Thank you to everyone who attended our "Building Elevated Pedestrian Walkways While Protecting Trees" webinar. The questions and engagement from the audience made for a great session, and many of you stayed on for an extra 25 minutes of Q&A!
In case you missed it, here is the full webinar recording from November 4, 2026.
You can browse the topics discussed and main takeaways using the sections and time stamps below (see detailed key takeaways at the bottom):
- 00:00 - Introduction and Speaker Bios
- 03:00 - What Roots Do: The Bones and the Belly of the Tree
- 04:50 - Types of Roots: Taproot, Lateral, Fine, and Sinker Roots
- 07:35 - Why Most Roots Live in the Top Three Feet of Soil
- 13:45 - Mycorrhizal Fungi and How Trees Communicate
- 17:15 - Why It All Comes Down to Soil
- 21:15 - Soil Compaction and 95% Proctor
- 24:30 - Critical Root Zone and the Drip Line
- 26:00 - Design Strategies for Protecting Trees
- 28:30 - PermaTrak System Overview
- 33:55 - Case Study 1: Wake Forest University
- 39:00 - Case Study 2: Shoreline, Washington
- 42:00 - Case Study 3: Zilker Botanical Garden, Austin, Texas
- 48:45 - Q&A Session
Check out:
- Our full article on How Much a Boardwalk Costs, including how foundation type affects price.
- Download the slides used during the webinar: (Linked here)
Building Elevated Pedestrian Walkways While Protecting Trees - Detailed Key Takeaways
[00:03:00] What Roots Do
The best way to think about tree roots is as the bones and the belly of the tree. Woody roots provide structure and stability, while fine roots handle water and nutrient intake, hormone signaling, defense against pathogens, and communication.
There are four main root types:
- Taproots: Most young trees start with one, but most trees lose their taproot as they reach maturity. Exceptions include trees in very sandy environments, such as longleaf pine and bald cypress.
- Lateral roots: The most common structural roots. They act as guy wires and main arteries, running at or just below the soil surface.
- Fine roots: Hair-like roots that live mostly in the top 12 inches of soil, where water and oxygen are available. They are short-lived, from a few weeks to a season, and regrow when soil conditions are good.
- Sinker roots: Woody roots that drop vertically from lateral roots, acting like soil nails for extra stability.
[00:07:35] Most Roots Live in the Top Three Feet of Soil
About 90% of trees encountered in urban environments have a lateral root system, and most tree roots exist in the top three feet of soil because roots need oxygen to grow. This opens up creative installation options. For example, boring three feet under a tree can avoid most roots, including the structural ones.
Buttress roots close to the trunk are critical for stability and act as the main collector pipes for water and nutrients. Cutting them weakens the tree and makes it more vulnerable to wind damage.
[00:13:45] Mycorrhizal Fungi
About 90% or more of plants on earth have a symbiotic relationship with mycorrhizal fungi. Trees spend between 2% and 20% of the sugar they produce feeding the soil and these fungi. In return, the fungi improve nutrient uptake, help with drought resistance, defend against pathogens, and link trees into a connected community that shares nutrients across a forest. Protecting the tree means protecting the roots, and protecting the roots means protecting the fungi.
[00:17:15] Protect the Soil, Protect the Tree
Healthy soil has space for air and water to move. Compaction closes those spaces, making it difficult or impossible for roots and fungi to survive. Heavy machinery can leave a hard pan about 12 inches down, even if the surface is later loosened.
Key points:
- A 95% Proctor specification, common for walkways, compacts soil to roughly the hardness of brick, and tree roots cannot grow in brick.
- One-time accidental compaction can often be fixed with air tools and organic-rich backfill, as long as woody roots were not damaged.
- A path never stops compacting. Surface sidewalks next to trees cause long-term damage and encourage foot traffic into the root zone.
- Vibration compaction spreads beyond the work area and reduces the soil volume available to the tree.
[00:24:30] Critical Root Zone and the Drip Line
Roots grow in a bullseye pattern, densest near the trunk and extending well beyond the drip line. The percentage of root zone impacted is only part of the picture. Impacting 1% of the root zone at the edge of the canopy is very different from cutting a structural root that feeds 30% of the tree.
[00:26:00] Design Strategies for Protecting Trees
- Bring an arborist in at the concept stage. Arborists can tell you whether a tree can withstand construction and help structural engineers place foundations.
- Use raised walkways around high-value trees. Keeping the walkway off the soil prevents compaction and allows air and water to reach the roots. Raised walkways are the gold standard for protecting large trees.
- Place piers deliberately. Footings on grade avoid going into the ground, and piers can be placed beside structural roots rather than through them.
- Choose footer size strategically. Smaller footers can be shifted around surface roots, while longer spans with larger footings can avoid sensitive soil entirely.
[00:28:30] PermaTrak System Overview
PermaTrak began in Australia and came to the United States in 2010, with more than 750 structures installed since. The system works like a set of building blocks: precast concrete abutments, stackable piers (base, riser, and cap), beams, and treads, with optional curbs, handrails, or guardrails. Treads can be ADA compliant and include integral color and a slip-resistant texture. Handrails are typically required by code once the walkway is 30 inches or more above the adjacent ground.
[00:33:55] Case Study 1: Wake Forest University
Wake Forest needed a safe walkway for students heading to the dining hall along a busy parking lot, next to heritage trees whose roots were being compacted by foot traffic. The landscape architect and arborist identified where piers could and could not go, and PermaTrak designed the beam lengths and slopes around those locations while meeting the 8.3% ADA slope limit.
- Precast piers sat on the soil with nothing drilled into the ground
- Pairs of piers were spaced roughly every 10 feet
- An 18-inch air gap was kept between the beams and the ground for tree health
- Beams rest on the piers without a mechanical connection so the boardwalk can move with the soil
- The university has since used PermaTrak in two more locations on campus
[00:39:00] Case Study 2: Shoreline, Washington
When the Washington State Department of Transportation planned a sidewalk along a busy road, local residents organized to save the mature evergreen trees along the corridor. The arborist mapped the critical root zones, PermaTrak located the 24-inch-diameter piers around them, and the arborist and reviewing consultant approved the layout. Only the six-inch-tall base was used, creating a low-profile walkway with a concrete bumper curb instead of a handrail.
[00:42:00] Case Study 3: Zilker Botanical Garden, Austin, Texas
Visitors at Zilker Botanical Garden were walking along the road or cutting through the grass near heritage trees, compacting the soil. Austin's tree protection rules required load distribution panels inside critical root zones and exploratory air spading to expose the roots. Using that information, the team went through several iterations with the arborist to place helical piers every 10 to 16 feet. The foundations extend below the frost line, and the handrail is mounted to the treads so the only ground disturbance came from the helical piers.
[00:48:45] Q&A Session
Can you use a curb instead of a handrail if the boardwalk is more than 30 inches above the ground?
- Code requires a handrail above 30 inches, with the bottom horizontal member less than four inches from the walking surface
- Some owners have chosen to use curbs only and accept the liability, but that does not meet code
How do you handle frost heave when piers sit on the soil?
- One option is to take foundations below the frost line, as at Zilker
- Precast piers on grade will move with seasonal frost, which is why the modular system is designed to move rather than crack like a sidewalk
- Designers need to estimate the likely heave and decide whether it is acceptable
What is the maximum vehicle weight for piers on grade?
- A 24-inch precast pier next to a tree can practically support up to an H10 load (a 20,000-pound truck)
- 10,000 pounds is more realistic for most tree root crossings
- Boardwalks 10 feet wide are designed for at least a 10,000-pound maintenance vehicle
What beam lengths work for tree root crossings?
- PermaTrak beams can range from about four feet up to 50 feet
- For tree root crossings, plan for spans of 20 feet or less
Is there a rule of thumb for the critical root zone?
- Critical root zones are both biological and regulatory, so start with your local development code
- Many codes base it on trunk diameter at breast height (DBH), often one foot of radius per inch of diameter
- Katie-Rose's personal rule of thumb is three times the DBH, and an arborist should look at each specific tree
How do you protect the ground when machinery is needed for helical pier installation?
- Contractors can be restricted to a set footprint using load distribution panels
- The most restrictive approach is top-down construction, building from the top of the boardwalk as it goes
- Katie-Rose recommends 12 inches of fresh arborist wood chips topped with plywood or load distribution panels
What does PermaTrak cost per square foot?
- Material cost is very consistent, but foundations and handrails can double or triple the total depending on soil conditions
- Precast piers are the most economical foundation type
- See our full article on How Much a Boardwalk Costs.
Can PermaTrak be designed for flooding?
- Yes, with mechanical connections between pieces to resist hydraulic forces
- PermaTrak has designed for flood velocities of 10 to 15 feet per second
Where is PermaTrak manufactured?
- Components are precast offsite and shipped by flatbed truck from plants in Redding, CA; Houston and Dallas, TX; Indianapolis, IN; Charlotte, NC; Union, SC; and Orlando, FL
