Drainage at the building threshold

Effective drainage must be ‘front of mind’ for new builds seeking to achieve the increasingly popular blending of indoor and outdoor environments. Rob Butcher of ACO Water Management discusses the key considerations.

The integration of outdoor and indoor spaces is a crucial element for any building, and this couldn’t be more applicable to the current trends in home design. A growing number of homeowners embrace the idea of having a step-free level walkthrough from their indoor space onto the patio. This continuity between indoor and outdoor environments encapsulates the ‘bring the outdoors in’ design movement. 

Given the desire for aesthetically-pleasing houses, more emphasis is required on threshold drainage to avoid unwanted – and often dangerous – water ponding. When considering threshold drainage, there are a number of different measures to navigate the water-sensitive divide between indoor and outdoor spaces. From gradients to channels, here are three key tips for housebuilders and developers.


Before approaching threshold drainage, the damp-proof course (DPC) is a construction element that needs to be kept in mind. Integral to protecting the property from structural damage, it must be positioned correctly for effective water management. 

As outlined in Approved Document C of the Building Regulations, the DPC needs to be positioned no lower than 150 mm above the threshold when building a level patio to internal floor. This is to prevent the instance where rain hits the ground and splashes back up over the DPC. If it is placed too low or the ground is being built up too high, damp can infiltrate the dwelling as rainwater breaches the DPC.

In projects where the patio is lower than the internal floor, the DPC should be 150 mm above the floor level. Following this practice will futureproof the building in case a homeowner wishes to retrofit a level threshold later down the line. 

A common misconception is that a drainage channel can be installed directly against the DPC to prevent this problem arising. As the grating would be at the same level, rainwater will continue to splash and permeate the brickwork above – proving to be a counterproductive approach. Furthermore, such a measure has the potential to make matters worse, as an environment is created between the wall and the channel for vegetation to grow. Moss may start to develop along the wall which could eventually breach the DPC and leave the brickwork in an increasingly vulnerable condition. 

It is clear that being reactive can in fact cause further issues, which is why it is vital that housebuilders adopt a proactive stance here. While legislation offers a guideline, it is recommended practice for developers to set the DPC higher than the minimum 150 mm above-ground requirements for level thresholds. This is also applicable to properties where the homeowner is likely to create a level threshold as part of future renovations. Implementing an elevated DPC provides an added buffer that can decrease the risk of breaches and rising dampness. 


The seamless integration seen in open-plan design requires level access between indoor and outdoor spaces, which allows water to pool at the threshold and surrounding area. It’s for this reason that measures must be put in place to transport surface runoff – namely a gradient to ensure water travels away from the property entrance. 

Developers should be aware of the requirement for a patio or paved area to slope away from the property, as stated in Part H of the Building Regulations. At the same time, Document Part M should be kept in mind as regulations prohibit 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. 


It must be understood that sloping is the bare minimum and will need to be supported by other components to form a competent threshold drainage system. This is where threshold interceptor drains also play a crucial role in home design. Such a solution prevents accidental flooding of the external area from spreading into the dwelling without compromising the aesthetic. 

This is especially pertinent in cases where level threshold access is to be implemented. Bi-fold doors are a common staple in the entrance design, however, the running track at the bottom leaves a slot for rainwater to collect when the doors are closed. Therefore, a correctly-specified channel should be installed at the front of the running track to percolate the water out to a suitable outlet. 

On the subject of disposal, this element cannot be forgotten when installing drainage channels. ‘Phantom drainage’ is a prevalent example of poor practice across threshold drainage, whereby the linear channels are connected to nothing. 

Without a suitable outlet there is nowhere for the water to drain and it will continue to collect until the channel overflows. This is why housebuilders must always determine where the collected surface water will be disposed of when implementing threshold channel drains. 


The parameters discussed are a vital part of home design in ensuring building thresholds are both attractive and functional parts of the property. When placing these considerations front of mind, it is equally important that housebuilders consult drainage experts to ensure the right solutions can be put in place. To this end, housebuilders and developers will be better equipped to implement effective drainage systems across their projects.

Rob Butcher is design services manager at ACO Water Management