I indicated in my previous message, I would probably opt for pressure treated material for posts and stuff with ground contact. For the other material, I would use a naturally decay resistant wood. I would use different grades of material -- appropriate grades for the structural applications (eg beams) and clear (clearer) grades for handrails and such, and then use the least acceptable grades for things like decking or pickets.
If you can designate a few woods, I have some test data from Oregon that will give you some comparative numbers on some West Coast species. Potentially some of this data may have applicability to some of your local woods.
Stanley Niemiec -- Wood Technologist
Paint is a wonderful protectant. Besides kids like colorful thiings, and once the swing set becomes an embaressment to your teenagers you can properly dispose of it as being too decayed to save..
Use pine or some other cheap wood.. paint it and save the money for the kids college fund.
And I bet you could replace an 8' 2x4 30 times for the price of hardwood. Might not be a bad idea to make the decking from plain ol 2bys, and save the $$ for the more structural or ground contacted pieces.
well actually it depends..
I pay the same or more for cheap pine/fir then I do for white oak.. Oak is 80 cents a bd.ft. and so is pine..
I just bought 230 bd.ft. of Black walnut today for $50.00 It was a veneer grade log with a defect in it so instead of 12 feet of veneer grade I got 8 feet plus four feet with a defect in it..
I needed 4/4x8" boards and that's what they sawed for me.. granted I got two twelve foot long flawless boards but I also got the rest with a knot hole on one end leaving me with 8 foot long flawless boards...
start buying from a sawmill instead of a store, you'll be amazed at the differance in price..
"Paint is a wonderful protectant."
For UV radiation yes but unless you go back to the lead based paints of old, they provide little or no decay resistance augmentation. About the only thing they do in this regard is to facilitate the shedding of water which can be also accomplished with paraffin based treatments.
You are absolutely correct about softwood species being a less costly option in most cases.
A certain amount of care needs to go into the design and selection of materials for children's play structures primarily because of liability issues. A person may design a play structure for a limited number of years based on the use expectations of one's children and their current ages BUT unless you thereafter tear it down immediately, it can become a danger to other children especially if it begins to decay or someone uses non-galvanized fasteners that rust away. My guess is that the porch collapse that recently happened in Chicago will likely be attributed to rotted non-treated, non-decay resistant wood and rusted fasteners. There is a rule of thumb relative to decayed wood vis a vis its strength properties -- if the wood has lost 10% of its weight, it has lost 90% of its strength.
If someone knowingly opted for inappropriate materials solely on the basis of cost minimization, I think they would be a sitting duck for any opportunistic lawyer were someone hurt on it. And with this as a consideration, it might not be a bad idea to go to the library and find a book with some plans so that you can model your structure after something that is already published.
what you say is certainly true were you to design swings for others.. I know of no way for a kid that gets hurt on a swing set designed by daddy to collect damages..
Further any parent would see failure long before it became a serious safety issue (especially with his own kids at risk)
Frankly most kids use a swing set for at best 5 years before it's outgrown. It then hangs around forlornly untill the space is needed for the "fort" or tree house etc..
Regarding paint,,
most houses are painted to provide some measure of rot protection. Eastern white pine is sometimes used for trim (at least according to a recent article in Fine home building and with paint seems to hold up very well..) I believe that exposed to the weather it would last a couple of years at best..
Frenchy -- I really respect your pragmatic and hard learned knowledge but with regard to paint as a "measure of rot protection" I have to stretch to accept that.
Fungi need three things (sort of like the fire triangle) to grow. They need oxygen, moisture and a food source. Deep pilings are sunk so deeply into the soil that no oxygen is present so they can last for many centuries. If wood is below the fiber saturation point (ca 25% MC) there is generally insufficient water for the fungi to use for their metabolism. They need free water within the cell lumens. And if there is too much water as with sunken logs, especially if the water has a very low oxygen content, the fungi cannot survive or do much damage. Finally with regard to a food source, the presence of either naturally toxic extractive substances or impregnated man-made chemicals creates an environment/food that is toxic/unedible.
The only thing that paints do is retard moisture adsorption (either as a film to diminish water vapor transmission or as a film to facilitate the shedding of liquid water. In and of themselves, paints, unless there is a fungicide inclusion, will not prevent decay. In fact, with regard to millwork products like window sills, rot can occur under the paint.
The most effective way most buildings avoid excessive decay is to insure that non-decay resistant woods are more than 18" from ground line. In the county I live in Oregon, it is part of the building code and strictly enforced.
Well made arguement. Your points are certainly validand accurite... I'll grant that it's a strectch to accept paint as a decay preventer, yet an unpainted piece will decay sooner than one that is painted.. So indirectly I guess it would be OK to make the statement.
But you are correct, paint in and of itself does little to prevent decay.. I mean you can't set a gallon of paint next to a board and prevent rot.. However a well applied film of paint on all suraces will delay decay..
Hi all! Mike, I hear you. Ipe is the wood of choice for my kids. Tough, hard, and is very SPLINTER resistant, if one rounds over cut and treats the ends with Anchor Seal, or similar. That being said, PORT ORFORD CEDAR is an excellent second choice. The blonde of the cedars, is is very hard (think Oak), decay resistant and has a wonderful, sweet smell when worked. Ipe dust is a a horror show by comarison. PORT ORFORD CEDAR is much less expensive than IPE or red cedar. If there is drawback to P.O. Cedar is that it will check if not treated, not bad, certainly not like P.T., but more than IPE, the other drawback is that Yellow Jackets seem to like it. They don't eat it, but they seem to land on it and scratch it somehow, leaving faint marks somewhat like a weld, but they don't seem to break the surface of the materiel. It's weird I know and could be related to the fact that I live in a fairy rural area. (My deck railings are P.O.Cedar, the deck itself is IPE) Other than that, it is pretty bulltproof, hard, smooth, greys up nice. Won't take paint though. Good choice for a swingset. For ground contact, I would suggest Trex or similar composite. Good Luck!
John
Stanley's tretise, below, is (as usual) an authoritative and complete description of the factors in wood rot.
One strategy I use with wood in the ground is to use a decay resistant species (redwood here in the grand state of confusion) and further treat the base, the end that goes into the ground. I wrap paper towels around the base from the bottom to a couple of inches above the ground level and soak with copper naptheneate solution (get it in hardware stores). Then, I put a plastic bag around the soaked towel and keep it soaked for a week or so. Then, you've used lower hazard natural preservative above ground and more effective technological preservative below ground.
Good luck. Digging holes is not my favorite job!
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