The short answer
A conservatory gutter overflows when water arrives faster than it can drain away. The most common causes are leaves and debris blocking the outlet, a sagging run holding water, joints and brackets that have slipped, or a gutter too small for a roof that sheds water quickly. Clearing the blockage comes first. Resetting the fall, replacing failed sections or upsizing the gutter fixes the rest.
Blockages are the first thing to check
The most common cause is also the easiest to fix. Conservatory gutters are low, small and often sit under overhanging trees or beside a house wall that catches leaf fall. Leaves, moss from the roof, seed and the grit shed by an ageing roof surface collect at the outlet, which is the hole where water drops into the downpipe. Once the outlet narrows, water backs up along the gutter and spills over the front lip or, worse, back against the frame.
A polycarbonate roof is especially good at creating grit. As the sheets age they shed fine particles that settle in the gutter and form a paste that blocks the outlet over time. A glass or tiled roof sheds less, but still carries debris in from nearby trees.
Check the downpipe too. A gutter can be perfectly clear while the pipe underneath is blocked, or the drain at its foot is silted up. If water comes out of the gutter joints but not the bottom of the pipe, the blockage is below the gutter. Clearing both and flushing with a hose shows quickly whether the run is working.
Falls, sagging and slipped joints
Even a clean gutter overflows if it holds water. A gutter needs a slight fall towards its outlet so that water keeps moving. Fall is the gentle slope along the run. A fitting guide for one manufacturer's small gutter range recommends roughly 1:350, which is a very small drop and easy to lose.
Over the years brackets loosen, fascia boards move and the gutter sags between supports. Where it dips, water pools, debris settles and the standing water adds weight that drags it further. Once there is a dip, the water stops flowing even when the run looks clear. The same guide sets fascia brackets at no more than 500 mm apart, so a run with wider gaps will tend to droop.
Joints are the other weak point. Plastic gutters are joined with rubber seals and clips. These dry out and the joint starts to drip, or the clip lets go and two lengths drift apart. A drip at a joint is easy to mistake for overflow. Resealing or replacing the joint fixes it, but if the run is old and brittle, replacing whole lengths is usually the better answer.
When the gutter is too small for the roof
Sometimes nothing is wrong with the gutter except its size. This tends to show up after a roof change. A solid conservatory roof sheds water faster and in a more concentrated flow than the roof it replaced, and the gutter that coped with the old roof may not cope with the new one.
The run of roof matters as well. A long, wide slope delivers more water to each metre of gutter than a short one. Where a conservatory roof also drains the slope of the house above it, via a valley or abutment, the load on one gutter can be much higher than it looks from the ground. Heavy rain then does what it always does and finds the weak point, which is usually the corner or the point where one section feeds into another.
The fix is a larger gutter, a second downpipe, or both. Adding an outlet halves the distance the water has to travel along the run. If the conservatory has a hipped or complex roof shape, the layout of the outlets is part of the design, not an afterthought.
Where the water is going when it overflows
It matters where the overflow lands. Water spilling over the front lip falls clear of the building and is mostly a nuisance and a stain. Water running back behind the gutter is more serious, because it soaks the fascia, the frame and the dwarf wall below. On a conservatory that can show as damp patches inside, mould on the frame and rotting timber at the eaves.
A roof leak and a gutter overflow can look the same from inside the room. If staining shows up high on the frame in wet weather, it is worth finding out which one it is before paying for the wrong repair. Our guide to conservatory roof repair goes through how leaks are traced.
Guttering also fails at the end of its life, as materials do. One source, a gutter cleaning firm, puts typical uPVC guttering at 10 to 20 years, so a run that is many years old may be due for replacement. MyBuilder's 2026 guide reports typical UK guttering costs of about £30 to £70 per metre; the quote you receive for your own roof follows the survey.
How we put it right
We start by finding out whether it is a maintenance fault, a fall problem or a capacity problem, because each has a different answer. A blocked outlet is cleared. A sagging run is taken down and reset at the correct fall with new or tightened brackets. Failed sections are replaced, and if the gutter is too small we fit a larger profile or add an outlet.
If the conservatory roof itself is being replaced, the gutter is the best moment to deal with it, since the eaves are already exposed. On a lightweight system the eaves detail is part of the design, and the page on a lightweight tiled roof shows how the layers meet the gutter.
Our survey carries no charge, and its photographs show your own gutters and eaves. Check them for a gutter that falls the wrong way or sags between brackets, as standing water at one end usually marks where the overflow into the room starts. We cover everywhere within 30 road miles of Bath; phone 01225 285181, send a WhatsApp message or fill in the website form to arrange a visit. Our conservatory roof replacement page sets out the wider job in full. For the wider picture of planning a new roof, see our guide to conservatory roof planning.
