Perhaps you are considering building a new deck. It will cost $15,000 or $25,000 or more (Read more about cost in Remodeling magazine’s Cost vs Value article), and you wonder about the longevity of such an investment. You’ve read that the decking companies warranty their new synthetic decking for 25 years or even for life. But will a deck really last that long?
Or maybe you already have a deck, but it is 8 or 10 years old and looks shabby, and you wonder about upgrading it. Replacing the flooring and rails could cost $8,000 or $12,000 or even $18,000. The new synthetics look good and promise low maintenance. If you’re tired of re-treating it every year or two, then “low maintenance” sounds tempting. 
Before I begin: I put a summary at the end of this article for you busy executives: [CHART].
Normal people can just read on.
The first issue is your deck’s frame – the skeleton that supports the floor and rails. All deck frames should be made of pressure treated lumber, which is guaranteed for life not to rot. Or to be eaten by termites. Good: I hate those creepy terrorists.
So this is encouraging, let’s look further.
As a Roman poet noted: Finis origine pendet.
Hold on: Is that Latin? You’re throwing Latin at me? I hated that dead language in high school, and now…
Sorry. Let’s just say that the end depends on the beginning. If your deck was originally built properly — with good, exterior materials like rot-proof wood and galvanized bolts and screws, was flashed and bolted properly to your house, has adequately sized joists and beams, etc. — then the frame should last decades — or longer. Really? “Decades” is a long time. And “longer” is even longer. I like this.
But how can you tell if the frame was, indeed, properly constructed? Well, do the floorboards sag when you walk on the deck? Does the deck wobble if a few people sway in unison? Do the stairs shake when your kids run down? Are any of the frame members rotted? There’s a good checklist in my article about deck safety. If you are unsure, have a professional deck builder inspect your deck. Don’t fasten new decking and new rails to a weak or shaky frame.
Ground contact PT returns from Exile
As I mentioned, your deck frame should be made of pressure treated wood. Arrogant contractors call it simply “PT”. But not all PT is created equal. The original PT, introduced in the early 1970s, was treated with CCA (chromated copper arsenate – wow, that sounds nasty) to protect it from rot and termites. The copper prevents rot, and termites cannot digest wood bonded with good ol’ As2CrCuOg. (What??) That original PT had enough CCA to be certified for “ground contact” use. For life. But in late 2003, the PT industry voluntarily eliminated the chrome and arsenic (thanks) and changed the chemical formula of the preservatives to ACQ, Copper Azole, MCQ, and MCA . But in doing so they also reduced the retention levels of the chemicals in all but the largest timbers from “ground contact” to merely “above ground use only”. Lowering the retention levels reduced the cost of PT lumber, but it also reduced its protection and longevity.
Now it’s Chemistry you’re slinging around? I hated Chemistry in high school. What does all this bunk with dates, “retention levels”, and that alphabet junk mean to me and my deck?
Sorry, I got carried away. More simply: If your deck was built before 2003, then its frame will resist rot longer than more recently built decks. Happily, the higher protection levels have returned to all framing and decking lumber last summer (2016). So newly built decks are better protected.
So if your deck frame is solid, let’s look higher: at the flooring and rails.
Deck Floors: PT
One of the oldest decking options remains one of the longest lasting: PT. It has a written lifetime warranty and comes in Premium – a grade better than #1. PT decking is also the least expensive. It does misbehave somewhat — split, cup, and splinter in places. But how long will a PT deck last?
I’d like to say “indefinitely”, but my personal experience began only a couple of decades ago. Here’s a 1993 photo of PT deck we built that year:
And here it was a month ago:
Ouch! What happened?
A violent windstorm toppled a monster oak tree onto this 23-year old all-PT deck.
The trunk of the oak tree was 16” in diameter and hit with major force. But because the deck was well constructed, damage was limited to parts of the side rail, some decking, a joist, and one brace.
Prior to that storm, the PT deck was solid and had aged well into its 20s. We repaired the damage. Here is the deck today:
Aside from Mother Nature’s anger, a properly built PT deck will last many decades. And with regular care, PT decks can look good for a long, long time. Some say it will last 40 years.[Ref 1] Well, I’m not sure about 4 decades, but we did just see a deck that’s two decades old, going on three decades.
Cedar has a great reputation for longevity. Indeed, cedar is very popular as house siding. The cedar siding shingles on my house lasted over 60 years, until I tired of re-painting. But don’t be misled, house siding is a vertical surface. It drains easily and lives high enough to enjoy the breezes. Your deck is flat, so rainwater sits around and soaks into the wood. And cedar has changed. Cedar earned its great reputation in the last century and earlier, when it available as old growth heartwood from the dense center of those magnificent giants.
But the remaining old cedar trees are protected now (thank you), and the cedar we can buy for decking is new growth, and mostly less dense sapwood. This cedar still resists rot, but not nearly as well as its parents; it contains less protective resin, which leaches out over time. Untreated, today’s western red cedar decking will last 10 to 15 years. Regular cleaning and treating can extend that life to 20, or perhaps 25 years.
People have used fir for deck floors since…wow, since before there were decks. In the 1940s and 50s fir was THE choice on stoops and front porches. Fir was (and is) straight, long, clear – and easy to work with. So carpenters love it. But Mother Nature does not. No. Fir has no natural resistance to rot, so take away that porch roof, and Mother rains directly on the fir floor, and it rots.
But not right away. If your fir floor is spaced for proper ventilation (1/4” between boards), gets good air flow below (at least 2 ft) and sunlight above (thanks Mother), and – this is key — you apply a preservative every year or two, then your fir deck should survive into its mid-teens, 15 or 16 years. I have seen fir last as long as 17 years, under ideal conditions. Notice that word “ideal”. Subtract some of those conditions, and fir rots sooner, much sooner — like 12 years or even eight. And when fir rots, it teases you. A couple of small spongy areas appear one year. The next year you replace two boards. The following year you need to replace five. And then ten,… and you realize your fir floor cannot be saved. It is gone. Too bad, those 1×4 boards had a classic look. Too bad, because there was a better alternative that is rot resistant, knot-free, and costs about the same.  Mahogany. I am not talking about furniture grade mahogany, from Honduras. I’m talking about Meranti mahogany from Indonesia or Cambara mahogany from Brazil. For more information about cedar decking, see my article “Which Kind of Cedar for your Deck?”.
My experience with mahogany decks is nearly as long as with PT – about 21 years. Mahogany decking survives well, especially if treated every year or two, except under the worst conditions. I’ve seen 1×4 mahogany flooring begin to rot after 16 years IF it’s close to the ground and gets no sunlight or ventilation.
Here’s a 16 year old mahogany deck. Note the total shade:
Without sunlight, dampness never leaves this low deck. So rot fungus creeps in and sets up housekeeping. Here the mahogany stair treads – only 6” and 12” above the ground — have begun to rot:
You can replace a few decking boards this year, and more next year, but the rot will win. The best solution is to replace all the flooring with a harder wood or with synthetic.
Here’s another mahogany floor that failed prematurely:
Lack of ventilation is killing this low mahogany deck after 17 years, but Mother Nature is not the only culprit here. No, the homeowner laid the mahogany floor boards too close together, and that trapped moisture and encouraged rot. Wood flooring should be spaced 3/16” on decks with good ventilation underneath and ¼” (or more) on decks with poor ventilation.
So what wood lasts longer than mahogany?
Who? Ipe. It is a very hard wood from Brazil, pronounced “E-pay”. Ipe is so dense it sinks in water. Really.
Not impressed? Then consider this: ipe has a Class A fire resistance rating, the same as steel and concrete. [Ref 6]
(Warning: here comes some technical stuff.)
On the Janka Hardness scale, Ipe rates an amazing 3,510 lb/f; so it is about 3 times harder than oak (Red oak rates 1220 lb/f and white oak is 1360 lb/f). And flex your muscles: to crush it, you’ll need 13,600 lbf/in2 of force. For more details about this amazing wood, see [Ref 4]
Such hardness translates into extreme durability and popularity for high traffic areas. Boardwalks by the ocean are especially harsh environments for decking; the salt spray, unrelenting sun, sand, and heavy foot traffic combine to wear out all but the strongest wood. Not surprisingly, ipe has been the choice for many large oceanside boardwalks — Atlantic City, NJ; Jones Beach in NY; Long Beach in NY; New London in CT; Disney World FLA, Canalside in Buffalo, etc. Parts of the ipe boardwalk at Jones Beach in New York was recently resurfaced – after sixty years. Impressive.
Ipe’s density also protects it from insects and rot. Insects cannot consume or bore into it. And neither can rot fungus penetrate Ipe. So distributors who sell Ipe warranty it against insect infestation and rot for 20 or 25 years.
As we’ve seen, rot is the primary killer of wood decks. What’s going on? Rot fungus invades wood and attacks its structure until the wood just falls apart.
Healthy wood cells Partially damaged
I would show you actively rotting wood, but it’s rather disgusting. The hypha tentacles break into….Well, OK, I’ll show you, but I warn you this photo is not safe for children or household pets:
Rot fungus needs four conditions to do its damage: water, but not too much; oxygen from the air; moderate temperatures, 70º to 90º F; and food — the wood itself. Remove any of these, and rot fungus dies. Winter temperatures limit the rot season in most areas, but oxygen is always present in the air. Treating your deck with a penetrating preservative will temporarily keep moisture out, but the best defense against moisture is to allow sunlight and ventilation to dry the wood. If you can, give your deck a few feet of air below and a few hours of daily sunlight above, and it will last beyond its teenage years and into its twenties.
If you’re not intimidated by a little organic chemistry, there’s an authoritative explanation of rot here.
PLASTIC TO THE RESCUE
In the 1990s, plastic was combined with wood sawdust to create the first rot resistant, low-maintenance decking, Trex. Over time competitors eliminated the wood fibers and offered homeowners all-plastic decking, which will never rot. Remember those 4 elements that rot requires? Water cannot penetrate plastic, and without wood, there is no food to sustain rot. It dies. Nice.
Twenty years of competition has brought us some very good looking all-synthetic decking. Here’s some:
The promise that originally established synthetics in the decking market was “No Maintenance”. Homeowners did not need to regularly treat this decking to restore its color or to extend its life, like they did wood. But “No” turned out to be too strong a promise. Decks still needed to be hosed off and sometimes scrubbed to remove mildew, which can grow anywhere. So a class action lawsuit quickly forced manufacturers to relabel their decking “Low Maintenance”. To understand more about the caring for low maintenance decks, see my article “No Maintenance, Low Maintenance, Mo’ Maintenance”.
Perhaps best of all, plastic cannot rot, so manufacturers warranty their decking for a modest 25 years to a more noteworthy “Lifetime”. Impressive. So if you put state-of-the-art synthetic decking and rails on a solid, well-build PT frame, your deck should last 40 years or more.
Unless Mother visits.
- Back to Main Text For an explanation of just how much maintenance synthetic decks need, see my article Synthetic Decks: Low maintenance, no maintenance,…
- Back to Main Text You remember the poet Marcus Manlius, don’t you? This quote is from his fourth book of Astronomica, written in the first century AD.
- Back to Main Text If you really want to know: ACQ is short for ammoniacal copper quat, CA is copper azole, MCQ is micronized copper quat, and MCA is the friendly name for micronized copper azole. Interested in their chemical formulae or their 27-character International Chemical Identifiers? Neither am I. But if you wish, you can read even more exciting details about the chemicals in deck preservatives in an article in Fine Homebuilding.
- Back to Main Text Some years mahogany (Meranti or Cambara) 1×4 costs more than Douglas Fir, some years it’s less.
- Back to Main Text Actually, there are about 20 species in the ipe family. As a group, then, the specific gravity varies from 0.91 to 1.10. The specific gravity of water is 1.0, so some ipe boards sink and some barely float.
- Back to Main Text Moisture, but not too much – 20 to 30% is adequate. In general, the moisture content of outdoor wood averages 14% or less. For a detailed chart of equilibrium moisture content by city and state, by month, see http://www.woodweb.com/knowledge_base_images/zp/equilibrium_moisture_content.pdf
Ref 2: Overview of PT and Cedar: http://www.kompareit.com/homeandgarden/deck-compare-cedar-vs-pressure-treated.html
Ref 6: Back to Main Text Fire rating of ipe, see: http://www.deckmagazine.com/design-construction/ipes-fire-rating-locating-utilities_o
Ref 7: http://ironwoods.com/wp-content/uploads/2013/01/Iron-Woods-Ipe-Warranty.pdf
Ref 8: An authoritative explanation of rot. Old, but still valid. http://web.mit.edu/parmstr/Public/NRCan/CanBldgDigests/cbd111_e.html
Ref 9: Meranti: http://www.wood-database.com/dark-red-meranti/
Ref 10: Cambara: To purists, Cambara is not in the mahogany family, although its properties for exterior use are very similar. See: http://ironwoods.com/products/our-species/cambara/ . See also: http://www.mataverdedecking.com/hubfs/Cambara_Decking_Sell_Sheet_20Nov15-1.pdf?t=1493407081515
Ref 11: How to find your way among dozens of types of mahogany — probably more than you’ll ever want to know about mahogany: http://www.wood-database.com/wood-articles/mahogany-mixups-the-lowdown/
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