Drains Flooding in Heavy Rain Horsham | Why It Happens

When the rain really comes down in Horsham, some drains back up fast. Not because they're blocked in the usual sense, but because the system they connect to simply can't shift water quickly enough. That's an important distinction, and it changes what you should do next.

Two different drainage systems, one big confusion

Most Horsham properties drain into one of two systems, sometimes both.

Foul drains carry wastewater from your sinks, toilets and appliances to the sewage treatment works. They're not designed to handle rainwater.

Surface water drains take rainwater from roofs, driveways and paths, usually straight to a watercourse or soakaway.

Older parts of town, including much of Roffey, Denne, and the streets around the town centre, often sit on a combined sewer. One pipe takes everything. That's fine most of the time, but in a heavy downpour, surface water floods in from roads, car parks and gullies all at once. The pipe fills. Somewhere in the network the pressure backs up, and the nearest low point, quite often a garden drain or a ground-floor toilet, is where it comes out.

Southern Water manages that combined network. When the problem is in the public sewer, it's theirs to fix, not yours.

Why Horsham gets it worse in certain spots

Geography matters here. Low-lying areas around Broadbridge Heath, Littlehaven and the Arun flood plain near Pulborough and Billingshurst sit close to the River Arun and its tributaries. When those watercourses rise, there's nowhere for surface water to discharge quickly. The network backs up from the outfall inward.

Tower Hill and parts of North Heath are slightly higher, but some roads there have ageing clay laterals connecting to Victorian-era brick sewers. Those don't always drain freely under load.

Soakaways, common on newer estates in Southwater and Warnham, can become saturated in prolonged wet weather. When the ground is already waterlogged, a soakaway stops working almost completely. Water has nowhere to go.

What you can actually control

Not all of it is out of your hands.

• Keep your gullies clear. Leaf matter blocking a road gulley outside your boundary directly affects what happens at your drain. Give it a check after autumn.
• If you've paved over your front garden (common on Holbrook and Trafalgar estates), you've added to the surface water load hitting your private drainage. A permeable surface or a channel drain connected properly helps.
• Check your downpipes aren't discharging onto a soakaway that hasn't been maintained. A CCTV drain survey will show whether yours has collapsed or silted up.
• Non-return valves can be fitted to prevent sewer water pushing back up into ground-floor gullies during flood events. Not a permanent fix, but practical protection.

If it keeps happening after every significant rain, a survey is worth having. Sometimes a partial root ingress or a sagging pipe section slows the drain just enough that normal flows are fine but storm flows aren't. You'd never spot that without a camera.

If your neighbour's drive floods into yours, or a road gulley is the clear culprit, that's a call to West Sussex County Council's highways team rather than a drain engineer.

Drainsco covers Horsham and the RH12 and RH13 postcodes, with engineers available day and night. If rain is backing up through your gullies or you want a survey to find out why it keeps happening, call us for a clear price before anything starts. No call-out fee.