How to Use the LABC Foundation Depth Calculator

If your proposed extension sits on clay soil and there are trees within 20 metres, the required foundation depth depends on the species, height and proximity of each tree. The free LABC foundation depth calculator takes you through all of these variables and gives an indicative depth recommendation.

The tool is available at fdc.labcwarranty.co.uk. Here is how to use it.

Step 1: Ground Slope

The first question concerns the slope of the ground at the site. A sloping site can cause foundations to work harder (particularly in downhill positions) and affects the depth calculation. Enter the approximate slope of your site.

Step 2: Postcode

Enter the postcode for the site. The calculator uses this to determine the predominant soil type in your area using the British Geological Survey national shrinkage potential map. If your site is on clay (high or very high volume change potential), the tree calculations that follow will generate deeper requirements.

Step 3: Volume Change Potential

Based on the postcode, the calculator suggests a volume change potential (VCP) rating — low, medium, high or very high. This reflects how much the clay at your site is likely to shrink and swell with seasonal moisture changes. On a high-VCP site, tree root influence extends further from the trunk, and foundation depths increase accordingly.

Step 4: Adding Trees

For each tree or large shrub within 20 metres, you enter:

– Species (from a dropdown list covering common UK trees and large shrubs)
– Status — whether the tree will be retained or removed
– Distance from trunk to the nearest proposed foundation (in metres)
– Height (actual measured height or estimated height)

If a tree is to be removed, it is still entered in the calculator — a removed tree does not eliminate its influence for several years.

Step 5: View the Recommended Depth

Once all trees and shrubs are entered, the calculator outputs a recommended minimum foundation depth. This depth is the minimum to the underside of the foundation concrete below existing ground level.

Note: this is the depth to the foundation base, not to good bearing strata. Your Building Control Officer still needs to confirm that the soil at that depth is adequate on the day of inspection.

Using the Output in Your Structural Design

Take the recommended depth from the calculator and provide it to your structural engineer. It feeds directly into the foundation specification on your drawings. If the result is deeper than 1.5 metres, heave protection and a suspended floor are likely to be required — your engineer will confirm based on the specific values.

Get Your Foundation Designed

PorthouseDean designs foundations for clay sites with trees. Get a quote to have your structural drawings produced with the appropriate depth for your site conditions.