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 breeching 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 2×4 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
