It’s the Decking that Limits how low a Deck can be


I love the convenience of low backyard decks. A perfect bridge between enjoying your house to enjoying your yard. A low backyard deck allows you to transition fluidly from yard to house and back with minimal steps. But how high should a deck be off the ground?

Both composite and wood decking needs to be at least 7” (175mm) above ground. You can upgrade the joist for ground contact, but still, the underside of the decking needs ventilation and separation from the ground. Anything lower than 7” (175mm) you should consider a concrete patioOpens in a new tab., which is better designed for ground contact.

First, we need to discuss why decking needs to be raised. Followed by construction considerations with building a deck so low. Finally, cheating to win, what can be done to help a ground-level deck survive and be enjoyed for years.

Decking must have ground clearance

Both composite and wood decking need space between the groundOpens in a new tab. and the underside of the decking. Regardless of decking material, all decks need to be designed with open clearance below the decking. The good news is that most decks have 2×8 joist, providing 7 ¼” (184mm) of clearance if the joist are not buried. The height required for the decking is built right into the joist.

“I frequently go back and speak with homeowners and their concerns. Mold is a serious issue for ground level decks.”

DennisOpens in a new tab.

Most treated wood decking is treated for non-ground contact. Ground contact increases moisture and fungi levels beyond most decking treatment level. A hoodie will not protect you against the freezing winter winds. You need a winter jacket. The same is with decking. Yes, it is treated to last for years outside but not touching the ground. It’s not treated to that level. In relationships to ground contact 6” (150mm) is considered clearance. Anything closer has the potential for water and fungi to transfer from the ground.

Composite decking also has limits for ground clearance.

“Minimum ground clearance required 50mm over concrete, 150mm over dirt. Drainage and airflow beneath the deck is also very important to ensure a good looking long term deck.”

TrexOpens in a new tab.

Always a good idea to follow the manufacturers’ recommendations. I see it no difference with Trex’s products the number one seller of composite decking. Trex’s decking must have 6” (150mm) ground clearance. Raising the decking to at least 7” (175mm) from the ground after accounting for the decking material, which is often 1” (25mm) or thicker.

TrexOpens in a new tab., FirberonOpens in a new tab. and TimberTechOpens in a new tab. require only 1 ½” (38mm) sleepers on concrete surfaces. Reducing the clearance by ½” (12mm) not sure about the discrepancy but either way, clearance is required over concrete.

Deck height because of moisture

The main reason that composite decking should be raised is moisture. Fungi rot needs water, dirt holds water, being to close increases the moisture level of the decking, encouraging rot in organic material. With composite decking, there have been some major strides made to help protect the decking from moisture.

Trex and many other composite decking have moved to capped decking. A huge step forward in preserving the decking and prevent mould. But most capped decking is only capped three sides. Leaving the bottom uncapped and unprotected. With raised decks, this is not an issue, as the decking itself shelters the bottom. Leaving the bottom un-capped reduces cost, capping is expensive but also allows the material to breathe, exhaling moisture while adapting to temperature change.

Deck Height for Ventilation

Equally important with decking is ventilation. Decking needs to be removed from the water but also have sufficient ventilation both to move excess moisture from the underside of the decking but also temperature.

Without free-flowing air under the decking, the temperature under the deck can become extremely hot. Think of your car on a hot summer day, with the windows closed. Without the cooling effect of air movement, composite decking can over expanding causing buckling and decking to pop up leaving the face screws behind. Rasing the decking allows for air movement.

Construction considerations for Ground Level Decks

If a 7” (175mm) deck works well for your yard, here are a few things to consider for construction. A surface area less then 7” (175mm) a concrete patio will much better serve you. Providing a durable outdoor space for you to enjoy that can survive ground moisture without ventilation.

The ground below a low deck needs to prep before building a deck.

Levelling the ground will prevent pooling of water under the deck, decreasing moisture levels. All plants need to be pulled to prevent weeds from sneaking up between deck boards.

To help with weed suppression, cover the ground with a good quality landscape fabric and cover with a 2” (50mm) of gravel or non-organic material. The landscape fabricOpens in a new tab. will stop weeds from growing and the gravel will keep the fabric in place and lower air temperature under the deck. For a fuller discussion of under deck, treatment read this article Best Ground Cover Material for Under a Deck.

Also, divert all downspout away from the deck as not to keep the deck as dry as possible. Any additional water could cause damage to the decking or rot the joist from underneath. Replacing the top few inches of topsoil with gravel will assist in draining rain away from the deck.

Consider Increasing decking gaps

Ventilation is so important for lowering moisture and temperature under a deck. With limit ventilation under the low joist increasing gapping will allow more hot air and moisture to escape. With slightly larger gaps, more moisture and heat will dissipate, a good thing for a low deck. Trex recommends a maximum of 3/8” (10mm) for air ventilation. Gapping decking greater than  3/8” (10mm) will create tripping hazards and will not look good.

Consider Flush beams to support Joist

Drop beams are the easiest way to support deck joist, but with minimal space flush beams may be better. Flush beams integrated into the deck joists will require no additional space. Hangerring the joist off a doubled or tripled up rim. The beams can still be supported with concrete footings below the frost level but without additional trenching for a drop beam.

Reinforced concrete beams with sleeper plate

Deck beams don’t have to be wood. With the proper steel reinforcement, a concrete grade beam can support a ground-level deck. Supporting the deck joist unaffected by water or fungi. Ideally, the concrete grade beam will be tied in with concrete columns extending below the frost level.

Increase Treated Ground Contact Joist

The joist in contact with the ground requires additional levels of treatment to prevent rot. Ground contact lumber needs to be treated with ACQ (alkaline copper quaternary) to 0.40-PCF or higher. The ground with higher moisture levels and fungi need additional treatment to counter.

Cheating to Win, What can be done to help a grade-level deck survive

Water and lack of ventilation are the greatest risks to a low deck. Here are a few items that will help to keep your deck dry and ventilated.

Trex Rain Escape, normally installed to create a dry storage space under a deck but can be used to divert water away from the under the deck. The membrane covers over the joist, protecting them from rain but also draping between the joist carrying the water to a trough and diverting the water away from under the deck. Keeping both the joist and the ground below dry.

With wood decking sealing, the decking, especially the underside before installing, will decrease moisture and rot. Decking is treated for fungi growth, not water. By sealing the decking before installation will help to keep the decking dry, adding years to your decks’ life.

Some composite decking like Fiberon cap all four sides of the decking. The additional capping increases moisture protection for the underside of the decking, which is good for decking close to the ground, but it is advisable to seal end cuts. Wrapped all four sides the cut ends can soak up additional water and mushroom.

Consider using PVC decking manufactured without any organic material. A fully synthetic material that will not rot with increase moisture or ground contact.

Floor register style vents between joists will allow additional air to circulate under the decking. This can be further increased with rim joist vents. Drilling 3” round holes in every joist cavity will increase circulation, pushing more air through the joist cavities.

Whatever you do with your low deck, keeping it dry is essential. With the added challenge of proximity to the ground, extra steps will need to be taken to divert water away and enhance air circulation.

Do you need a permit to build a deck on the ground?

Before you start construction, always check with your municipality if permits are required. In Calgary, where I live and build decks any decks under 24” (0.6m) don’t require permits but I still like to build them to code. As good construction, practise and standards will only increase the decks’ life. No permit required. Good construction always required.

Ryan Nickel

A Red Seal carpenter, passionate about building decks to be enjoyed.

Recent Posts