1Your gutters are blocked
This is by far the most common cause. Leaves, moss, roof grit and general debris build up in the channel and pack down into a solid layer. Rainwater can no longer travel along the gutter to the outlet, so it backs up and pours over the edge instead.
You'll usually notice it worst in autumn and after a windy spell, and often at one specific point along the run rather than the whole length. Plants or grass growing out of the gutter are a clear sign the blockage has been there a while.
2Your downpipe is blocked
A clear gutter still overflows if the water has nowhere to go. Debris often washes along the channel and jams at the outlet or in the bend at the top of the downpipe, forming a plug.
The giveaway is water spilling over close to the downpipe while the rest of the run looks fine, or a downpipe that stays silent and dry during heavy rain when it should be running. Tapping the pipe can also reveal a dull, solid sound where the blockage sits.
Clearing the outlet and flushing the pipe through is part of every gutter cleaning job I do, so a blocked downpipe is usually sorted on the same visit.
3There's too much moss and roof debris
Older roofs, and any roof shaded by trees, shed moss and tile grit every time it rains. That material washes straight into the gutter and settles as a heavy sludge that slows the flow long before it fully blocks it.
If the gutters keep filling again within months of being cleared, the roof is usually the source. Dealing with the moss on the roof itself, alongside the gutter clean, breaks the cycle rather than treating the symptom.
If moss is the root cause, a roof clean and moss removal alongside the gutter clear is the longer lasting fix.
4Your gutter is sagging
Guttering needs a slight, consistent fall towards the outlet. When a bracket fails, a fixing pulls loose from the fascia, or the weight of trapped water and debris drags a section down, the fall is lost or even reversed.
Water then sits in a dip and spills over at the lowest point every time it rains, even after the gutter has been cleaned. A visible dip in the line, or standing water left behind once the rain stops, points to this.
5The guttering or joints are damaged
uPVC becomes brittle with age. Joints can lose their rubber seals, brackets crack, and sections can be knocked out of line by ladders or storms. The result is a steady drip or a fine spray from one spot rather than water pouring over the front edge.
Leaks at a union or a corner are worth catching early, because the water usually runs down the wall behind the pipe and starts to stain the render or soak the brickwork.
While I'm up there I can also give the soffits and fascias a wash down, which is when staining from a long-running leak usually shows up.
6Heavy rain is exposing an existing problem
A partly blocked or slightly sagging gutter can cope perfectly well with light drizzle and only fail during a downpour. That's why plenty of people only notice a problem two or three times a year, then assume it was just the weather.
It rarely is. If your gutters overflow in heavy rain and your neighbours' don't, the rain is showing up an underlying issue rather than causing it.
7The problem could be further along the drainage system
Sometimes the gutter and downpipe are both clear and water still backs up. In that case the issue is at the bottom: a blocked gully, a silted-up drain, or a soakaway that can no longer take the volume during sustained rain.
You'll see water rising at the base of the downpipe or pooling on the ground beside the wall. It's worth checking before assuming the guttering is at fault, and it's the first thing I look at when everything up top is clean.
What should you do if your gutters keep overflowing?
Start from the ground, safely. During the next spell of rain, watch where the water actually comes over: a single spilling point near the downpipe usually means the outlet is blocked, an overflow in the middle of a run points to a blockage or a dip, and a fine drip from one joint is a seal or bracket problem. Check the gully at the bottom of the downpipe too, and see whether the pipe is running at all.
Please don't go up a ladder to look. Most overflowing gutters are cleared in under an hour with pole and vacuum equipment from ground level, and a proper clear-out plus a flush through the downpipe will tell you straight away whether the guttering itself needs any repair work.
Think your gutters could be the problem?
Paul can take a look and let you know what needs doing. No obligation, and no charge for the quote.