Building Near Trees

Working Within the CRZ of a Heritage Tree

By Keith BrownPublished June 13, 2011Updated August 1, 2026

Published June 2011.

The city of Austin is very demanding that you do not impact what is defined as 1/2 of any heritage tree’s critical root zone (CRZ) during construction and development processes. And, rightly so. If you are building around a tree and expect it to live, rooting area needs to be preserved. Unfortunately, sometimes the city’s CRZ protection requirements can really restrict design ideas and site use. During one particular project I worked on with David Carroll with LZT Architects, the design successfully avoided breaching the 1/2 CRZ of a heritage tree, however, the contractors needed a few feet of access around the edge of the new building. The problem was the design built right up against the 1/2 CRZ, which meant workers would need to work on top of the 1/2 CRZ; this is a no-no. To mitigate the access problems, the builder put down a stage of CRZ ground protection. This consisted of an 8” layer of mulch directly on the natural grade with a stage made of 2x4 sleepers attached to plywood. This stage effectively buffers foot traffic from the root zone to protect the tree and allow workers to do their job.

Here is a pdf of the stage sketch

Common questions

In 2011, what portion of a heritage tree's critical root zone did the City of Austin require construction to stay out of?

Writing in 2011: the city of Austin is very demanding that you do not impact what is defined as 1/2 of any heritage tree's critical root zone (CRZ) during construction and development processes. And, rightly so. If you are building around a tree and expect it to live, rooting area needs to be preserved.

In 2011, could construction workers walk on the half of a heritage tree's critical root zone the City of Austin protected?

Writing in 2011: working on top of the 1/2 CRZ is a no-no. On one project the design successfully avoided breaching the 1/2 CRZ of a heritage tree, but the contractors still needed a few feet of access around the edge of the new building — the design built right up against the 1/2 CRZ, which meant workers would need to work on top of it. To mitigate the access problems, the builder put down a stage of CRZ ground protection.

What was the CRZ ground protection stage on that project made of?

Writing in 2011, about one particular project with David Carroll of LZT Architects: the builder's stage consisted of an 8" layer of mulch directly on the natural grade with a stage made of 2x4 sleepers attached to plywood. This stage effectively buffers foot traffic from the root zone to protect the tree and allow workers to do their job. That is what one builder did on one site in 2011, described as it was built.

In 2011, did Austin's root zone rules limit what you could design on a lot?

Sometimes, yes. Writing in 2011: unfortunately, sometimes the city's CRZ protection requirements can really restrict design ideas and site use. On the project described here the design did clear the 1/2 CRZ, and what still had to be solved was contractor access around the edge of the building — which was solved with ground protection rather than with a redesign.

Common questions

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