Construction Requirements of a Deck Over Tree Roots

A deck around a tree is a beautiful blend of nature and outdoor living spaces. Integrating the convenience of a deck with the beauty and serenity of nature. It is also a fantastic compromise between cutting the tree down in the yard to have a deck or leaving the tree but missing the deck. But every tree has roots. So ever deck around a tree, must be built over tree roots.

There are a few things to keep in mind when building a deck over roots.

A deck over tree roots must be built high enough off the ground to provide room for the roots to grow. Using flush beams to allow maximum distance between the tree trunk and footings as not to damage the tree’s roots. With the joist and beams positioned to provide room for future tree growth.

Balancing the needs of the tree and deck. To ensure the deck does not kill the tree or the tree the deck. What would be worse than going to all the work to design and build a deck around a tree only to have it die?

A deck around a tree must not only look good and providing an enjoyable outdoor living space but also must be good for the tree. Not destroying the roots during construction or hinder the tree after.

The key to not damaging the critical tree’s roots is to support the deck in a way that will not damage the roots. Constructing the deck substructure in a way to not infringe on the tree’s roots while sufficiently supporting the deck.

Keeping the deck and tree healthy is accomplished in three ways.

1. Deck ground clearance for root growth

90% of trees rootsOpens in a new tab. are in the top 2′ (0.6m) of dirt. Meaning often as the tree grows, the roots will push out of the ground. A deck to close to the ground will not only be damaged by the roots but will damage the tree. By hindering the growth of the roots and possibly causing damage by constraining the roots.

A deck’s joist and beams should be a minimum 6″ (150mm) above the roots. Which will ensure that regardless of how long the deck covers the roots, there will always be enough room for growth.

Achieving deck ground clearance is best done with flush beams. With the beams integrated at the joist height providing the most clearance under the deck for the tree’s roots and growth.

Plan for Trunk Growth with the deck design

Not only will the tree roots grow, but the tree trunk will also. Build the substructure as to accommodate any future growth. You can Google the average tree trunk diameter for the species of tree, but on average, this is going to be in the range of 2-3′ (0.6-0.9m).

Leave as a minimum 32″ (812mm) between major structural joists. Hanger the rim joist supporting the joist in line with the tree, providing a sufficient box for the tree to grow into without damaging the tree.

The decking overhanging the joist box around the tree should be supported with framing members on flat. With only a 1 1/2″ (38mm) of material below the decking. Then it can be trimmed back from the tree with a jigsaw as the tree grows. Keeping a clean deck opening roughly 2″ (50mm) around the tree trunk. Keeping the tree healthy, refreshing, and enhancing the deck space.  

2. Footing Placement in the Tree’s Structural Root Zone

Positioning of deck footings around a tree is a little more complicated than simply building a deck in the backyard. Not only do you need to create the substructure as not to damage the tree’s roots but also not harm the tree’s structural roots during construction. Which potentially is most likely to happen while digging the footings.

Deep Green Permaculture gives this great equation for figuring out a tree’s structural root zone. To read the full discussion of Deep Green Permaculture, click hereOpens in a new tab..

Structural Root Zone radius = (Diameter above Root Crown x 50)^0.42 x 0.64

For our purposes, here is a simple chart to gauge a tree’s structural roots zone. Just like deck beams and joists chart, round up to the nearest trunk measurement. Read over to the left the approximate size of the tree’s structural roots. If the tree’s diameter is 9″ read across the 12″ tree diameter. Always round up never down. Better to be safe than sorry.

Trunk DiameterStructural Root Zone (Metric)Structural Root Zone (Imperial)
6” (0.15m)1.50m4’11”
12” (0.30m)2.00m6’7”
18” (0.45m)2.36m7’9”
24” (0.60m)2.67m8’9”
30” (0.75m)2.93m9’7”
36” (0.90m)3.17m10’5”

As you can see, this place’s a reasonably large circle around the tree which roots should not be damaged while building a deck. Of course, you want the deck around the tree, and it will need to be closer than 5′ (1.52m). This is a problem. If the roots surrounding a tree are structural and essential for the tree’s health, how can you build a deck around it?

Fortunately, there are some solutions for deck footings around a tree. As not to damage the roots.

Floating deck and footings

If the roots are in the ground, then if you do not dig in the footings, you will not damage the roots. Simple enough and can be done in most States if your deck is less then 30″ (0.76m) and is not attached to the house. In most of Canada, the required deck height is lower than 24″ (0.6m) to be built without footings below the frost level.

Floating footings do come with some risk. As the name suggests, your foundations are floating. As the ground rises and falls with the seasons, so will your deck. It also has the potential for roots to grow under the footing, pushing it up or damaging the root.

If you go with floating footings. Place 6″ (150mm) of drain gravel under the footing to allow water to drain away, minimizing the effects of frost on the deck. It’s not really the ground that expands and contracts but the water in the ground. Drain it away, and the deck will be more stable.

Digging in deck footings by tree roots

For a raised deck or one that demands a firmer foundation correctly, dug in footings are best. Six inches (150mm) below the frost line. Which in Florida is easy enough with no frost but a little more challenging up where I build decks in Calgary, Alberta, where footings are required to be 48″ (1.2m) below grade.

If you’re in another Canadian city, I have a chart of 22 significant Cities and their footing requirements. Click hereOpens in a new tab. to see the table and read about footings in Canada.

First, no gas augers when digging footings by a tree. They could rip through or damage the root before you even know you have hit a root. A clamshell post hole digger will allow you to realize when you have hit a significant tree root quickly. Whenever you hit something, stop and look before cutting through it. If it a tree root more than 2″ (50mm). You need to adjust the footing location.

No grabbing the Sawzall and cutting the root. Yes, it may be easy, but that could kill the very tree you are trying to incorporate into your outdoor living space. Or worse, weaken it that in the next windstorm it will topple over destroying your deck or even your house.

This is the root structural root zone. Just like your deck, if you cut through the beam, your deck could collapse. The same with the tree, the roots are what keep it from falling down.

Adjusting Deck Footing Locations around Tree roots

Be wise and leave the largest area around the tree as possible. Do this with large joists and beams, providing the greatest spans possible over the roots. The further away from the tree trunk, the footings are the less structural roots you will encounter. That’s a good thing.

Next, the easiest way is to do design the deck flush beams for 8′ (2.4m) footings spacing but plan to dig 6′ (1.8m) footing spacing. Creating the freedom to adjust the footings around the roots as they are discovered.

For example, with a 12’x12′ (3.6mx3.6m) deck around a tree with a double 2×10 flush beam supporting the joist, with three footings. One on each end and one in the middle, space 6′ (1.8m) apart. Lay it out around the tree and start to hand dig the footings. If you encounter a root, adjust the footing along the beam, all without adjusting the beam location. On the ends, the beam can overhang up to 16″ (400mm) past the footings. The centre footing can move either way up to 2′ (600mm) from 6′ to 8′ (1.8-2.4m) from the end footings. Navigating around whatever roots encountered.

3. Tree Root Requirements Under a Deck

The good news about ground coverings under a deck is that what is good for the deck’s health is also good for the tree.

  • Drainage
  • Ventilation 

A deck built over tree roots must not block water or limit ventilation. What you cover the ground under a deck is critically important to allow water and oxygen to get to the tree’s roots. To read more about good ground coverings under a deck, click hereOpens in a new tab..

How large of area around the tree needs to be protected for tree health?

Tree Protection Zone (TPZ)

The Tree Protection Zone is the area around a tree that arborist consider a no-go zone for construction as not to damage roots that are critical to the tree’s health. There are three common ways to determine the tree’s protected root area.

TPZ = canopy drip line + 3’ (1m)

The simplest way is to determine the trees canopy drip line, extend out 3′ (1m), and everything within this area needs not to be damaging to the roots of the tree. Stand on the ground, lookup, if you see branches, you are under the tree’s canopy. Take a few more steps away from the tree until you can see the sky. Then take three more steps. Simple but the least accurate as the tree’s canopy can be trimmed and shaped as not to reflect the roots below.

TPZ = DBH x 12 

A more common and accurate method to determine footing locations around a tree is with a little math. Measure the tree’s trunk diameter at breast height, roughly 55″ (1.4m). Yes, our breasts are at different heights but roughly. Take the tree trunk diameter measurement, and times it by 12. This is the radius around the tree in which the roots need to be cared for.

This is super easy when using Imperial. Whatever the truck diameter is in inches, the roots are in feet from the centre of the tree. My tree in the front yard is 22″ wide; the roots extent out 22′ from the tree.

Basically, half my yard has roots in it from that tree. Wow? That a lot of roots.

TPZ = Height x 1/2

The third method is more an exception then common practice. It is only suitable for tall skinny trees, like a hybrid poplar tree, tall and thin with a small canopy. Measure halfway up the tree. Use this same measurement as the radius around your tree to avoid placing footings within. 

This area surrounding the tree needs to be able to drink up rainwater when it falls and be ventilatedOpens in a new tab. to allow the tree roots to breathe. The decking of a deck surrounding a tree needs to be gapped enough to let rain and air to get to the roots. Keeping the tree healthy, shading and beautifying the deck for years to come.

Conclusion of a Deck Over Tree Roots

A deck built around a tree covering the roots must be designed to have minimal effect on the tree roots, both during construction and after. Allowing the roots to support the tree while not damaging the deck. This is done by positioning the deck high enough above the roots with room around the trunk for growth with sufficient deck drainage and ventilation. By doing these, I have you can enjoy your deck and tree for years to come.

Ryan Nickel

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

Recent Posts