Fitting an Outside Tap: Backflow Prevention, Pipe Routing & Building Regs
An outside tap must include a double-check valve (fluid category 3, per Water Supply Regulations 1999) on the supply side to prevent garden hose contamination siphoning back into the mains. Connect to the rising main (not a branch from the hot water supply) with 15mm copper or plastic pipe, insulate the external run, and install a servicing valve inside the property. The connection to the mains water supply is notifiable to your water undertaker under the Water Supply (Water Fittings) Regulations 1999.
Summary
Fitting an outside tap is one of the most common add-on plumbing jobs in UK residential properties, and it is also one of the most frequently done without appropriate backflow protection. The consequence of fitting a tap with no double-check valve is that a hosepipe connected to the tap, left lying in a contaminated puddle or bucket with fertiliser dissolved in it, can siphon back into the mains water supply under negative pressure events (such as a main burst nearby). This is a genuine public health risk and is why the Water Regulations specifically require backflow prevention at fluid category 3 or above at any tap connected to hosepipes or garden irrigation.
The job itself is straightforward for any competent plumber. The key decisions are: where to connect to the existing pipework; how to route the pipe through the wall; what isolation arrangement to provide inside the building; and what backflow prevention device to fit and where.
Key Facts
- Water Regulations requirement — Outside taps are notifiable to the water undertaker (water company). In practice, self-certification by a WRAS-registered plumber is accepted
- Backflow protection minimum — Double-check valve (DCV) — fluid category 3. A single check valve (one non-return valve) is insufficient
- Location of DCV — Inside the property, upstream of the tap. Not in the frost zone. Can be combined with the service valve in a single fitting (e.g., Surestop + DCV combinations)
- Connection point — To the rising main (cold supply), ideally below the stopcock but above the meter. Do not connect downstream of any softener
- Pipe material — 15mm copper or WRAS-approved plastic (Hep2O, Speedfit) internally. Copper or stainless steel externally in exposed locations
- Frost protection — External pipework must be insulated. Internal service valve allows isolation and draining down in winter. Drain-down taps are available that allow the external pipe to be drained from inside
- Notifiable to WRAS — Under Regulation 5 of the Water Fittings Regulations, any new connection to the supply must be notified to the water undertaker before work begins (10 working days notice required unless self-certifying through an approved scheme)
- Surestop valve — A remote-operated service valve (pull cord, toggle, or electronic) that allows the outside tap to be isolated from inside without a separate visit to the internal stopcock — popular addition for ease of use
Quick Reference Table
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Try squote free →| Component | Requirement | Notes |
|---|---|---|
| Double-check valve | Mandatory | Must be WRAS-approved |
| Service valve | Required | Inside building, accessible |
| Drain-down facility | Recommended | Prevents frost damage |
| Pipe insulation (external) | Required | BS 5422 lagging |
| Wall entry sealing | Required | Weatherproof and damp-proof |
| Tap type | Hose union bib tap | WRAS-approved fitting |
| Fluid Category | Risk | Example | Protection Required |
|---|---|---|---|
| 1 | None | Drinking water supply | None |
| 2 | Slight (taste/aesthetics) | Softened water | Single check valve |
| 3 | Significant (chemical) | Hosepipe in garden chemicals | Double-check valve |
| 4 | High (toxic) | Chemical dosing systems | RPZ valve |
| 5 | Very high (microbiological) | Irrigation submerged emitters | Break tank |
Detailed Guidance
Choosing the Connection Point
The best connection point is the rising main in the kitchen — typically 15mm copper — with a T-fitting and ball valve. Position the T as close to the rising main stopcock as practical. Avoid connecting to:
- Waste pipes (obvious)
- Hot water supply (unnecessary heat loss, water wastage)
- Downstream of a water softener (hardened water is preferred for garden use; softened water has elevated sodium content)
- A gravity-fed cold supply (pressure will be too low for effective hose use)
If the kitchen is on the opposite side of the house from the garden, consider connecting at the nearest point to the outside wall rather than routing 5–10m of pipework.
Installing the Double-Check Valve
The double-check valve must be installed:
- Inside the building (not frost-exposed)
- In an accessible location (it needs annual inspection)
- Upstream of the service valve (between the mains supply and the service valve)
- With the arrow on the valve body pointing toward the tap (in the direction of flow)
Many plumbers use a combined service valve and double-check valve unit — this simplifies the installation and takes up less space. Ensure the unit is WRAS-approved (look for the WRAS logo on the product or check the WRAS product approval database).
Routing the Pipe
Through the wall: Drill a 28mm hole through the masonry (to allow 15mm pipe with lagging to pass). Angle the hole slightly downward to the outside (5–10°) to prevent water tracking inward along the pipe. Seal around the pipe with a suitable waterproof compound inside (silicone) and weatherproof compound outside (frame sealant or lead-free flashing tape). If the wall is a cavity wall, use a sleeved fitting that spans the cavity.
External pipe length: Keep the external pipe run as short as possible. Every metre of external pipework is a potential frost damage point. The external pipe should drop straight from the wall entry to the tap body — no horizontal runs that collect water.
Pipe material externally: Copper is traditionally used for external exposed pipework. MDPE (blue poly pipe) is suitable underground. CPVC or WRAS-approved plastics can be used externally if protected from UV degradation (wrap or paint). Chrome-plated copper looks better on finished masonry.
Tap Selection
Use a WRAS-approved hose union bib tap. Features to look for:
- ½" BSP male thread (standard UK garden tap connection)
- Solid brass construction (not chromed zinc — zinc is less durable externally)
- Integral backplate for fixing to wall surface (or use a separate wall plate)
- Optional: integral double-check valve (backup protection, not a substitute for the DCV on the supply side)
Frost Protection
The standard approach for UK outside taps:
- Install an internal service valve (ball valve or lever valve)
- At end of gardening season: close the service valve inside, open the outside tap, allow the external pipe to drain
- A drain-down tap (with a small drain-off valve in the body) allows this to be done from inside — turn the internal valve to drain, water drains from the external pipe through the internal drain-off
Alternatively, foam pipe lagging on all external pipework (minimum 25mm wall thickness) provides frost protection to approximately -5°C.
Notification
Under the Water Supply (Water Fittings) Regulations 1999, any new connection to the mains supply must be notified to the water undertaker before installation commences. In practice, if a plumber is registered with an approved scheme (WaterSafe or WIAPS), they can self-certify the installation without prior notification. The householder does not need to notify separately if using a registered plumber.
Non-notified installations are technically illegal and may create issues on property sale (solicitors are increasingly asking for WaterSafe certificates for plumbing installations).
Frequently Asked Questions
Does a double-check valve need annual inspection?
Under the Water Regulations, WRAS-approved double-check valves should be inspected periodically to confirm they are functioning correctly (no flow in the reverse direction). In practice, most are never inspected and most work correctly for 10+ years. The key is ensuring one is installed at all — the vast majority of existing outside taps in the UK have no backflow protection whatsoever.
Can I fit an outside tap myself as a homeowner?
Yes — the Water Regulations do not prohibit homeowners from doing their own plumbing. However, you must notify the water undertaker before doing the work if you are not using a registered plumber. Get it right — a tap with no DCV is a regulatory breach and a potential public health issue.
My new tap has low pressure. Why?
Outside taps should be on mains pressure (typically 2–5 bar). Low pressure indicates: (1) connection to a gravity cold supply rather than mains — resite the connection to the rising main; (2) too-small pipe (10mm pipe creates excessive pressure drop); (3) the internal service valve is not fully open; (4) the double-check valve is partially blocked — replace it.
Do I need planning permission for an outside tap?
No — fitting a garden tap does not require planning permission.
Regulations & Standards
Water Supply (Water Fittings) Regulations 1999 — Notification requirement and backflow protection standards
WRAS Water Regulations Guide (Section 15) — Backflow prevention fluid category classification
BS EN 13959:2020 — Double check valves (product standard)
BS 5422:2009 — Thermal insulation of pipes (frost protection specification)
WaterSafe (WIAPS) — Find approved plumbers and self-certification scheme
WRAS Product Approval Database — WRAS-approved fittings and valves
WRAS Water Regulations Guide — Backflow risk categories and protection requirements
water regulations — Full water supply regulations overview
push fit fittings — Fitting selection for pipework
compression fittings — Compression joint techniques for connection to existing pipework
site setup — Working on live water mains
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