Water Regulations and Outdoor Taps in London: The Double Check Valve Requirement
Every outdoor tap in London must be fitted with a WRAS-approved double check valve under the Water Regulations. This guide explains what the requirement is, why it exists, and what happens if your outdoor tap does not have one.
The Legal Requirement for a Double Check Valve on an Outdoor Tap
The Water Supply (Water Fittings) Regulations 1999 require that any outdoor tap installation in England, Scotland, or Wales must incorporate an appropriate backflow prevention device. For an outdoor tap used with a hosepipe — the standard use case — the required device is a double check valve complying with the WRAS (Water Regulations Advisory Scheme) specification. This is not a guideline or a recommendation; it is a statutory requirement. Installing an outdoor tap without a double check valve is a criminal offence under the Regulations, and Thames Water has the right to disconnect any water fitting that does not comply.
In practice, enforcement against individual homeowners is rare. Thames Water and other water companies focus their compliance activity on commercial premises and on larger residential projects where independent inspectors are involved. However, the legal requirement exists and applies to all installations — including existing outdoor taps that were fitted before the requirement was widely understood. If you have an outdoor tap that was fitted without a double check valve, you are operating a non-compliant installation that could be the subject of a notice from Thames Water requiring rectification.
Why the Regulation Exists: Backflow Risk
The backflow risk from an outdoor tap is specific and well-understood. A hosepipe connected to an outdoor tap represents what the Water Regulations classify as a fluid category 4 risk — a risk of the fluid entering the mains supply containing chemical or biological contaminants that could be harmful to health. The scenario that the regulation addresses is as follows.
Imagine a hosepipe is connected to an outdoor tap and left submerged in a paddling pool treated with a small amount of pool disinfectant, or in a garden pond, or in a bucket of concentrated weedkiller solution, or even a bucket of car washing soap. In normal circumstances, mains pressure keeps water flowing out of the tap and through the hosepipe. But if mains pressure drops suddenly — for example, because a main bursts nearby and water supply pressure across the street temporarily reverses — the submerged end of the hosepipe can siphon back, drawing contaminated water from the pool, pond, or bucket back through the hosepipe, back through the tap, and into the mains water supply. This contaminated water then flows to neighbouring properties and potentially into the wider distribution network.
This is not a theoretical risk. Incidents of mains contamination via unprotected garden hoses have been documented in the United Kingdom and are the primary reason the double check valve requirement was introduced into the Water Regulations.
What a Double Check Valve Does
A double check valve contains two independent spring-loaded check valves installed in series within a single body. Each check valve allows water to flow in one direction only — from the supply to the tap — and closes under spring pressure if flow stops or reverses. If either check valve fails to close, the other provides a redundant barrier against backflow. The double configuration is required specifically for category 4 risk applications because a single check valve alone is not considered sufficient protection for a hazard of this severity.
WRAS-approved double check valves for outdoor taps are compact, inexpensive devices that are fitted on the supply pipework inside the building, downstream of the indoor isolation valve. They require no maintenance and have a long service life. The entire assembly — isolation valve, double check valve, and pipework to the wall — occupies no more space than a standard push-fit fitting under the kitchen sink.
Retrofitting a Double Check Valve to an Existing Outdoor Tap
If you have an existing outdoor tap in London that was fitted without a double check valve, the retrofit is straightforward. The supply to the tap is turned off at the kitchen cold supply or at the indoor isolation valve (if one was fitted). A short section of the supply pipe inside the building is modified to insert the double check valve body in line. The supply is then turned back on and the valve is tested. The job takes 30 to 60 minutes and costs from £80 including parts. Prestige Engineers retrofits double check valves to existing outdoor taps across all London boroughs. Contact us to bring your outdoor tap installation into compliance with the Water Regulations.