Grade beams are a type of foundation that landscape architects can specify to spend tree roots in order to save them.
Grade beams are useful foundation structure to use if your site has large existing trees. If new structural work is being done within the drip line of the tree, a great beam can span the roots and avoid cutting roots of mature trees.
Many cities in California require great beams to be used to protect historic and large caliper trees. You can find the city’s or municipality’s individual regulations in the tree protection plan and ordinances.
How Are Grade Beams Designed?
A grade beam is a load-bearing beam foundation that is above grade and is supported by piers. Grade beams are meant to span tree roots while supporting a structure above. Additionally, grade beams can be incorporated with more traditional types of foundations and used only where tree roots must be preserved.
Grade beams are an alternative construction method that reduces pruning damage to roots and allows structures to be built closer to the root collar than other tree preservation techniques.
Instead of cutting a trench through the drip line of a mature tree (which causes considerable damage to the tree) a grade beam is a foundation built at or above the existing grade. The mass of the foundation is supported by strategically placed reinforced concrete piers. The piers transfer the load to the soil and prevent toppling. Walls and other landscape structures can be built on top of a grade beam foundation.
You can see an illustration of a grade beam in figure 320-63 of Timesaving Standards for Landscape Architecture.
Why Protect Mature Trees During Design and Construction?
Protecting mature trees preserves years of growth and beauty and protects habitat in rural, suburban, and urban environments. Trees sequester carbon, reduce the heat island effect, and give much-needed shade for people.
Construction is tough on trees.
During construction trees often suffer from mechanical injury to the roots trunks or branches from construction equipment.
The soil gets compacted by construction equipment which reduces root functioning and inhibits the development of new roots.
Changes in existing grade can cut or suffocate the roots. Changes in grade can affect the water table by either raising it or lowering it.
Construction can also change the microclimate which can expose sheltered trees to sun and wind.
The California Supplemental Exam has a strong bias towards protecting existing large trees. Native oak trees and other native species are strongly favored over new development.
There is a simple reason for this. Trees resent have their roots disturbed and the soil compacted by construction equipment during construction.
Landscape architects play an important role in protecting trees through proper design, specifications, and construction observation.
What do I need to know about grade beams for the California supplemental exam?
For the California Supplemental Exam (CSE) for landscape architects, you need to know that grade beams exist are one solution that landscape architects can use to build structures around existing trees (especially native oak trees) while protecting the root system.
There is seldom a good reason to disturb the drip line of an existing tree (especially native oaks) on the California Supplemental Exam.