Box Gutters: Built-In Gutters, How They Fail and How to Fix Them

A box gutter is a gutter built into the structure of the roof instead of hung on the outside of it. You'll find them hidden behind a cornice on older houses, between two roofs where they meet, and behind parapet walls on flat-roofed buildings. The trough is framed in wood and lined with sheet metal or a roofing membrane, and when the lining fails, the water goes straight into the building. That's why box gutters have a bad reputation, even though a well-lined one can last decades.

The term also gets used for a square-profile hang-on gutter, sold mostly for commercial buildings. This guide is about the built-in kind, which is where the questions and the leaks are.

Where box gutters are used

• Behind a cornice. Common on Victorian and early 20th-century houses. From the street you see a decorative cornice; the gutter is hidden in its top.
• Between roofs. Where two roof planes slope toward each other, a box gutter collects water in the valley between them.
• Behind parapets. On low-slope roofs, water runs to a box gutter along the parapet wall and out through scuppers or internal drains.

How they fail

1. Lining corrosion or splits. Old tin (terne-coated steel) linings rust through if they're not painted. Copper linings last longer but crack at seams when they can't expand and contract.
2. Bad slope. Many box gutters were built nearly flat. Standing water speeds up every other problem.
3. Overflow into the wall. A clogged outlet in a built-in gutter backs up over the lining's edge and into the soffit or wall.
4. Seams. Soldered seams on metal linings, or lap seams on membranes, open up over years of thermal movement.

Signs to look for: water stains on the ceiling or wall just inside the eaves, peeling paint on the cornice, and rot visible at the soffit.

Repair options

Option	What it involves	When it fits
Patch and coat	Clean, patch holes, paint the lining with a roof coating	Small leaks in a lining that's mostly sound
Membrane reline	Line the trough with EPDM, TPO or modified bitumen, terminated up the walls	Most common fix; lower cost than metal
New metal lining	Copper or terne-coated stainless, soldered with expansion joints	Historic houses, long-term fix
Rebuild with slope	Reframe the trough with tapered blocking, then reline	When ponding is the root cause
Convert to hung gutter	Remove or roof over the box gutter and install a hung gutter	When the cornice isn't historically important

With a membrane reline, the membrane is usually carried up the back wall and fastened with a termination bar, then counterflashed so water can't get behind it.

Keeping one working

• Clear the outlets and leaders at least twice a year, more under trees.
• Paint metal linings before they rust. Terne needs regular painting; copper and stainless don't.
• Add an emergency overflow (a scupper or a second outlet) so a clog spills outside instead of inside.
• Don't let anyone walk in the gutter in hard-soled shoes; dents and punctures start leaks.

If you own a historic house in a district with design review, check before converting a built-in gutter to a hung one. Relining keeps the original look and usually gets approved faster.