Relaying my patio. Can limestone screening damage a main tree root on which it sits? Sand hasn't to date.?

Question:The original patio was laid on a base of 8" of sand. I'm replacing this with 2" of soil, then 4" of limestone screening, then 2" of sand on top. The main tree root - about 10" in diameter - has not been damaged by the sand, nor, apparently, by having the stone sitting directly on top of it. I just want to make sure, before I replace some of the sand by limestone screening, that in doing so I won't be damaging the tree.

Answers:
Yes it may. Limestone screenings will raise the pH. Most trees & shrubs prefer a slightly acidic soil. If the limestone screenings raise the ph too high, nutrients are locked up in the soil and your trees can suffer nutrient deficiencies. Iron & Magnesium are two prevalent nutrients most commonly affected by high ph soils, but others can be affected as well.

You are correct in rebasing a patio on sand alone. The proper base would be 6" of #9 sand based gravel compacted in multiple lifts with just a 1" sand base top layer. Limestone screenings contain too much fine material and hold too much moisture in areas with freezing winter temps. The 6" gravel base helps to distribute loads over a larger area and eliminate settling. Gravel will also allow more oxygen infiltration to your trees roots, an essential element to the health of your trees.