Rainwater (surface water) system components

Rainwater drainage, also called surface water drainage, collects rainwater run-off from roofs and paved areas and carries it away before it can soak into the building or pond around it.

The journey from roof to ground

At roof level, gutters run along the eaves to catch water shedding off the roof. Use the selected system’s line and fall instructions. Some layouts are rated level; any specified fall is towards the outlet, and local sags are avoided.

At each outlet, a downpipe takes the water vertically down the wall. Size gutters, outlets and downpipes for the effective roof area and design rainfall, and provide the required safe overflow route for blockages or rainfall beyond the design.

At the bottom, the water is delivered somewhere suitable: a gully, a rainwater shoe to a surface-water drain, or a soakaway that lets it disperse into the ground. The right destination depends on the site and local drainage arrangements.

For revision, picture the journey: roof to gutter, gutter to outlet, downpipe to ground, then to a soakaway or surface-water drain. Confirm sizing and disposal options against Approved Document H.

At the gutter

• Running outlet: the fitting with an opening in the base of the gutter, connecting it to the downpipe below
• Stop end: closes off the open end of a gutter run so water is directed along to the outlet instead of running out of the end
• Union bracket: joins two straight gutter lengths and seals the joint, leaving the marked gap for the plastic to expand and contract
• Angle fitting: turns the gutter around a corner while keeping the run continuous, unlike a straight union
• Fascia support bracket: fixes the gutter to the fascia at the designed height and line; a union bracket has the different job of joining and sealing lengths
• Deep-flow profile: a deeper section can provide greater rated capacity; check the manufacturer’s figures for the actual outlet layout
• Valley gutter: sits in the internal angle where two roof slopes meet, collecting and concentrating the water from both - harder to inspect than an eaves gutter, so a blockage there is more likely to send water into the building rather than over an open edge

At the downpipe

• Swan neck (offset): bridges the gap between the gutter outlet at the eaves and the downpipe fixed to the wall below
• Pipe clip: fixes the downpipe to the wall at intervals, carrying its weight and the wind load on an exposed run, and keeping the socket joints aligned
• Push-fit sockets: commonly permit movement as PVCu expands and contracts; assemble and support the selected system as instructed, keeping its specified movement allowance
• Shoe: fitted at the foot of the downpipe to direct the discharge into a gully, clear of the wall
• Hopper head: gathers the flow from more than one pipe or gutter at high level into a single downpipe below

Keeping debris out

A leaf guard or balloon grating fitted at a gutter outlet, or a first-stage filter fitted before a storage tank, limits material entering the downpipe or storage. Guards and filters still need clearing, and must suit the system’s rated flow; they are not drinking-water treatment.

A blocked gutter or outlet can make incoming water overflow. Its escape path depends on the layout; water reaching the building can cause damp and damage, so inspect outlets and overflow routes.

Materials

• PVCu (uPVC) is the standard material for domestic gutters and downpipes: light, low-cost and it does not rust, though it expands and contracts noticeably with temperature
• Cast iron and aluminium give a traditional appearance suited to period properties; cast iron is far heavier than plastic, so its fully loaded weight, profile and manufacturer’s specification determine the supports and fixings

Discharge points

• Where a downpipe discharges onto a lower roof and more than 25 square metres of upper-roof effective area drains through it, a distributor pipe is fitted to the shoe to spread the flow so it does not over-top the receiving gutter
• A soakaway taking roof water is sized for the area draining to it and sited on ground permeable enough to accept the water, at least 5 metres from a building or road

Combined systems, disposal order and gullies

A combined system carries foul water and surface water together in the same drain to the sewer. A separate system keeps them apart: rainwater goes to its own outfall, such as a soakaway, a watercourse or a surface-water sewer, while foul water follows its appropriate wastewater disposal route. In a separate layout, do not add roof flow to the foul system without approval: its intended capacity and pollution controls may be affected.

Where rainwater has to be disposed of, the preferred order is to soak it into the ground, for example with a soakaway, where that is practical; then to a watercourse; and to a sewer only as a last resort.

Where roof drainage connects to a combined drain, a trap is required. A trapped (back-inlet) gully holds a water seal that stops smells rising from the drain, and its grating and sump catch leaves and grit before they reach the underground pipework.

Rainwater and the wholesome supply

Keep harvested rainwater separate from drinking-water pipework, including where a roof downpipe fills a garden butt. A direct connection could let a fall in mains pressure draw stored rainwater into the wholesome supply. Return excess rainwater to the appropriate outfall. Where mains backup is provided, it needs category-5 separation, commonly a Type AA or AB air gap; a label or ordinary check valve does not replace that protection.