GROUNDWORKS 69
DRAINAGE AT THE BUILDING THRESHOLD
Eff ective 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.
T
he 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.
DPC
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 fl oor. 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 infi ltrate the dwelling as rainwater breaches the DPC. In projects where the patio is lower than the internal fl oor, the DPC should be 150 mm above the fl oor level. Following this practice will futureproof the building in case a homeowner wishes to retrofi t 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.
GRADIENTS
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.
DRAINAGE CHANNELS
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
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