Drought and clay series · October 2026

Trees next to your house on clay: how far is safe?

Trees are valuable, for your home too. But on clay soil a tree too close to the façade can dry out the ground under your foundation.

Trees and greenery along a stream

A tree drinks more than you think

On a hot summer day a large tree can draw up to about 500 litres of water from the ground. On clay it dries the soil out to a distance of about 1 to 1.5 times its height, and down to 5 m deep. When it finds no more water around its trunk, the roots go further: to the moister ground under your house, or to a leaking drain.

Willows, poplars, oaks and limes in particular take up a lot of water. The problem often only appears years after construction, once the tree has grown large.

What distance is safe?

On plastic soils Buildwise recommends a distance of at least 1 times the tree height for a single tree, and 1.5 times the height for a group of trees or a hedge of shrubs. A tree that will grow to 15 m is best planted at least 15 m from the façade.

If that is not possible, the foundation has to go deeper: within the influence zone of trees about 3 m has been assumed so far. For trees taller than 12 m less than 6 m from the house, Buildwise advises considering a pile or pier foundation.

The tree is already there: what now?

  • Prune regularly. A smaller crown evaporates less water. It only works if done every year, ideally before a dry summer.
  • A root barrier. A vertical wall of mortar or lean concrete, at least 2 m deep, between tree and house keeps roots away from the foundation. It takes specialist work so as not to destabilise the tree or the building.
  • Felling, with care. After felling, the ground gets wet again and swells, sometimes by as much as it settled before. Felling is only advisable while the damage is still limited.
  • Deepen the foundation. If the damage is serious or the ground keeps moving, the house is founded deeper, below the zone the tree dries out.

Sources: Buildwise (Belgian Building Research Institute), Cracking in buildings due to shrinking or swelling of plastic soils, WTCB-Dossiers 2021/5.10 and WTCB-Contact 2018/3; M. De Vos, Special problems when building on Tertiary clay, TI-KVIV study day 2004; BRE Digests 240–242, 298 and 412. This article summarises those sources in our own words and does not replace advice from an engineer.