Drainage fall calculator

Getting drainage falls right on every job

Drainage fall is one of those things that seems straightforward until you get it wrong. Lay pipes too flat and you end up with blockages and callbacks. Too steep and the water races ahead of the solids, leaving deposits in the pipe. Getting it spot on means fewer problems down the line and a job that passes Building Control first time.

This calculator takes your pipe run length and drainage type, then works out the minimum fall per metre, the total drop you need from start to finish, and confirms whether your gradient meets Approved Document H.

Building Regulations Part H: the key numbers

Approved Document H of the Building Regulations covers drainage and waste disposal for buildings in England and Wales. The two numbers every drainage engineer needs to know:

• Foul water: 1:40 minimum (25mm drop per metre). For a single WC on a short run, 1:80 may be acceptable.
• Surface water: 1:100 minimum (10mm drop per metre). This is for rainwater drainage from gutters, gullies, and downpipes.

Scotland follows Technical Handbook Section 3, and Northern Ireland uses Technical Booklet N. The gradient requirements are broadly similar, but always check the specific document for your region.

How to set out drainage falls on site

Before you start digging, work out your invert levels at each end of the run. The invert level is the lowest internal point of the pipe at any given location. Mark these on your drawings and transfer to site using a laser level or dumpy level.

For a typical domestic extension with a 10m foul water run at 1:40, you need 250mm of total drop from the first connection to where it meets the existing drain or manhole. That is the invert at the house end minus 250mm equals the invert at the manhole end.

Dig your trench to the correct depth, add a bed of 10mm pea gravel (typically 100mm deep), and lay your pipes on the gravel. Use a spirit level and straightedge to check the fall as you go, or stretch a string line between your two invert points and work to that.

110mm vs 160mm pipes

Standard domestic drainage uses 110mm diameter pipes, which handle the flow from a typical house without issue. 160mm pipes are used for larger developments, longer runs, or where multiple properties connect into a shared drain. The bigger pipe has roughly double the flow capacity at the same gradient.

If you are running a long drain from a new development or connecting several properties, check with Building Control whether 160mm is needed. The flow capacity figures in this calculator help you decide whether 110mm will cope with the expected load.

Common mistakes to avoid

The most frequent issue is not accounting for the depth at the connection point. If the existing drain is shallow, you might not have enough depth to achieve the required fall over a long run. Measure the existing invert level before you start and work backwards to check your new pipe will have enough fall.

Another common problem is uneven bedding. If the gravel bed is lumpy, the pipe can develop belly points where water sits. Take the time to get the bed right and you will avoid callbacks.

Pricing drainage work

Drainage jobs involve groundwork, materials, and often Building Control sign-off. Use the concrete calculator to price up any concrete surrounds or bed material, and the quote generator to build a detailed breakdown for your customer.

Once the job is done and signed off, send a professional invoice through InvoiceAdept with itemised costs for excavation, pipe materials, backfill, and any testing or inspection fees.