What is a box gutter?

A box gutter is a concealed gutter that runs inside the roof line, usually in the valley where two pitches meet or behind a parapet. When one fails it drains water into the roof structure rather than off it, so it often shows up as an internal leak rather than an obvious drip.

Why does the fall matter so much?

A box gutter only sheds water if it runs downhill to its outlet. Over the years a gutter can lose its fall, so water sits in it, finds the smallest weakness and works back under the roof covering. Rebuilding the deck to a proper measured fall is the real fix, not just a new covering over the old shape.

Why EPDM rubber over felt or lead?

EPDM rubber is laid over the rebuilt deck as a single flexible membrane with very few joints, dressed up under the slate on both sides. It handles the movement a long gutter sees far better than felt, carries a long life expectancy, and unlike lead there is nothing worth stealing.

How long does a box gutter rebuild take?

A box gutter strip, re-deck to fall and rubber, with the slates eased back and re-dressed, is usually about a week on site depending on the length of the gutter, access and the weather.

Case Study 06

Stoke Goldington: a box gutter rebuilt.

A tired box gutter between two slate pitches, stripped out and rebuilt to a proper fall, then lined in EPDM rubber so it finally sheds water the way it should.

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Old failing box gutter between two slate roof pitches before the rebuild

01/Project brief

Project brief

A box gutter that had run out of fall.

The property is stone and slate, with a concealed box gutter running in the valley between two pitches. Over the years the old covering had tired and the gutter had lost what little fall it had, so water sat in it and worked its way back under the slate. The brief was to rebuild it properly: strip it out, re-deck it to a real fall, and line it in a material that will outlast the next patch by decades.

DurationAbout a week
GutterEPDM rubber
The fixRebuilt to a fall

02/Project specification

Specification

The specification.

Every line item, on the record.

Finished EPDM rubber box gutter running between two slate pitches — Project card
Stoke Goldington Box Gutter Rebuild
About a week on site · EPDM rubber, rebuilt to fall · Stoke Goldington

01 The brief

ClientPrivate residential ownerLocationStoke Goldington, near Newport PagnellPropertyStone and slate residentialDurationAbout a week on site

02 The build

StripOld covering and tired gutter stripped outDeckBox gutter re-decked and rebuilt to a measured fallCoveringEPDM rubber membrane, dressed up under the slateSlateSlates eased back and re-dressed into the new gutter

03 Sign-off

FallFall checked so water runs to the outlet, no pondingRecordWritten photographic recordRecordWritten photographic record throughout

Old tired box gutter with no fall before the rebuild

Finished EPDM rubber box gutter with a proper fall between slate pitches

03/ Before & after

About a week. A gutter that drains again.

Before After

LocationStoke Goldington

MaterialEPDM rubber

TimeframeAbout a week

ScopeStrip, re-deck, rubber

04/Work breakdown

Stage by stage

Five stages, from strip to rubber.

A box gutter only works if it has fall. The order of operations that took this one from ponding to properly draining.

Old tired box gutter with no fall running between two slate pitches before the rebuild

Stage 01

01 Strip the gutter

Strip out the old gutter.

The tired old covering was stripped out of the box gutter and the slates along both edges eased back, so we could see the deck and work out exactly how much fall had been lost.

Box gutter stripped out and opened up, old deck taken back to a sound timber base

Stage 02

02 Deck back to base

Take the deck back.

The old gutter deck came up. Anything soft or past its life was taken out rather than covered over, back to a sound base to build on.

Gutter re-decked in fresh board and firred to a proper measured fall to its outlet

Stage 03

03 Rebuild the fall

Rebuild to a measured fall.

The gutter was re-decked and firred to a proper measured fall, set out so water runs the length of the gutter to its outlet instead of sitting in the middle. This is the part that actually fixes the leak.

Breathable membrane laid in the box gutter and edges prepped, slates ready to dress back

Stage 04

04 Membrane and prep

Membrane and prep.

A breathable membrane was laid and the edges prepared, with the slates ready to dress back down over the new gutter upstands on both sides.

Finished EPDM rubber box gutter dressed under the slate on both pitches, draining cleanly

Stage 05

05 Rubber and dress in

Rubber it and dress in.

The box gutter was lined in EPDM rubber as one flexible membrane, dressed up under the slate on both pitches so the water is carried cleanly off the roof.

05/Detail gallery

On site

The details that make the finish.

Finished EPDM rubber box gutter

Re-decked to a fall

Membrane and prep

Rubber dressed under the slate

06/Craft note

Why the fall

Why we rebuild the fall, not just the cover.

The tempting way to deal with a leaking box gutter is to lay a new covering straight into the old shape. It looks tidy on the day, but if the gutter had lost its fall then the new covering is holding water just the same as the old one did. The leak comes back, usually the following winter.

The honest fix is to rebuild the deck to a proper measured fall so the water actually runs to the outlet. It is more work, it means easing the slates back on both sides, but it is the difference between a gutter that drains and a gutter that ponds. On this one we set the fall out and checked it before a scrap of covering went down.

EPDM rubber is the right lining for a job like this. It goes down as a single flexible membrane that copes with the movement a long gutter sees, dresses neatly up under the slate, and unlike lead there is nothing on the roof worth stealing. Properly detailed, it is a very long-life answer.