Upgrades or Alterations to Rainwater Goods
When the rainwater system is regularly overwhelmed during heavy downpours, attenuation measures can reduce the flow of rainwater collected by rainwater goods. Additionally, upgrades or alterations of rainwater goods can be made in response to design or capacity shortfalls of the system.
Upgrades or alterations of rainwater goods could be necessary:
- where there is a complicated design, in which rainwater travels a long distance and/or via an intricate route before discharging into the below ground drain
- if rainwater goods sizes are too small or merely adequate when checked against rainfall intensity values
Changes may also be considered when planning other work to a building, such as external decoration or roof repairs. Upgrading rainwater goods at the same time would make cost effective use of high-level access equipment.
If rainwater goods have deteriorated beyond repair or are modern interventions that detract from the character of the historic building, this is a good opportunity to adapt or upgrade the system to improve its resilience to a changing climate.
It is essential to check whether below ground drains have sufficient capacity to cope with the increased flow generated by upgrading gutters and downpipes. Altering existing below ground drains, or installing new ones, will need to comply with Building Regulations and may require planning consent. Additionally, underground work in a location where archaeological interest is likely to be affected may require specialist investigation. If a building stands on a scheduled monument, scheduled monument consent will be needed before ground works begin.
Historically significant rainwater goods
Rainwater goods can make an important contribution to the significance of a building. For example, date-stamped hoppers can record when a building was completed, or when an important phase of work was carried out.
Decorative hoppers, gutters and downpipes may be part of the architectural interest of a building. Even relatively simple rainwater goods, such as lead downpipes or traditional iron gutters and downpipes, can make an important contribution. Sometimes, a minor change such as replacing a downpipe with a larger diameter can markedly alter a building's character and appearance.
In planning and Listed Building Consent terms, changes to rainwater goods to accommodate increased rainfall are likely to be acceptable in most cases. The design of an upgraded system should complement a building's architecture, and care should be taken to consider management strategies or designs that minimise impact on heritage significance. Changes should be limited to those needed to achieve the required performance. For example, an alternative to replacing rainwater goods may be to supplement them with additional pipework.
Historic England Advice Note Statements of Heritage Significance: Analysing Significance in Heritage Assets provides more information.
Each case needs to be individually assessed, and listed building consent, scheduled monument consent and/or planning approval may be required.
Where it is not feasible to retain historic rainwater goods, they should be photographed and recorded before being removed.
How to adapt or upgrade gravity-fed rainwater goods
When the current system cannot cope with the volume of rainwater from the roof, interventions that could be considered include:
Adding or widening existing external downpipes
Increasing the number or diameter of downpipes may be an option if:
- the eaves gutter or hopper outlet can accommodate an increased downpipe diameter
- the falls of eaves, box or parapet gutters can be adjusted to discharge into new downpipes
- below ground drains can accommodate wider and/or additional downpipes
Adding new downpipes may not only deal with capacity shortfalls but may also simplify complicated discharge routes.
Ideally, downpipes should be set away from the wall. This will reduce risks of leaks saturating the wall and enable regular cleaning and redecoration as part of a routine maintenance regime. Where the system design cannot accommodate this, extra vigilance will be needed to watch for leaks, which should be repaired immediately.
Changes should not adversely affect the significance of a listed or scheduled building.
Increasing gutter capacity
Increasing the dimensions of eaves, valley, box or parapet gutters should be considered where extensive roof works are proposed. In the case of valley, box or parapet gutters, their capacity could be increased at the same time as roof repairs or replacement works.
Changes should not adversely affect the significance of a listed or scheduled building.
Adding fail-safe systems
The design of rainwater systems should ensure that any failure is immediately obvious, to prevent water penetration and risks to the building fabric.
Sometimes, additional fail-safe systems can be incorporated. These include adding an overflow outlet to a hopper or installing a hopper to stop overshooting from a valley gutter. Such measures would give advanced warning of capacity issues in the rainwater system.
How to size and test gravity-fed rainwater goods
The upgrade of rainwater goods needs to balance the capacity and design requirements of the system. It should take into account the risks of damage due to spill over or overflow, whether changes would affect the building's significance and the cost of the works.
Rainwater goods can be sized according to the Building Regulations 2010 Approved Document H3 Rainwater Drainage or, more accurately, according to BS EN 12056-3:2000 Gravity drainage systems inside buildings: Roof drainage, layout and calculation. Both documents provide guidance based on the effective roof area and the rainfall intensity.
Effective roof area
There are two methods that can be used to calculate the effective roof area:
- According to Building Regulations 2010 Approved Document H3 Rainwater Drainage, the effective roof area is calculated using the roof plan area and adding an allowance for the roof pitch at 30°, 45° and 60°. This is a simplified calculation, and it is not completely accurate for roof pitches steeper than 70° (the same calculation is used for 75° pitch and 85° pitch)
- According to BS EN 12056-3:2000 Gravity drainage systems inside buildings: Roof drainage, layout and calculation, the effective roof area is calculated according to the roof pitch but includes a correction to account for wind-driven rain in an angle of 2:1 or approximately 26° from vertical. This is a more accurate calculation method as it accounts for all roof pitches
Rainfall intensity
Past guidance recommended fixed values of rainfall intensity, without considering regional differences and local climatic conditions. This is no longer considered best practice when taking into account resilience to climate change.
- Current guidance (BS EN 12056-3:2000 Gravity drainage systems inside buildings: Roof drainage, layout and calculation and Building Regulations 2010 Approved Document H3 Rainwater Drainage) includes rainfall intensity maps, where design rainfall intensity values vary depending on the geographical location of a building. These maps are not based on latest data. To account for risks of overflowing due to higher rainfall intensities than the values given, BS EN 12056-3:2000 includes a risks factor table for additional consideration
- A more accurate way to determine rainfall intensity, which accounts for a changing climate, is using statistical rainfall data where it exists. The Environment Agency's guidance Flood risk assessments: climate change allowances explains how to use the peak rainfall allowances map which accounts for anticipated changes in peak rainfall intensity. This information can also be consulted in a table format using the name of the management catchment where the building is located: peak rainfall climate change allowances by management catchment
Calculations of rainwater goods capacity and behaviour are always approximate, and it is important to test any new disposal system before it is signed off. Testing methods are included in BS EN 12056-3:2000 Gravity drainage systems inside buildings: Roof drainage, layout and calculation.
Choosing materials for gravity-fed rainwater goods
For functionality and longevity, the materials chosen for rainwater goods should always be robust, easy to maintain and of the best possible quality. The choice of materials can make an important contribution to a building's appearance and, therefore, its significance. Where relevant, historic materials should continue to be used.
When rainwater goods need to be replaced, this is an ideal opportunity to remove modern unsympathetic materials and reinstate original materials. The latter are normally more robust and will last longer, and will also enhance a building's appearance. In the case of partial replacements, any permanent new sections should have the same profile as the originals and be made of the same material.
Metal rainwater goods can be subject to galvanic corrosion, so their fixings and fittings must be chosen with particular care.
Lead and cast iron rainwater goods can, with appropriate maintenance and redecoration, last far longer than modern replacement materials.
Lead rainwater goods should be repaired with lead, except where the material is at high risk of being stolen and the local planning authority and Historic England (if statutory consultee) deem a substitute material to be more appropriate.
Wholesale rainwater system replacement in PVCu is not generally acceptable on a designated building. It not only impacts its appearance, but the lifespan of PVCu is much shorter than that of other materials. Powder coated aluminium or heavy cast aluminium may be appropriate in some circumstances when like for like replacement is not possible.
Part replacement may affect the integrity of the whole system, because it is difficult to connect powder coated aluminium or PVCu to lead and cast iron. Also, while aluminium may outlast plastic, both materials are essentially single use and difficult to maintain and repair.
Siphonic drainage systems
A siphonic drainage system is fitted with a baffle plate that restricts air flow and allows higher volumes of water to drain quicker than in a gravity-fed drainage system, which relies on the whirlpool effect created by air entering the system. It may sometimes be used where the number of downpipes cannot be increased (because it would harm the significance of the building, for example) or to improve the discharge within a rainwater system embedded in the building fabric.
A siphonic system must be designed in accordance with BS EN 12056-3:2000 Gravity drainage systems inside buildings: Roof drainage, layout and calculation. The effective roof area and rainfall intensity can be determined in the same way as for a gravity-fed system.
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