pro landscaper magazine
pro landscaper magazine

ACO Presents: Building Threshold Drainage Series

by | 05 Jan 23 | Promotion

As all landscapers should know, drainage forms a vital part of garden and patio design. In part two of this four-part series, Rob Butcher, Design Services Manager at ACO Water Management discusses how to correctly design gradients for gardens and patios.

To recap quickly, more and more, homeowners are looking for a step-free and seamless integration from indoor to outdoor spaces, often seen in open plan designs with bi-fold doors. However, with level access, water can pool at the threshold and the surrounding area if not planned correctly. For this reason, measures must be put in place to transport surface water runoff – namely a gradient to ensure water travels away from the property entrance.

Gradients are a legal requirement

Having a gradient, or otherwise known as a fall, for a patio or paved area, is a requirement of Part H of the Building Regulations, and the gradient must slope away from the property. At the same time, Part M, which governs access in and around buildings, should be kept in mind as under this section of the building regulations it prohibits a slope steeper than one in 12 metres when forming a ramp for level building access.

When it comes to measurements, the necessary ratio of the slope itself will depend on the surfacing and its roughness. Consulting drainage experts in this matter can help to determine the ideal gradient for effective surface water drainage.

Soggy gardens and preventing flooding

At the same time as planning gradients, it’s a good opportunity to check if surface water runoff is likely to flood the garden.

Some gardens suffer with boggy and waterlogged ground, especially after heavy rain. If the house is relatively new, turf may have been laid on ground compacted by heavy vehicle and equipment use during the building process. This means that when water seeps through and hits the compacted ground it cannot drain away properly. The issue can be made significantly worse if the compacted ground is clay.

To determine whether or not this is the cause of flooding, dig down into the soggy ground a few feet. If beneath the saturated area the earth is hard, then compacted ground is probably the issue. The problem can be alleviated by digging up the affected area, rotovating the hard ground, and replacing the topsoil and turf.

A good tip is to also look at the local topography. Is adjacent ground sloping in towards the wet area so that rainwater runs down saturating the ground? If that is the case, then a solution could be to install a French drain to divert rainwater away from the problem area to an appropriate outlet.

This is the second instalment of ACO’s Building Threshold Drainage Series. If you missed the first part of this series where we talked about the damp proof course, please find it here.

For more information on threshold drainage and the solutions available, visit

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