Loft Bathroom Calculator — Soil Pipe, Pump & Ventilation

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.

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 gravity soil pipe is impractical. WC macerators (Saniflo, Saniplus, Sanipro and similar) discharge through a small 22mm pipe, run horizontally up to around 50m on the entry-level unit or up to about 100m on a Saniplus, with a minimum 1:100 fall after any vertical lift. The 32mm discharge size applies to grey-water-only waste pumps (Sanivite/Sanispeed), not to WC macerators. Macerators add ongoing maintenance and noise considerations.

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.

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.

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.