On shrinkable clay, the trees most likely to affect a building are thirsty species, growing large, close to shallow foundations. The NHBC Standards (Chapter 4.2, Building near trees) are the guidance most widely used in the UK to judge this.
Water demand and zone of influence
NHBC places tree species into high, moderate or low water demand categories and gives a typical mature height for each. The distance over which a tree can dry the soil, its zone of influence, is a multiple of its mature height:
| Water demand | Zone of influence | Examples |
|---|---|---|
| High | 1.25 × mature height | Oak, elm, poplar, willow, eucalyptus, Leyland cypress |
| Moderate | 0.75 × mature height | Ash, lime, plane, sycamore, horse chestnut, most conifers |
| Low | 0.5 × mature height | Birch, hornbeam, holly, hazel, magnolia |
Where a tree’s species has not been identified, high water demand should be assumed. For species not in the table, NHBC advises using the category for the genus, or high demand if the genus is not listed either.
The zone also depends on how shrinkable the soil is (high, medium or low volume change potential) and on the climate: drier eastern and southern areas need deeper foundations than wetter western ones. Foundation depths in NHBC Chapter 4.2 combine all of these.
How the postcode check uses this
The Forest Research map gives the height and outline of each tree canopy, but not the species. The report therefore applies the high water demand multiplier (1.25) to the tree’s current maximum canopy height, and measures the distance from the pin to the edge of the canopy. A tree is flagged when the building pin is inside that distance. Because a growing tree can get taller, this is an estimate rather than a maximum.
If you know the species, add it in the report’s site tree register. The report then uses the NHBC water demand and mature height for that species.
Heave after removal
Felling a large tree on clay is not a simple fix. The soil it had been drying can take up water and swell for years, pushing foundations up. NHBC notes that a semi-mature tree may already have a water demand close to that of a mature tree. Always get advice from a structural engineer or arboriculturist before removing a tree near a building, and check for Tree Preservation Orders and conservation areas with your council.
NHBC water demand and mature heights
From NHBC Standards Chapter 4.2, Table 3. The last column is the resulting zone of influence.
| Species | Water demand | Mature height | Zone of influence |
|---|---|---|---|
| Elm, English | High | 24 m | 30 m |
| Elm, Wheatley | High | 22 m | 27.5 m |
| Elm, Wych | High | 18 m | 22.5 m |
| Eucalyptus | High | 18 m | 22.5 m |
| Hawthorn | High | 10 m | 12.5 m |
| Oak, English | High | 20 m | 25 m |
| Oak, Holm | High | 16 m | 20 m |
| Oak, Red | High | 24 m | 30 m |
| Oak, Turkey | High | 24 m | 30 m |
| Poplar, Hybrid black | High | 28 m | 35 m |
| Poplar, Lombardy | High | 25 m | 31.2 m |
| Poplar, White | High | 15 m | 18.8 m |
| Willow, Crack | High | 24 m | 30 m |
| Willow, Weeping | High | 16 m | 20 m |
| Willow, White | High | 24 m | 30 m |
| Cypress, Lawson | High | 18 m | 22.5 m |
| Cypress, Leyland | High | 20 m | 25 m |
| Cypress, Monterey | High | 20 m | 25 m |
| Acacia, False | Moderate | 18 m | 13.5 m |
| Alder | Moderate | 18 m | 13.5 m |
| Apple | Moderate | 10 m | 7.5 m |
| Ash | Moderate | 23 m | 17.2 m |
| Bay laurel | Moderate | 10 m | 7.5 m |
| Beech | Moderate | 20 m | 15 m |
| Blackthorn | Moderate | 8 m | 6 m |
| Cherry, Japanese | Moderate | 9 m | 6.8 m |
| Cherry, Laurel | Moderate | 8 m | 6 m |
| Cherry, Orchard | Moderate | 12 m | 9 m |
| Cherry, Wild | Moderate | 17 m | 12.8 m |
| Chestnut, Horse | Moderate | 20 m | 15 m |
| Chestnut, Sweet | Moderate | 24 m | 18 m |
| Lime | Moderate | 22 m | 16.5 m |
| Maple, Japanese | Moderate | 8 m | 6 m |
| Maple, Norway | Moderate | 18 m | 13.5 m |
| Mountain ash | Moderate | 11 m | 8.2 m |
| Pear | Moderate | 12 m | 9 m |
| Plane | Moderate | 26 m | 19.5 m |
| Plum | Moderate | 10 m | 7.5 m |
| Sycamore | Moderate | 22 m | 16.5 m |
| Tree of heaven | Moderate | 20 m | 15 m |
| Walnut | Moderate | 18 m | 13.5 m |
| Whitebeam | Moderate | 12 m | 9 m |
| Cedar | Moderate | 20 m | 15 m |
| Douglas fir | Moderate | 20 m | 15 m |
| Larch | Moderate | 20 m | 15 m |
| Monkey puzzle | Moderate | 18 m | 13.5 m |
| Pine | Moderate | 20 m | 15 m |
| Spruce | Moderate | 18 m | 13.5 m |
| Wellingtonia | Moderate | 30 m | 22.5 m |
| Yew | Moderate | 12 m | 9 m |
| Birch | Low | 14 m | 7 m |
| Elder | Low | 10 m | 5 m |
| Fig | Low | 8 m | 4 m |
| Hazel | Low | 8 m | 4 m |
| Holly | Low | 12 m | 6 m |
| Honey locust | Low | 14 m | 7 m |
| Hornbeam | Low | 17 m | 8.5 m |
| Laburnum | Low | 12 m | 6 m |
| Magnolia | Low | 9 m | 4.5 m |
| Mulberry | Low | 9 m | 4.5 m |
| Tulip tree | Low | 20 m | 10 m |