Preventing below Grade Deck beams from Rotting


So you want to build a deck close to the ground, having less than 15” above grade. Well, that’s going to create some challenges. No worries with challenges come opportunities. Our three main challenges will be water, ground contact and frost. All can be managed to build a low deck less than 15” from grade.

A deck beam below-grade needs to account for additional water, ground contact and the effect of ground-level frost. Therefore below-grade beams should be built from wood treated for ground contact. Also, remove dirt around and below the beam to limit ground and water contact.

Let me explain better what I mean, and some alternatives to deck beam below grade. Starting with the challenges and then some alternatives.

Challenges with Deck Beams below Grade

When building a deck, addressing the three essentials to allow you to enjoy your deck for years to come.

  • Water
  • Ground contact
  • Frost

All three play into the durability of your deck. Both water and ground contact will cause your deck joist and decking to rot. Which is never a good thing. Your deck’s relationship to frost will determine how stable your deck is. A deck must be supported from the concrete below the frost level. Without below frost support, the deck will rise and fall with the seasons. Causing undue stress on the material, causing potential failure of the deck. Also, an uneven deck is not very enjoyable. You do not want to be walking uphill to the deck railing.

A deck beam being below grade increases these risks. With the beam’s proximity to fungi carrying soil and water. A deck beam below grades is most likely to rot. A rotten beam is an unsafe deck. If the beam goes, everything goes because the deck beam supports the entire deck load. This is why so much attention is given to the beam’s strength in design and construction. It is the critical piece of a deck and one of the harder to replace if it begins to fail. Not like decking which can be pulled and replaced. Repairing a rotten beam under a deck is challenging to say the least. I have done it before but it’s a lot of work. A beam below grade is going to increase the challenge all the more. Do it right the first time.

Well then, how can we get it right the first time?

What are the keys to installing a deck beam below grade that won’t have issues in the future?

Use wood treated for Ground Contact for the beam

Many things can be done to help protect a beam below grade but this is the simplest and most promising. Not all treated wood is equal. Most outdoor wood is not treated for ground contact. Ground contact wood must be treated to 0.40-PCF. The good news is often on the barcode. It will be labelled ground contact or not.

A lower level of treatment is merely surface level and will quickly deteriorate with a higher level of water and fungi contact. Often to achieve this higher level of treatment will involve incising the wood, which will reduce its span strength. As the wood fibres have been cut to infuse more treatment. More plys maybe require to offset the weaker wood but lasting 50 years or more. It makes it worth it.

Limit ground contact with Deck Beams below grade

Just because the beam is below grade doesn’t mean it has to be buried. Trenching around and below the beam provides separation between the wood and the ground. Removing ground will do three things.

  1. Trenching separates the fungi in the ground from the wood. Preventing the beam from rotting. Technically, wood separated 6” (150mm) from the ground is not in contact. No, we still want to use ground-contact wood, but the separation will decrease rot.
  2. Waterlogged wood is weaker. A below-grade beam sitting in water will be weakened with increased moisture levels. Digging out below the beam provides a place for water to drain before coming in contact with the wood. Ensuring the beam stays dry and strong.
  3. Water expands when it freezes. Providing space between the beam and the ground allows ice to form without lifting the beam. Providing a more stable deck surface.

Landscape fabric and Gravel around Deck Beams below Grade

Just trenching around the beam creates the potential for the edge of the trench to collapse. With dirt falling on the beam. Adding gravel supports the edge of the trench preventing collapse. As all whiles, before placing gravel lay landscape fabricOpens in a new tab. down to stop weeds and erosion around the beam. The fabric prevents dirt from running into the trench during a rainstorm. The gravel holds the fabric against the trench sides. Man, I love teams!

Alternative to a Wood Beam Below Grade

Before trenching for the beam, Would a flush beam work?

With a simple square or rectangle beam, a flush beam may be a better alternative. If there’s room for the deck’s joists, there’s room for a flush beam. Doubling or tripling up the rim and then hangerring the joist will save you on digging. While raising the beam away from the ground and water.

A little more care will need to be taken in footing placement. A beam and footing on the edge of the deck will need to be measured exactly. A drop beam under the deck is hidden allowing you to be slightly off without affecting the look of the deck. But with an exposed flush beam appearance is everything. All will see how the deck and footings line up. Take your time, measure twice. It will take less time than digging for a below-grade beam.

Why not Concrete instead of wood for the beam?

Concrete isn’t affected by water or dirt. Concrete is safe in contact, both with ground and water. They build concrete dams all over the world, holding back water for years. That being said beavers build dams out of wood and mud. Dam beavers ruining my example.

Requiring less trenching concrete grade beams are strong and durable below grade. Build forms for the concrete beam and reinforce it with rebar. After pouring the concrete set a treated sill wood plate on top for nailing the joist to. Concrete lasting longer then wood and with proper steel reinforcement is stronger.

Below Grade Deck Beam Questions

Should you tar the beam for added protection?

Tarring is an option, for wood submerge for an extended period of time like railway ties and piers often are. It will protect the wood for an extended length of time. Few yards are wet enough to warrant the addition of tar. PWF wood is incised and treated to last over 50 years. Very few decks will last that long. PWF beam is sufficiently treated without the mess of tar.

Will wrapping a below-grade beam in Joist Membrane extend its life?

I am a big fan of joist membrane, especially on joist supporting composite or PVC decking. The adding protection helping the joist to last as long or longer than the decking. Well worth the extra time and money.

With a beam below grade, I am not so confident. Membrane between the beam and joist will help to seal around the joist nails preventing water from getting past the treated out shell of the beam. That might be of value.

A beam below-grade though is exposed to water and fungi on the sides and bottom. Requiring the beam to be wrapped entirely in membrane to prevent contact. Initially sounding like a good idea. My biggest concern is drainage. You can’t stop water. A well-designed deck allows water to drain off, both the decking and each individual part. Drainage is the key not damming. Wrapping a beam is like putting it into a bag and tying the bag closed. Excellent if no water gets into the bag, but if any water gets in. It is now trapped, unable to escape. Trapped water will only penetrate the beam more, increasing rot.

I think its best to use higher treated wood but allow the beam to dry.

Additional treatment of Wood to prevent below-grade beam rot

Before installing the beam coat with end cut treatment or wood sealer. This is a given for all end cuts as the cut exposes untreated end wood. The idea is if a little is good, more is better. The same as raising outdoor wood from 0.15 PCF to 0.40 for ground contact.

It will increase the wood’s resistance to both water and fungi. Unfortunately, it will not be as consistent as the pressure applied treatment. Also, wood sealer are only effective for a limited time, at best five years. Meaning they will extend the woods life but will not protect the wood for life. I think trenching around the beam will do more to limit rot then additional treatment but it will do no harm to treat the wood on site.

Ryan Nickel

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

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