Most deck joists are wood. Fungi and water increase rotting, so the question is, how much clearance should you have between the ground and the deck joist? What is the minimum deck joist ground clearance?
Deck joist should have a minimum of 6″ (150mm) clearance between the joist and the ground. Deck joists will rot in proportion to contact with moisture and fungi. The higher the level of moisture and fungi, the faster the joist will rot. Dirt contains a high level of fungi and water; contact will accelerate wood rot.
There are several reasons for joist ground clearance and ways to minimize the distance. Six inches (150mm) clearance is a quick rule of joist clearance from the ground, but this distance can be reduced with certain construction technics. Steps can be taken to reduce deck joist ground clearance requirements, but I will explain a few later.
Why do deck joists need clearance from the ground?
Dirt contains a high level of fungi designed to breakdown organic material. Fungi is an excellent thing for plant growth and the planet overall. We are all dependent on micro-organisms breaking down organic material to feed plants and clean up waste. The problem is these beneficial fungi don’t know the difference between a deck joist needing to last for years and a twig, which needs to be broken down.
With wood being natural, we need to look to nature for the solution. How does nature prevent wood from rotting? Separation, distance between the wood and the fungi prevents the fungi from contacting the wood. Notice how long a standing dead tree can last in the forest, but within a few years of falling down, it will be decomposed into beneficial soil. Six inches (150mm) between deck joists and the soil prevents fungi from moving from the ground to the joist.
Think of clearance like UN PeaceKeepers, not to get political, but they prevent conflicts by creating space between two parties. The air between the joist and ground is this safe no man’s land preventing conflict.
“However, because the spores are so small – 10 microns long – air drag brings them to a stop in a mere 3 millimeters. When thousands of spores are ejected at the same time, however, some can travel more than 100 millimeters, or 4 inches.”Berkeley University of California
By installing deck joist 6″ (150mmm) above the ground, they are out of the reach of fungi spores. The fungi will grow in the ground and release its spores, but with the joist higher then 4″ (100mm), the wood cannot be seeded, preventing rot. The extra 2″ (50mm) is a safety factor. Just in case, we have some super flyers.
Protecting Joist from Water
Fungi spores are only one element of joist rot. Fungi, like most living organisms, need water. Protecting the joist from water is important in preventing joist rot. But of course, deck joists are outside, for decks are our bridge between our houses and the outdoors. Clearance between the ground and the joist is also about ventilation.
Providing sufficient air movement under the deck joist allows them to dry out after a rain or even after washing your deck off. It’s not water that damages joist but saturation. As long as joist can dry to below 19% moisture level, for fungal cannot grow.
“Wood always contains a certain amount of moisture; air-dried wood may contain as much as 18 per cent, although it is generally considered that wood in this condition is immune to fungal attack.”National Research Council Of Canada
I realize that decks are rained on, but again the ground clearance is for ventilation and drying, not stopping water. Wood dried to low moisture content level in less than 14 days will not rot. Most fungi take 14 or more days to sprout. In short, if the wood dries in less than two weeks, rot cannot develop. There are many factors in how soon fungi will grow, but drying out joists quickly after rain will prevent rot. The sooner, the better. The more air circulating under the deck, the faster the joist will dry.
Joist ground clearance of 6″ (150mm) or more provides sufficient ventilation to dry the underside of the joist. With air natural flowing under the deck removing excess moisture and helping the deck joist last for years.
Cheating to Win! What can be done to protect joists having ground contact?
For the sake of our conversation all joist within 6″ (150mm) of the ground, we will consider having contact. Regardless if the joists are 5″ (125mm) above grade or buried in the dirt, steps need to be taken to prevent rot.
Increase Wood Treatment to Ground Contact Level
Treatment level of the joist is the easiest way to prevent rot with joist in contact with wood. All treated wood in North America are stamped if intent for ground contact or not. Deck joist less than 6″ (150mm) from the ground should be treated for ground contact.
Most appearance grade treated wood is treated to 0.15 PCF sufficient level to prevent rot with non-ground contact wood. Joists in contact with dirt need a higher level of treatment to counter the increased level of fungi and moisture. Wood treated for ground contact will be treated to 0.40 PCF. Check the bar code, it’s clearly stamped. Timber Town in Calgary, where I buy most of my decking material, clearly print on their invoices if treated for ground contact.
In short, treatment is poison for fungi. More fungi need increased levels of poison to prevent growth. Remember those old spaghetti westerns where the hero has only six bullets left and twenty guys surrounding him. Give your wood enough bullets to live. Increase the treatment wherever there are higher levels of fungi.
- Wood will come in contact with soil, vegetation, leaf litter, and debris that can hold moisture for extended periods of time.
- You don’t expect good air circulation, especially on the underside and between decking boards.
- Wood components are installed less than six inches above the ground and are supported by treated wood or concrete without a moisture-transferring barrier/break.
- Wood is in direct contact with material that already shows signs of decay.
- Wood gets wet on a very frequent, recurring basis and doesn’t have time to dry adequately.
- You are building in a tropical climate.
Increasing wood treatment for lower joist meet many of these requirements. A good reminder that clearance includes leaves and debris, not ground per se, but containing high levels of fungi and moisture.
Other Then Treatment Preventing Rot of Ground Contact Deck Joist
Placing gravel over landscape fabric below ground level decks can also minimize joist rot. Remember fungi are living, breathing organisms covering the ground with landscape fabric with non-organic material prevent fungi growth. The gravel not only improves the look under the deck but also prevents fungi growth. Fungi need organic material. Gravel is non-organic, removing the environment required for fungi. Remove the habitat, remove the fungi.
Landscape fabric not only prevents sunlight from reaching the weeds but also provides an additional level of separation. The fabric is acting as a net catching whatever spores are released, preventing joist contact.
Porous material must be used to cover the ground under the deck. Using plastic under the gravel will prevent drainage. Trapping water under the deck, increasing the risk of rot with the additional water level. Fungi multiply rapidly in wet wood. Water drainage is essential to prevent wood rot.
If you would like to learn more about ground treatment below decks. Click on this link to read more about ground cover material under a deck.
Not just ground clearance, but there are many other ways to prevent joist rot. If you would like to learn more, click this link to another article on prevent joist rot.
Don’t forget the decking.
Protecting ground contact joist is only part of the solution. Bring the deck joist in contact with the ground can raise other issues. Consider the decking also. The underside of the decking can be at risk with ground contact. Most decking is not treated for ground contact and can prematurely rot. Remember, within 6″ (150mm) of the ground is considered ground contact in terms of construction.
Using 2×6 joist treated for ground contact installed at ground level places the underside of the decking 5 ½” (140mm) from the ground. Increasing the potential for rot and mould on the decking from close proximity to the dirt. Even if the joist may not rot, the decking can.
Conclusion for deck joist ground clearance
Ideally, deck joist should be 6″ (150mm) or more above grade to provide adequate separation and ventilation. If the deck height does not have sufficient ground clearance than the joist treatment level should be increased for ground contact. Along with ground contact treatment level landscape fabric and gravel will help to minimize rot with ground contact material.