Plumbing and drainage drawings in London

Drain gradients: the numbers that make waste flow

Approved Document H gives minimum falls for foul drains. A one-hundred-millimetre pipe carrying under one litre per second of peak flow can run at 1 in 40, and when peak flow is above that the guidance allows 1 in 80 (minimum 1 WC). A seventy-five-millimetre pipe allows 1 in 80 above one litre per second.

Larger pipes are shallower still: a one-hundred-and-fifty-millimetre drain can be laid at 1 in 150 (minimum 5 WCs). Any minimum number of fittings stated is part of the rule, because a flat fall with too little flow leaves solids behind.

We check each run against these figures on the section as well as the plan. In a London terrace with a shallow existing drain, the fall is often the limit on where a new bathroom can go.

Building over or near a drain

Many extension drainage questions start with one. The building-over requirement of Approved Document H covers building over or close to public sewers and lateral drains, and its distances are specific. The guidance looks at work within 3 metres of the centreline of a drain.

A pipe passing under a building should not run more than 6 metres beneath it. Building over is not advised where the drain is deeper than 3 metres, or where the pipe is larger than 225 millimetres across.

In a built-over case the water company usually has to agree, and the foundations are designed around the pipe. The drawing marks the drain, the offset to the new walls and the protection proposed. If the foundations are involved, our structural engineering service coordinates with the drainage layout.

Connections, sewers and who owns what

Since the transfer date of 2011-10-01 (year, month, day), many sewers that once belonged to groups of houses have been owned by the water company. That changes who you ask when a shared drain is affected, so we confirm ownership early.

Under section 106 of the Water Industry Act 1991, the sewerage undertaker has 21 days to respond to a notice of a proposed connection. Section 185 of the same Act deals with moving pipes: Duty on the sewerage undertaker to move or alter its pipe or apparatus on notice from a person with an interest in the land.

These are statutory periods and duties, not promises about any one water company. We prepare the connection plan, and you or your contractor submit the application.

Hot and cold water: efficiency and safety limits

Regulation 36 sets a calculated water use for new dwellings. The standard is 125 litres per person per day, and 110 where the optional requirement applies. We pick taps, showers and cisterns to hit the figure and record the calculation.

Once the work is complete, the person carrying it out must tell the building control body the resulting consumption within 5 days.

The bath outlet is limited to 48 degrees Celsius, and the schematic shows the control that achieves it.

Party walls and excavation near neighbours

New drains and foundations can trigger the Party Wall etc. Act 1996 when they dig close to a neighbour's building. Section 6 applies if excavation is within 3 metres of a neighbouring structure and deeper than its foundations, or within 6 metres where a line at 45 degrees from the foundation crosses the new work.

Notice must be served at least 1 month before the work starts. Our party wall notices service covers the paperwork, and the drainage drawing gives the surveyor the depths and distances.

Price, scope and how the sheets are checked

We do not publish a starting price for plumbing and drainage drawings. A bathroom move and a new-build drainage layout are very different jobs, so we run a free constraints check and send a fixed quote.

We design and draw. We do not install, pressure-test or certify, and we do not replace the plumber's or groundworker's sizing where a specialist system needs it.

A second technologist checks the sheets, and a senior review signs them off before they join the construction drawings package.

Plumbing and drainage drawings by borough

Ground conditions, flood risk and sewer ownership vary across London, and so do the designations that limit external pipework such as soil stacks on a front elevation. Our council pages set out the local figures for each borough.

Start with the borough where the house stands. Architectural drawings in Croydon shows the format, and architectural drawings by area lists every place we cover.