Expert: Arshad Khan - 12/14/2009

Question
QUESTION: Dear Arshad Khan,
I hope that you are fine and keeping good. Sir, I am designing a building inwhich the architect requies to grow a tree (Religious reasons..) I have chekced the raft footing and provided a viod in the raft and detailed and sized the section accordingly. My question is that what else considerations do i need to take into account. Like i was thinking that the when the roots will grow bigger they might do a problem. do i need to provide a small toe wall at the end of the void??/

Thanks in advance
best regards
Muhammad Umar

ANSWER: Hi Muhammad,

This really depends on the type of tree, as some have roots that grow downwards whilst others have roots that spread outwards, horizontally. It is the latter ones that cause most problems for buildings. The natural water table also has an effect on this matter.

You have not indicated whether the void is in the middle of the raft or along one edge- in anycase, it will be wise to incorporate a vertical RC wall, at least 300-400mm thick and about 1m deep (but this depends on the nature of the tree/root growth, etc), all around the void, to prevent the roots spreading directly under the soffit of the raft.

---------- FOLLOW-UP ----------

QUESTION: thank you sir,
the tree is near the edge  of the building. Basement retaining wall is present near to the void!

Answer
Hi Muhammad,

If you have a basement and the raft is at say 3m depth, then there may not be a need to provide a further toe wall. Insure that the basement wall is thick enough and that any external tanking/waterproofing is well protected against root damage. (by either a brick wall or second skin RC wall).

Another thing I forgot to mention earlier is that tree damage can also be due to subsidence caused by changes in moisture content of clay soils (as roots suck out the water), especially expansive clays- so if the water table remains high, there is less chance of problems associated with subsidence, especially if you have a basement with deep foundations.

Please also bear in mind any future potential damage to adjacent properties. Normally the zone of influence (extent of roots) of trees is equal to the footprint of the extent of the branches/leaves.