Building outdoor woodworking to resist rot
While it’s true that trees thrive in the great outdoors, the material woodworkers know and love isn’t perfect for living outside. Wood used outdoors needs to be carefully protected if it’s going to last, because its one big enemy is rot – something indoor wood doesn’t usually face.
Teak (top), cedar (middle) and white oak are all species of wood that are naturally very resistant to rot. Using these woods doesn’t mean you can design a piece poorly, but they will certainly give you a leg up compared to most of the other woods on the market.
There are many factory treatments for woods that will increase their ability to remain rot free for years to come even though they’re outdoors.
Cut and Paste
Once mitres were cut in the ends of these deck boards the cuts were treated with preservative to slow rot.
Not Too Deep
When screws are driven below the surface of the wood the resulting cavity is great at collecting water and bringing it into the wood, increasing rot. Screws driven so their heads are flush with the outer surface of the wood reduce the chance of rot.
Top It Off
End grain will soak up water faster than any other surface of wood. Fence posts, for example, could be capped with wood, plastic or metal caps to keep water from getting into the wood and starting the rotting process.
What causes rot?
To prevent rot, it’s important to know what causes it. Rot is an advanced level of decay that can be identified by discolouration and softening of the wood. It requires four elements to develop:
By removing any one of these four requirements, rot progress is prevented and the fungus goes dormant. However, if at any time all four are present, decay will pick up where it left off and advance.
How can we prevent rot?
Option 1: Reduce oxygen to less than 20%
Unless we’re building furniture for use below the waterline or outer space, this is very difficult and not at all practical. Even a carefully selected and applied finish will not stop oxygen from getting into wood.
Option 2: Keep temperature below 4°C or above 40°C
If you happen to live in a part of the world where it never gets above 4°C or below 40°C, it may bring you a little slice of happiness knowing you don’t need to worry about rot. But for the rest of us, we shall keep looking for a good rot prevention method.
Option 3: Guard the food source
The heartwood of most species is naturally more decay resistant, so avoiding the use of sapwood can be a good starting point. The heartwood of some species is more resistant to rot than others, containing extractives that are either repellent or toxic to fungi. Many oily or exotic woods, such as cedar, redwood and teak, fall into this category, as do some hardwoods we are all familiar with such as black walnut and white oak. Some common woods with little or no resistance to decay include beech, birch, ash, hemlock, ponderosa pine, red oak and maples.
Unfortunately, not all of us have the budget or desire to use black walnut or teak for our outdoor projects. Applying a chemical wood preservative to wood can be an effective way to deter decay. There are numerous options, but the most important thing to understand is that penetration is key because if the preservative doesn’t penetrate, it doesn’t protect. Some species of wood are more receptive to this sort of treatment, and end grain absorbs more readily than long grain. We can buy pressure-treated lumber at lumberyards in which the chemicals have been forced into the wood by positive pressure or vacuum pressure. Wood preservative is also available for application by brush, dipping or soaking.
Whether you buy pressure-treated lumber or treat it yourself, it’s vital to apply wood preservative to any surfaces that are cut, drilled or otherwise shaped since penetration is usually limited to the outside 1/4″ or less and any machining you do to the lumber is likely to cut through the preserved wood. Remember to treat the wood before it is put in place; getting preservative into joints and around screws and bolts is extremely difficult at best.
One last note: even wood that has been carefully treated with a preservative is rot resistant, not rot proof.
Option 4: Keep moisture content below 20%
Keeping wood below 20% moisture content is an effective way of preventing rot, which is why rot is not usually an issue for interior woodwork. This is the most effective way to prevent rot and what we’ll focus on below.
A quality exterior finish will go a long way to keeping moisture out of wood’s pores, but it won’t form a true waterproof barrier between the elements and the wood so other techniques should also be used. Also, even the best exterior finishes wear off and expose wood to moisture. Reapplying a finish when necessary will go a long way to protecting the piece of woodwork from the elements.
For items like patio furniture, the best way to make them last is to bring them inside or keep them sheltered during the wet season. Outdoor structures like decks, stairs, planters and pergolas need to stay outside for obvious reasons. They require different techniques to keep the moisture content down. Rain will increase the moisture content of the environment and everything it falls on. However, most exposed surfaces will dry quickly enough that decay is unable to make much progress, so fortunately putting canopies over all woodwork isn’t necessary.
What we do need to be aware of are areas where water sits, or is trapped, allowing more time for it to absorb into the wood. Trapped water is harder to see and to remove, so the key is to anticipate where it may occur and address it in the design phase. Two 2×4s ganged together may be cheaper than a 4×4, but they have a lot of potential to trap moisture between the two pieces.
To prevent or reduce pooling water, slope horizontal surfaces by at least 1° and position any cupped boards cup-side down. In cases where pooling water can’t be avoided, such as on deck joists, apply a metal or elastomeric sheathing prior to laying decking. Applying a water-repellent finish is a great way to reduce moisture absorption.
When laying decking consider that wide gaps allow tree debris to fall through more easily than narrow gaps, and don’t butt the ends of boards tightly together. Be careful when screwing down deck boards because overdriving a fastener will dimple the wood, leaving a cavity in which water can pool. Consider attaching boards from the edges or from below to prevent this problem. This also creates a longer-lasting, aesthetically pleasing deck without any visible metal fasteners on the top surface to burn unsuspecting bare feet on hot summer days.
In addition, plants and debris on wood surfaces tend to attract and hold moisture. Elevate plant pots off decking with feet, and don’t let vines or other plants grow on wood structures, unless that’s their purpose, like a trellis. Line wooden planters with a barrier to keep wet soil away from the wood and place them on gravel or concrete instead of directly on soil.
Capping posts can be an effective way to shed water and has the added benefit of covering the end grain which absorbs water more quickly than long grain. Don’t forget that the bottom of posts will also readily absorb water, so it’s best to keep them elevated off a surface that can hold water by using pier blocks or gravel, for example.
If you’re building outdoor chairs, cabinets, planters or other woodwork consider adding a metal or plastic foot to the underside of any wood that comes in contact with the ground. This will greatly reduce moisture wicking up the end grain and encouraging the rotting process.
Although there are many uncontrolled variables outside, building outdoor furniture and structures that will last a long time doesn’t require expensive equipment, materials or complex techniques. Just remember that careful workmanship, design and material
How Destructive Are UV Rays?
The ultraviolet portion of the solar spectrum degrades wood. This process is called UV weathering and will wreak havoc on the wooden projects we make for the great outdoors. The sun’s rays mainly damage the organic polymer between the walls of the wood’s fibres. This process causes the fibres to lose structural rigidity, and then they decompose. Once the wood breaks down, the finish that’s on the wood will crack and degrade, allowing moisture to get into the wood and cause rot. A specialized finish that will stand up to UV rays is critical in preventing the finish from breaking down and exposing wood to even more moisture.
There are many exterior finishes on the market. Evo Home Finishing has a product called Uvio, which is part of their Ligna product line. It’s an anti-UV primer that delays the effects of sunlight on wood. This is a colourless product and must be top coated with a water-based finish. They recommend applying their Hybri-Deck, which is a water-repellant penetrating oil, on top of the sealer for maximum protection. Hybri-Deck is available in a wide range of colours. Visit EvoFinition.ca for more information.
Livos Canada has a couple of exterior products that will go a long way to protecting exterior structures and woodworking projects from UV rays. Alis Decking Oil and DonnosOutdoor Wood Oil are both formulated to stand up to the elements and are available in a wide range of colours. Visit Livos.com for more information.
Chris Wong - [email protected]
Chris is a sculptural woodworker and instructor.