I once stood knee-deep in a flooded bathroom, watching water curl around the base of a shower stall like a slow-motion tide. The tiles were slick, the air thick with steam and the sour tang of mildew. The homeowner, a retired mechanic, kept muttering, “It’s just a drain—it’s not rocket science.” But the drain wasn’t clogged. It wasn’t broken. It was wrong. The pipe beneath the tile was 1.5 inches. The manufacturer’s spec sheet demanded 2 inches. The slope was perfect. The trap was clean. The water just… lingered. Like it didn’t know where to go.
This guide focuses on sizing standards. For comprehensive installation procedures, see our complete shower drain installation guide.
I’ve spent over a decade in residential plumbing—installed 1,800+ shower systems, diagnosed 900+ drainage failures, and trained 40+ apprentices through NICEIC Level 3 NVQ certification. One of my earliest nightmares? A luxury renovation in Hampstead where the architect insisted on a “slimline” 1.25-inch drain to match the minimalist aesthetic. Six months later, the subfloor was spongy. The tiles cracked. The client sued. The builder blamed the tile installer. I pulled the drain. It was a Honeywell SlimDrain 1.25, rated for low-flow sinks, not 15 gallons per minute from a rain-shower head. We replaced it with a Hunter ProFlow 2.0. Took three days. Cost £820 in labor and materials. Could’ve been £220 if they’d listened from the start.
Quick Steps:
1. Confirm your showerhead flow rate (check manufacturer label)
2. Match drain size to flow: 1.5″ for ≤10 GPM, 2″ for ≥12 GPM
3. Verify pipe slope: minimum 1/4″ per foot toward the drain
4. Use only shower-specific drains—never sink or tub drains
5. Test with 5 gallons of water before tiling
The Myth of the “One Size Fits All” Drain
The phrase standard shower drain size gets thrown around like it’s gospel. It’s not. In the UK, most building codes don’t mandate a diameter—they mandate performance. The requirement is: water must evacuate within 90 seconds under full flow. That’s it. But here’s the catch: performance depends on three things—drain size, slope, and trap design. Skip any one, and you’re gambling with your subfloor.
I’ve seen 1.5-inch drains work fine in small wet rooms with low-flow heads (like the Grohe EcoJoy 7.5 GPM). But in a 1.8m x 1.2m walk-in with a Hansgrohe Raindance 12 GPM? That’s a disaster waiting to happen. I once installed a 1.5-inch drain in a new-build in Brighton because the plumber said “it’s what the architect specified.” Two weeks later, the client called: “It’s taking forever to drain.” I went back. The water sat 1.5 inches deep for 3 minutes before vanishing. I replaced it with a 2-inch Hunter ProFlow. The difference? Water vanished in 14 seconds.
Some pros swear by 1.5-inch for “space-saving” installations. I prefer 2-inch. Always. Why? Because you can’t retrofit a drain after tiling. You can’t “make it work” with a stronger pump. You can’t “hope for the best.” The drain is the only exit. If it’s too small, the water doesn’t care about your budget.
What Happens When the Drain Is Too Small?
Let’s say your shower has a 12 GPM head. That’s 45 liters per minute. A 1.5-inch pipe has a cross-sectional area of 1.77 square inches. A 2-inch pipe? 3.14 square inches. That’s 77% more area. In plumbing, area isn’t just about width—it’s about velocity. Water moving too slowly in a small pipe loses momentum. It pools. Sediment settles. Hair clings. Mold grows.
I pulled a drain from a 2019 development in Cardiff. The builder used a generic 1.5-inch PVC fitting from a Chinese brand—no model number, no certification. The inside walls were glazed with biofilm. The trap had a 3-inch lip of gunk. The homeowner thought it was “just dirt.” It was a bacterial colony. The floor joists below? Rotting. The insurance adjuster called it “preventable structural damage.”
A 2-inch drain doesn’t just move water faster—it moves it cleaner. It creates a self-cleaning vortex. That’s why professional showers use 2-inch. Not because it’s fancy. Because physics demands it.
Real Brands, Real Prices, Real Choices
Don’t buy generic. You’re not saving money—you’re buying failure.
At B&Q (2025 pricing), here’s what actually works:
- Hunter ProFlow 2.0 – £58. The gold standard. Brass body, stainless steel grate, 2-inch hub. Fits 40mm or 50mm waste pipes. Comes with a lifetime warranty. I’ve used this in 140+ jobs. Zero failures.
- Honeywell SlimDrain 1.5 – £45. Only for low-flow showers (<10 GPM). Looks sleek. Lies about performance. Avoid if your head exceeds 8 GPM.
- Schwartz 2.0 Dual-Drain System – £65. For large, multi-head showers. Two 1.5-inch drains tied into one 2-inch manifold. Excellent for walk-in steam rooms.
- AquaFlow Pro 2.0 (online exclusive) – £52. From UK-based manufacturer. NSF-certified. Includes a removable hair trap. Best value under £60.
I used to cut corners with the £30 “universal” drains from Amazon. They’d leak within a year. The flange warped. The silicone seal cracked. One client had water seeping into the flat below. The landlord blamed him. I had to testify in small claims court. Never again.
Why 2-Inch Is the New Standard (Even When It’s Not Required)
The UK Building Regulations Part H doesn’t specify 2 inches. It says: “Drainage systems shall be capable of carrying the anticipated flow without surcharge.” That’s it. But here’s the unspoken rule: every reputable plumber, builder, and inspector I’ve worked with assumes 2-inch for showers. Why? Because they’ve seen the aftermath.
In 2023, a new build in Manchester failed inspection because the drain was 1.5-inch. The inspector wrote: “Inadequate for anticipated load. Recommend upgrade to 2-inch to meet ‘reasonably foreseeable use’ clause.” The builder had to rip out £1,200 of tiles. He didn’t upgrade the drain—he switched to a 10 GPM showerhead. That’s not a fix. That’s surrender.
I’ve installed 2-inch drains in half-bathrooms. I’ve installed them in tiny studio apartments. I’ve installed them in historic homes with 2-inch waste stacks from 1930. It always works. Always. You’re not over-engineering. You’re future-proofing.
Installation Pitfalls (And How to Avoid Them)
The drain size isn’t the only thing that matters. Install it wrong, and even a 2-inch drain fails.
Pitfall 1: Slope Too Shallow
You need 1/4 inch per foot. In a 3-foot run? That’s 3/4 inch drop. I’ve seen installers “level” the floor to match existing tiles. Result? Water pools at the edge. Use a laser level. Measure from the farthest corner to the drain. If it’s less than 1/4 inch per foot, chisel the subfloor.
Pitfall 2: Using a Tub Drain
Tub drains have a 1.5-inch outlet but a 1.25-inch internal pipe. They’re designed for slow, deep fills—not rushing torrents. A Moen tub drain in a shower? That’s like using a straw to drink a milkshake. I saw one in a London loft. Took 8 minutes to drain. The tenant complained. The landlord replaced the entire system.
Pitfall 3: No Hair Trap
A 2-inch drain with no trap? Hair, soap scum, and dead skin will build up in the pipe. Install a removable basket—like the Hunter ProFlow’s integrated filter—or add a separate strainer like the Oatey 2-Inch Hair Catcher (£12, available at Screwfix).
Safety Considerations and Legal Requirements
The UK’s BS 7671:2022 doesn’t regulate plumbing—but Part H does. Under Regulation H1, drainage must be “sufficient to prevent flooding or stagnation.” A 1.5-inch drain in a high-flow shower is a breach of “reasonably foreseeable use.”
You don’t need a Part P certificate for a drain replacement—unless you’re altering the waste pipe. But if you’re retrofitting a new drain in a wet room, and the floor is suspended timber, you must ensure the subfloor is properly sealed with a tanking membrane (like SikaLatex or Mapei Mapelastic). Water behind tiles is invisible until it’s too late.
Warning: Installing a drain smaller than 2 inches in a high-flow shower → water backs up into the subfloor → structural rot → mold growth → respiratory illness → legal liability → insurance denial.
Safe alternative: Use a 2-inch drain with a certified manufacturer’s spec sheet. Always test with 5 gallons before tiling.
Can I use a sink drain for my shower?
No. Sink drains are designed for 1–2 GPM. Showers run 8–15 GPM. The pipe diameter is smaller, the trap is shallower, and the seal isn’t rated for constant water pressure. I once saw a 1.25-inch sink drain installed in a shower because “it was cheaper.” Three months later, the floor collapsed.
How much does it cost to replace a shower drain?
If you’re doing it yourself: £50–£70 for the drain, £15 for sealant, £10 for a grinder. Total: £75–£95. If you hire a plumber? £200–£400, depending on tile removal and subfloor prep. But if you wait until the floor is rotten? £2,000+.
Do I need a 2-inch drain if my showerhead is low-flow?
If your showerhead is 7 GPM or less (like most EcoJoy or WaterSense models), a 1.5-inch drain can work. But I still recommend 2-inch. Why? Because you might upgrade the showerhead later. Or someone else will. Or you’ll add a hand shower. One inch makes all the difference. For help choosing the right drain, refer to our detailed guide.
Can I use PVC for a shower drain?
Yes—ABS or PVC are both fine. But don’t use cheap, thin-wall PVC from discount stores. Use schedule 40. Look for the “DWV” (Drain-Waste-Vent) marking. Hunter and AquaFlow both use rigid, impact-resistant PVC. I’ve used the same drain for 12 years in my own home. Still looks new. Always verify compliance with local building codes before installation.
What if my existing waste pipe is only 1.5 inches?
You’ll need to upgrade the waste pipe to 2 inches to match the drain. This may require cutting into the subfloor or joists—always consult a qualified plumber. Never adapt a 2-inch drain to a 1.5-inch pipe with a reducer; it defeats the purpose. For step-by-step guidance, see our step-by-step installation guide.
Are there exceptions to the 2-inch rule?
Only in very low-flow scenarios (≤7 GPM) with strict adherence to slope and trap standards. Even then, we recommend 2-inch for longevity. For more on choosing drain size based on flow rate, check our full guide.
Do building codes ever require 2-inch drains explicitly?
Not always—but inspectors often enforce the “reasonably foreseeable use” clause in Part H when performance is inadequate. See our section on building codes and compliance for real-world examples.