Can I put a bathroom in a loft conversion?

Yes, a bathroom or en-suite is a common feature of loft conversions. The main challenges are routing the soil pipe (110mm for WC) to the existing drainage, providing adequate ventilation, and ensuring the floor structure can support the additional loads (especially a bath). Building Regulations Parts G, F and H apply.

Do I need a macerator pump for a loft bathroom?

A macerator pump (like Saniflo) is needed only if the WC is below the level of the nearest drainage connection, or if routing a full 110mm soil pipe is impractical. Macerator pumps allow a 32mm discharge pipe to be run horizontally up to 50m. They add ongoing maintenance and noise considerations.

How should the loft bathroom be ventilated?

Part F requires a minimum extract rate of 15 L/s (intermittent) or 8 L/s (continuous MEV) for a bathroom. An openable window providing 1/20th of the floor area is acceptable as an alternative. An internal bathroom (no window) must have mechanical ventilation with a 15-minute overrun timer.

Do I need waterproofing in a loft bathroom?

Yes. The floor and walls in wet zones (around shower, bath and basin) should be waterproofed using a tanking membrane, liquid applied waterproofing or sheet membrane before tiling. This prevents water penetrating the floor structure and causing damage to the ceiling below.

What soil pipe routing options are there?

Internal routing boxes the soil pipe inside the loft space, running it down through the airing cupboard or built-in wardrobe to connect below. External routing takes the pipe down the outside wall — simpler but visible. Both must connect to the existing soil stack or drainage at a suitable point.

TradeCalculator.co.uk

Loft Bathroom Calculator — Soil Pipe, Pump & Ventilation

Plan a loft bathroom with soil pipe routing, macerator pump requirements, Part F ventilation rates and waterproofing quantities.

Room Length

Bathroom length

Room Width

Bathroom width

Requires 110mm soil pipe or pump

Basin

32mm waste pipe

Shower

40mm waste pipe

Bath

40mm waste, check floor loading

Soil Pipe Route

Ventilation Type

Below Drain Level (Needs Pump)

If gravity drainage not possible

Bathroom Budget (optional)

Total bathroom fit-out cost

How We Calculate This

This calculator helps plan a loft bathroom by determining soil pipe routing, pump requirements, ventilation and waterproofing quantities.

Key requirements

Ventilation: 15 L/s intermittent or 8 L/s continuous (Part F)

Waterproofing: floor area + 300mm wall splash zones

Soil pipe options

Internal: 110mm pipe boxed in within the loft
External: 110mm pipe down the outside wall
Macerator: 32mm pipe if gravity drainage not possible

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.