The short answer
Water gets behind a gutter when the roof covering does not overhang far enough into it, when the underlay has no eaves support tray to guide water over the lip, or when the gutter has dropped or been set too far from the fascia. Check how far the first tile or slate overhangs, look for felt hanging loose, and see whether the gutter has sagged.
Why water runs behind the gutter
Rain runs down a pitched roof, off the bottom edge of the covering and, if all is well, falls into the gutter below. Three things have to line up for that: the covering has to overhang the edge, the underlay beneath it has to lead water into the gutter and not behind it, and the gutter has to sit at the right height and distance from the fascia.
When one of those slips, water takes the easiest route. It drips off the edge and misses the gutter, runs back along the underside of the lowest tile or slate, or trickles down behind the gutter and onto the fascia. Over time that soaks the board, rots the rafter ends and stains the wall. A gap between roof and gutter is the visible form of this. It may be an actual space you can see daylight through, or it may be a tidy-looking gutter that simply sits too low or too far out.
The cure depends on the cause, so it pays to work out which of the three has gone wrong before ordering anything.
Check one: how far the roof covering overhangs
On a well-built roof the bottom row of tiles or slates (the eaves course) projects beyond the fascia so that water drops into the middle of the gutter. The right overhang depends on the covering and the gutter profile. If the eaves course sits flush with the board, or stops short, water has nowhere to go but behind the gutter.
This happens for a few reasons. The tiles may have slipped back, the fascia may have been replaced by a thicker board without adjusting the roof, or a deeper gutter has been fitted to a roof that was set up for a shallow one. A lower fascia can cause the same gap, since the gutter drops with it and no longer sits under the edge.
You can usually see the problem from a ladder or from a window: look along the edge in profile. If you cannot see the tiles clear the board, do not climb out to try. Take a photograph and ask someone with a tower or a scaffold to look.
Check two: eaves felt, support trays and drip trays
Underneath the covering sits the underlay, the sheet or felt that backs up the covering. At the eaves, the underlay should lead water over the fascia and into the gutter, not let it fall behind. In old roofs this is often a strip of felt that has dried, curled or sagged into the gap, creating a pocket that holds water against the board. On newer roofs it is a rigid eaves support tray, a plastic strip that props the felt up and carries it over the fascia lip.
The terms matter here, so a quick explanation helps. A drip tray or eaves tray is a plastic channel or strip that sits under the first course of tiles and over the back of the fascia, so moisture working beneath the covering is led out into the gutter. Eaves felt is the sheet of underlay that runs the same job. Either should lie flat, run into the gutter and stay free of tears.
If you can see felt hanging loose or ragged at the edge, or none at all, that is a likely culprit. Re-lining the eaves is a small job, often done at the same time as re-fixing a gutter, and it is cheaper to do when the gutter is already off. Sound felt that has simply slipped can sometimes be tucked back and dressed over the lip.
Check three: has the gutter moved?
The last cause is the gutter itself. Brackets that have loosened or screws that have pulled out of a soft fascia let the gutter sag or tilt away from the wall. A gutter set too low sits below the water's path, and one set too far out may hang just in front of the dripping edge. In each case the result is a gap, and rain misses.
Look along the run from the ground. The front lip of the gutter should be a little lower than the back so that water exits the roof over the back edge and is caught by the front, and the run should follow a gentle fall toward the outlet. Our guide to gutter replacement cost shows what a new run involves, and MyBuilder's guide, updated in August 2026, puts guttering replacement at £30 to £70 per metre.
If the gutter has moved because the fascia behind it has softened, re-fixing alone will not hold. That is the point to read our page on signs that a fascia needs replacing and to ask for a survey of the board.
Fixing it, and when to call a roofer
A small gap can be cured by resetting the gutter, tucking in the felt or adding a support tray, but all of those mean working at the roof edge. At first-floor height that needs a tower or scaffold and a head for heights, so it is not a job to attempt from a leaning ladder.
A survey should answer four questions: how far the tiles overhang, what state the felt is in, where the gutter sits and whether the board behind it is sound. We photograph each and send them with a written quote. The fix might be one of the small jobs above or, where the board has rotted, the replacement of the fascia and gutter together, a job covered in our guide to roofers who fit fascias. A shared gutter between semis adds its own question; see shared gutters.
Our workmanship is guaranteed for 10 years in writing, and we leave the site tidy. Contact us through the gutters, soffits and fascias page, by phone on 01793 626096 or on WhatsApp.