Geoff Wilkinson looks at the rules relating to roof rainwater drainage
One of the generally least ‘designed’ elements of domestic buildings is the rainwater disposal system. Almost invariably we see simple plastic rainwater goods tacked onto buildings, with little thought given to the aesthetics of the design or what it is trying to achieve. Let’s look at alternatives.
Small roofs under 6m2
In a previous version of Approved Document H (Drainage and Waste Disposal) there was a clear exemption from any requirement for rainwater gutters on small roofs with an area of 6m2 or less – porches, bay windows, and so on. This is less clear in the current version, but Table 2 in the guidance for roofs under 6m2 contains no requirement for gutters or downpipes, thereby continuing the exemption.
It’s still worth considering providing gutters in these cases to avoid water running down the face of the building and staining of the external finish. I would recommend at the very least a good overhang be created to discharge water away from the façade and any doors or windows.
For larger roofs this concept is extended with the option of an eavesdrop system. It is a fairly recent addition to Approved Document H so some architects may be unfamiliar with the systems. They have been used abroad for the past two decades and proven to be very effective, and are particularly useful when site constraints won’t permit soakaways.
Sloping surfaces should not direct water towards another building where it could lead to damage to that building or its foundations
An eavesdrop system allows rainwater from the roof to drop freely to the ground without a gutter being provided at all. Where they are used, they should be designed to:
- protect the fabric of the building from water ingress
- prevent water from entering doorways and windows
- protect people using doorways, etc, from falling water
- protect people and the fabric of the building from splashing, for example by provision of a gravel perimeter
- protect the foundations from concentrated discharges from valleys or valley gutters or from excessive flows from large roofs (ie where the area of roof per unit length is high)
- protect against discharge of water onto neighbouring properties.
Proprietary solutions exist, including angled slat systems that look similar to brise-soleil and direct rainwater away from the face of the building, having the advantage that they won’t get blocked with fallen leaves.
Hidden/secret gutters to boundaries
One of the most problematic drainage details we see is the hidden gutter. This is often employed in order to maximise the size of an extension – when needing to build right up to the boundary line, for example. It is not allowed to install guttering that would overhang the boundary line and you can’t employ the eavesdrop system either, as this would potentially discharge onto a neighbour’s land.
The common design solution is to create a hidden gutter by stepping back the roof and building a gutter on the outer skin of the brickwork similar to the detail below. This can cause a number of problems:
- the capacity is small and likely to become blocked by leaves, so it is difficult to maintain
- the detail can weaken the structure
- the detail can create cold bridging and pattern staining.
A better solution is to build up the external wall as a parapet and create a larger, lead-lined gutter within the boundary of the external wall.
Aj fw parapet drawings
Paved areas around buildings
It’s important to consider where rainwater meets the ground, often a requirement that’s forgotten by both architects and building control. Approved Document H requires that paved areas slope away from the building for at least 500mm to divert surface water away from the wall. Impervious surfaces should slope away from the building at a 1:40 to 1:60 incline. But remember that this should not direct water towards another building where it could lead to damage to that building or its foundations. Additionally, care should be taken to ensure that the soakage capacity of adjoining ground is not overloaded, especially in areas at high risk of ponding or flood.
Where this is not practical, it is important to incorporate gullies or channel drains that discharge into a suitable drainage system. This is especially important with extension designs incorporating flush patio doors, or where the client wants a level patio or deck area. The traditional detail of a render plinth painted with bitumen paint is another good way to prevent rainwater getting back into the building.
Geoff Wilkinson is an approved inspector and managing director of Wilkinson Construction Consultants www.thebuildinginspector.org