Wyes › Falls, bends and rodding access
Falls, bends and rodding access
A drain that falls correctly, avoids tight bends and can be reached for clearing will stay clear for years. Here is what the guidance says.
What is a fall?
A fall (or gradient) is how much a pipe drops over a distance, written as a ratio such as 1:40 (1mm drop for every 40mm length, or 25mm per metre) or as mm per metre. Too little and solids settle. Too much and water rushes ahead, leaving solids behind. The target is a steady slope that carries both.
Fall for drains (underground)
Approved Document H (2015) gives minimum gradients for drains based on peak flow:
| Peak flow | Pipe size | Minimum gradient |
|---|---|---|
| Under 1 litre per second | 75mm | 1:40 |
| Under 1 litre per second | 100mm | 1:40 |
| Over 1 litre per second | 75mm | 1:80 |
| Over 1 litre per second | 100mm (at least one WC) | 1:80 |
| Over 1 litre per second | 150mm (at least five WCs) | 1:150 |
For a typical house with a 100mm drain, 1:40 is a common target for the first length near the house, and flatter runs further downstream are acceptable if the flow justifies it. Where a flatter fall can't be avoided, ask building control.
Fall for branch pipes (inside)
Table 2 of the same document sets fall ranges for unventilated branch discharge pipes. For WC branches the range is 18 to 90mm per metre. For washbasin and bidet pipes it is roughly 18 to 44mm per metre for the shorter ones, up to 87mm per metre for longer ones, depending on length. Below the minimum, waste lingers; above the maximum, the trap can empty.
Bends
- Avoid bends in branch pipes where you can. Where unavoidable, use the largest radius practicable.
- The bend at the foot of a stack should be as large a radius as possible and at least 200mm at the centre line.
- In drains, bends should be limited to inspection chambers, manholes and the foot of stacks.
So underground you should not normally have an unprotected bend in the middle of a straight run. Use an inspection chamber or rodding point where the pipe changes direction.
Access: rodding eyes and chambers
Access points are needed at the head of each drain run, at bends, at changes of gradient or pipe size, and at junctions unless each run can be cleared from another access point. The maximum spacing in the guidance is 22m to small or large access fittings and to junctions, and 45m to inspection chambers and manholes.
A rodding eye is a capped pipe that comes up to the surface or sits in a small access box. An inspection chamber is a larger plastic or brick shaft with a channel at the bottom where pipes join. Chambers are easier to work with but take more space.
Rodding points in stacks
A discharge stack should have rodding access above the spillover level of the appliances it serves, so a blockage can be cleared from above. Many stacks use an access cap or a rodding eye on a bend.
Practical tips
- Use a spirit level on a straight edge, or a string line, to check the fall, and lay pipes on a bed of granular material.
- Start at the lowest point (the connection) and work back up.
- Do not use sand for bedding if gravel is specified by the pipe maker. Follow instructions.
- Test the drain, as required by building control, before backfilling.
See Approved Document H and building control for when to involve your council, and wye vs tee for junctions.
This is general information, not professional advice. The rules apply to England; Wales, Scotland and Northern Ireland have their own. Check with your local authority building control or a competent professional before starting work.