I have built many decks low to the ground, two feet or less. Inevitable, the question is always raised, about skirting. How can we close in the underside of the deck, beautifying the deck while hiding all that ugly underneath? Which has led me to give skirting some thought along with some research, ensuring that deck skirting is a net positive for the deck. Deck skirting ventilation being one of the critical elements for the well being of the deck.
Discovering that a deck’s skirting requires one square foot of ventilation for every 150 square feet of deck to allow moisture to escape from under the deck. Allowing the decking and substructure to dry, preventing rot, mould and decay from destroying the deck. Skirting ventilation is most effective with air moving from one side of the deck to the other.
One foot per every 150 feet of deck is a good quick rule of thumb, but the importance of venting varies according to region and precipitation. Areas with more precipitation require more ventilation to remove moisture from under the deck.
Well researching one place that regularly came up having issues with moisture destroying decks was Sydney Australia, which makes sense as they receive, on average 1309 mm [51.5”] of precipitation every year. Where I live and build decks in Calgary, Alberta, we only receive 471 mm [18.5”] of precipitation very year making ventilation less critical. But the more I research the more I am convinced it may be less critical but still important for the life of a deck, even in a dryer climate like Calgary.
Moisture Trapped Under Your Deck.
If you ever had the pleasure to visit the Conservatory at the Calgary Zoo, the first thing you notice as you walk in is the humidity. You go from the dry air outdoors into what feels like a sauna, dripping with water. The same can be happening under your deck.
Whenever it rains, the water runs off your decking, through the gaps and under your deck. Without ventilation creating a cross breeze, this water has no place to go. Pooling under your deck, evaporating, rising to form clouds of moisture trapped below the decking. Condensation forming on the underside of the decking, increasing the moisture level of the decking. This increased moisture then allows mould to grow or worse the decking and joist to rot.
“Wood begins to suffer damage if its moisture content remains at over 20% for long periods of time.”Wood Products
Decking boards with their capillary cells absorb and release water affecting their size. Translation, when wood gets wet, it gets bigger and shrinks as it dries. The potential increasing for decking damage when the deck boards moisture is not consistent. The top of the decking board dried by the sun but with the underside moist with the trapped water under your deck. This expansion of the underside and contraction of the top of the decking causing buckling, twisting and cupping.
On top of the potential damage to your decking, the water pooling below your deck becomes a breeding ground for mosquitos. Mosquitos larva requiring water to live, stagnant rain under your deck or pooling condensation will do if it’s undisturbed for a week or two. If under your deck is not drying out after a rain, these pesky little creatures have a window of time to flourish.
Annoying mosquitos hatching under your deck is not the only problem with increase moisture under your deck. Wood eating insects also thrive with increase moisture. They are eating away at the structural strength of your deck. Often unnoticed as the bore away inside the joist.
“Wood boring insects need a certain amount of moisture in timber of around 20% and higher to thrive.”Standard Heritage
It is becoming very apparent why wood is kiln dried to 19%. Being that magical moisture level preventing wood from rotting or becoming a home for damaging insects. Bringing us back to our question.
Do you need ventilation under a deck?
Yes, ventilation under a deck is beneficial in reducing moisture, extending the length of your deck.
In water climates, this needs to be more than the gapping between deck boards. Gapping is helpful to allow water to run off the decking while allowing room for expansions and contraction of the decking. But once the water gets below the deck, it needs air movement to take that moisture away. Air moving over the top of the deck is not enough; it must get under the deck.
I cannot explain the science of it, but another example from within my house and possibly yours is a compost bin. In Calgary, we are all given these little composite bins to collect food waste to be composted by the city. This bins lid has around 100 holes on the lid. Rotting food stinks. I cannot smell the compost when the lid is closed. Despite the holes, there is not enough air venting for the smell to overwhelm my kitchen. The gaps between decking boards are the same as the holes, but they do not provide enough air movement and circulation. The moisture is still trapped under the deck.
A deck needs fresh air moving under it, dryer fresh air blowing in from one side of the deck, collecting water from under your deck and taking it out. Therefore, a deck should have ventilation on multiple sides. Decks.com will go so far as to recommend that any decks 12” or less from the ground the perimeter should be left open to allow air movement.
With decks higher than 12” lattice, screens or ¾” gapping between skirting boards is essential, providing a cross breeze under your deck. Keep in mind a deck needs one square foot venting for every 150 of deck. Regardless of the decking used, ventilation is important.
Does a deck need to breathe?
Yes, as your deck breathes, water is taken out of the wood. An exterior deck needs to fully dry, returning it to the safe moisture level below 19% preventing rot and insects. Your decks’ breathing is simply bringing in fresh dry air in exchange for moist air under the deck.
I hope I have established the importance for all decks to have ventilation, but the reason will vary with the decking. Different decking material will respond differently to increase moisture.
Wood Decking needs Deck Ventilation
Wood decking can further be divided into two groups softwood and hardwood decking. Softwood decking commonly are cedar and Pressure Treated decking. Hardwood decking commonly are Ipe, Cumaru, Garpa and Machiche. Wood classification is not based on wood density but tree type, but with these decking, the hardwood is a denser wood than softwoods.
Softwood decking is least affected by moisture. Pressure-treated decking is treated to prevent mould growth and being a softwood will expand and contract with minimal damage. Old-growth cedar decking is naturally resistant to rot. Both being the most forgiving of moisture under your deck, to the point where it hardly matters.
Hardwood decking requires a consistent moisture level for a long life. If properly installed will provide one of the longest-lasting deckings. This Old House gives a life expectancy of Ipe decking at 40 years, many times longer than pressure-treated and even twice as long as composite decking. But ventilation is important with Ipe as contrasting moisture levels and temperature between the top and bottom of the decking will cause bulking and cupping of decking. With the underside expanding but the top contracting causing undue pressure on the decking, this pressure is released through cracking, twisting, or cupping of the decking.
Composite Decking requires Ventilation
Our main concern is moisture causing mould. Mould and fungi can grow on the wood fibres in composite decking. Blacking mould hard to remove from composite decking. Literally putting a black spot on your deck. Ventilation under a deck will remove the damaging moisture keeping your composite decking looking great for many years.
One solution for composite decking mould is capping the decking with plastic cap stock sealing the wood, preventing mould growth. Most quality composite is capped on three sides, the visual sides keeping their pristine look for years. Unfortunately, cutting of capped decking leaves removes the capping protection, with untreated end cuts.
PVC decking does not include any wood fibres. Mould grows on organic material, aka wood., No wood fibres in PVC decking, equalling no food for mould. PVC is the least likely of all decking to develop mould on the underside. Making it a great choice in environments with high humidity.
Decks need to breath and dry. Regardless of the decking material, a deck needs ventilation. The type of decking may affect the reason for ventilation, but all decks require ventilation. Take some time this summer, relax and take a deep breath, exhaling all of life’s stresses and let your deck do the same.