Outside Tap Installation: WRAS Backflow Protection, Double Check Valve and Freeze Protection

Quick Answer: An outside tap (garden tap, bib cock) must include a double check valve (DCV) to prevent backflow contamination of the potable supply, per Water Supply (Water Fittings) Regulations 1999. A self-draining bib tap or stop-and-drain isolation valve is required for freeze protection. Tap location should allow for winter isolation and drain-down. Internal isolation valve, dedicated 15mm copper supply, and lagged pipework through the wall are standard.

Summary

An outside tap is a tiny job that combines water hygiene, Building Regulations and basic plumbing competence. Done wrong, it contaminates the public mains and bursts every winter. Done right, it costs about £80 in parts, takes 90 minutes, and lasts 20 years.

The non-negotiable rule is backflow protection. The Water Supply (Water Fittings) Regulations 1999, Schedule 2, classify a garden tap and hose connection as Fluid Category 3 risk (organic substances) — meaning the potable mains must be protected by at least a double check valve (DCV) between the tap and the mains. Without a DCV, contaminated water from a hose end (left in a puddle, weed killer mix, swimming pool) can siphon back into the cold supply during a pressure drop, contaminating the household drinking water and potentially the wider mains.

The second non-negotiable is freeze protection. UK winters drop below 0°C every year and an unisolated outside tap with water in the pipe will burst. The fix is either a self-draining tap (water drains back when shut off) or an internal isolation valve combined with the tap's stop-and-drain feature. Skip this and the customer calls back in February with a flooded garage. See water pressure for supply considerations and legionella risk for backflow contamination risk.

Key Facts

• Water Regs requirement — Double check valve (DCV) for any tap with hose connection (Fluid Category 3)
• DCV location — between mains tee and the outside tap; internal where possible (frost protection)
• WRAS approval — all components must be WRAS-approved (check WRAS approved products database)
• Tap type — bib cock with hose union, or wall-mounted bib tap with self-drain
• Freeze protection — internal isolation valve + drain port, or self-draining tap, or both
• Wall thickness — typical 215mm cavity wall + render; tap spindle must reach through
• Pipe size — 15mm copper from internal tee, 15mm copper through wall (or proprietary outside tap kit)
• Internal isolation — quarter-turn ball valve with drain-down port
• Backflow categories — Fluid Category 1 (potable), 2 (low risk), 3 (low health risk, e.g. garden hose), 4 (significant), 5 (serious health risk)
• External through-wall sealing — silicone sealant around pipe and tap fixing screws; weatherproof
• Insulation — internal pipework insulated to point of penetration; external bib tap with insulated cover for severe winter
• Notification — WRAS notification not normally required for domestic outside tap, but water company may inspect on request
• Hose union — 3/4" BSP standard for garden hose
• Frost-proof / self-drain — proprietary taps (Kontite, Reliance, Pegler) drain water back when closed, leaving pipe dry
• Wall plate connector — secures tap to outside face; allows easy removal for repair

Quick Reference Table

Detailed Guidance

Step 1: Choose the location

The outside tap location is a balance between:

• Access to the internal supply (typically kitchen rising main)
• Convenience for outside use
• Distance from boiler vent or other obstructions
• External wall type (cavity wall, solid wall, render)
• Through-wall route (no joists or services to avoid)

Most domestic installations tap into the cold supply under the kitchen sink and pass through the rear elevation directly behind. Avoid penetrating bedroom walls or finished living spaces if possible.

Step 2: Plan the route

Plan the internal pipe route:

1. Tee off the rising main (or kitchen cold supply) downstream of the stop tap
2. Fit an internal isolation valve close to the tee — accessible for winter isolation
3. Fit the DCV after the isolation valve (some valves combine isolation, drain and DCV)
4. Run 15mm copper to the external wall
5. Drill the wall (22–25mm hole for 15mm pipe through cavity)
6. Sleeve through cavity wall to prevent contact with insulation
7. Wall plate elbow or stub to outside face

Optional: a single proprietary outside tap kit (Reliance, McAlpine) combines isolation, DCV, pipe and tap in one assembly — simplifies the job at slightly higher parts cost.

Step 3: Cut into the supply

With the stop tap closed:

• Drain down the supply at the lowest cold tap (kitchen or basement)
• Cut into the rising main at planned tee position
• Fit the tee (compression for easy work, soldered for permanent join)
• Fit the isolation valve immediately downstream
• Fit the DCV immediately downstream of isolation

The DCV must be a WRAS-approved product, marked appropriately. Verify the flow arrow direction — DCVs are directional.

Step 4: Wall penetration

Mark the penetration position on both sides of the wall. Drill a 22mm hole through with a long SDS bit on hammer drill mode. For cavity walls, drill from outside in (so spalling appears on the inside, where it can be plastered over).

For solid masonry, the pipe can pass directly. For cavity walls, fit a copper sleeve (typically 22mm copper, slightly oversized to allow movement) to prevent the supply pipe from contacting cavity insulation or mortar (vibration noise + freezing risk).

Pass the 15mm copper supply pipe through, leaving 50mm protruding outside. Seal around the pipe externally with silicone sealant, sloped to shed water away from the wall.

Step 5: External tap fitting

The wall plate elbow has a back mounting plate that screws to the external wall (typically 50mm × 8 gauge stainless screws into wall plugs in masonry). The pipe connects via compression to the elbow's bottom port; the outside tap screws into the elbow's top port (3/4" BSP).

Use PTFE tape on the tap thread (15–20 wraps clockwise) and tighten to seal but not so tight as to crack the elbow. Position the tap spout pointing downward.

For self-draining taps, fit the tap so the drain port falls freely (typically downward and outward).

Step 6: Insulate and pressure test

Insulate the internal pipework with closed-cell foam (25mm wall thickness for outside taps and supply runs in unheated space). The insulation should reach the external wall penetration.

Reinstate water supply, open all isolations, check for leaks (pressure test under static for 10 minutes — no pressure drop).

Operate the outside tap to clear air, check flow, close and check for drips.

Step 7: Test backflow protection

Inspect the DCV: check both check valves operate independently. A simple test: with the outside tap open and no water flowing, the DCV should hold pressure against backflow attempts.

Mark the location of the internal isolation valve on a label and explain its operation to the customer. The customer needs to know how to drain down for winter.

Step 8: Winter isolation procedure

Brief the customer on winter shutdown:

1. Close internal isolation valve (typically October/November)
2. Open outside tap to drain residual water
3. Open the drain port on the isolation valve (if fitted) to drain the section between valve and tap
4. Leave outside tap open through winter to relieve any expansion
5. Optional: fit insulated cover over the outside tap

Reopen in spring by reversing the sequence.

Frequently Asked Questions

Do I really need a double check valve?

Yes — it's a legal requirement under Water Regs Schedule 2, point 6.7. The water company can demand to see compliant fittings on inspection. A standard kitchen tap (no hose connection) is Category 2 and doesn't need a DCV; a garden tap with hose IS Category 3 and DOES need one. Failure to fit is also genuinely dangerous — a hose left in a puddle of weedkiller can siphon back during pressure loss.

Where should the DCV go — inside or outside?

Inside is preferred for freeze protection: an external DCV can freeze and crack. Inside the heated envelope, the DCV doesn't need protection. Fit it close to the tee with the isolation valve, ideally accessible for inspection.

My customer wants the outside tap on a brick pier in the garden (away from house). Can I run pipe underground?

Yes. Use MDPE blue (25mm typical) buried 750mm minimum depth (frost line) with insulation through the topsoil layer. Terminate in a frost-proof tap above ground with an internal stop tap in the house. The MDPE rises into a brick column or post and connects via a wall plate elbow. Drain valve at the lowest point for winter.

Can I use plastic push-fit through the wall?

Plastic push-fit (Hep2O, JG Speedfit, Polypipe) is acceptable for the supply run but copper is preferred for the through-wall section and the external connection — it's more robust, less likely to be damaged, and easier to insulate. Push-fit connections within walls or buried in screed are problematic (no access for failures) and should be avoided where copper or soldered joints are alternatives.

What's the difference between a self-draining tap and a frost-free tap?

Both terms are used interchangeably. The principle: when the tap is closed, water drains back from a chamber to either a manual drain or a one-way valve, leaving the visible spout and the section near the tap dry. Reliable products: Reliance Self-Drain, Pegler Anti-Frost, Kontite Frost-Free.

Do I need to notify Building Control?

No. Outside taps are not a Building Control matter (no structural or fire safety implication) and do not require notification under Part P (no electrical). The water company has the right to inspect and enforce Water Regs but generally does not require notification for a single domestic outside tap.

Can I use my outside tap to fill a hot tub?

A hot tub or paddling pool connection is acceptable provided the DCV is in place and the user understands they cannot leave the hose end in standing water. The Fluid Category 3 backflow protection (DCV) is adequate for these uses. Pool circulation or chemical handling may require Category 4 or 5 protection (RPZ valve or air gap) — that's the pool installer's responsibility.

What about a frost stat that drips when temperature drops?

Some installations use a frost stat (temperature-sensitive drip valve) instead of full isolation. The valve opens at ~3°C and allows a slow drip to keep water moving and avoid freezing. Effective but wastes water and only works if there's flow. Customer isolation and drain-down remains better practice.

Regulations & Standards

• Water Supply (Water Fittings) Regulations 1999 — primary regulation

• Schedule 2 — Requirements for Water Fittings — Section 6 — Backflow Prevention

• BS EN 1717 — Protection against pollution by backflow; classifies Fluid Categories

• BS 6700 (now superseded) and BS EN 806 (current) — installations supplying water for human consumption

• WRAS Approved Products — products certified to comply with Water Regs

• Building Regulations Approved Document G — sanitation and water efficiency

• CIPHE Technical Guidance — UK plumbing technical reference

• Water Industry Act 1991 — water supplier duties and powers

• Wholesome Water Quality Regulations 2016 — quality standards for drinking water

• BS 8558 — design, installation, testing and maintenance of services supplying water

• Water Regulations Approval Scheme (WRAS)

• Water Regs Approved Products Directory

• Approved Document G — sanitation and water efficiency

• Reliance Outside Tap Kits

• CIPHE — Chartered Institute of Plumbing and Heating Engineering

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