Box Gutters
Box Gutters for Dallas-Fort Worth Homes and Commercial Roofs
A box gutter is a rectangular trough built into the roof edge or hung from a commercial eave, and it carries more water than any K-style gutter off a truck. Architectural Metal Panels fabricates and lines box gutters in our Forney sheet metal shop for older homes and commercial buildings across Dallas-Fort Worth, sized for the inch-an-hour storms that overwhelm ordinary gutters.
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What is a box gutter and where will you find one?
A box gutter is a square-sided gutter, usually wider and deeper than a residential K-style, that is either built into the roof structure or fabricated as a large hung trough. Dallas-Fort Worth has two kinds.
Built-in gutters on older homes
Homes built before roughly 1950 in the older neighborhoods of Dallas and Fort Worth often have gutters built into the eave, hidden behind a crown molding or inside a boxed cornice. The trough is wood lined with metal, and the metal is what fails: original galvanized or terne liners rust through, patches get caulked over patches, and water soaks the cornice and the wall below. Replacing the liner, not tearing out the architecture, is the usual answer.
Commercial box gutters
Metal buildings, warehouses, schools and shopping centers use large fabricated box gutters hung at the eave or set behind a fascia, feeding oversized downspouts through conductor heads. These are shop-built from 24- or 22-gauge steel or .040 and .050 aluminum in long sections, with expansion joints and overflow scuppers designed in rather than hoped for.
How are box gutters sized for North Texas rainfall?
From the roof area draining to the gutter and the rain rate we design for, not from the width of the old trough. DFW receives about 36 inches of rain a year on roughly 80 rain days, May is the wettest month at close to 5 inches, and thunderstorms commonly drop 1 to 2 inches an hour with far heavier bursts. A box gutter has to pass that peak hour with margin.
- Roof area: eave length times the horizontal depth to the ridge, plus any upper roof that empties onto it, adjusted for pitch.
- Gutter cross-section: a wider and deeper trough for long commercial eaves. Built-in gutters are limited by the existing cornice, so the outlets do more of the work.
- Downspout and scupper sizing: we allow roughly 1 square inch of outlet per 100 square feet of roof as a starting point, then check it against industry sizing tables for the rain intensity and the length of the run. Commercial gutters get 4-inch round or 4×5 and larger rectangular downspouts.
- Overflow scuppers: a through-wall or through-fascia opening set just above the design water line, so a clogged downspout spills outside the building instead of over the inside edge of the gutter and into the wall.
- Conductor heads: the open collector box at the top of a downspout that lets a scupper discharge into it, adds air so the downspout runs full, and gives debris a place to stop before the pipe.
Fall toward the outlets matters too. We set liners and hung gutters at roughly 1/16 to 1/8 inch per foot and never leave a dead-level run, because standing water is what finishes a liner.
What liner options do box gutters have?
Two: a metal liner with locked and soldered or sealed seams, or a single-ply membrane liner. Metal lasts longer and looks right on a historic cornice; membrane is faster and suits commercial gutters that already sit under a membrane roof.
| Liner | Materials | Joints | Best for |
|---|---|---|---|
| Metal liner | 24-gauge Galvalume or 16-ounce copper; stainless for aggressive exposures | Flat-lock or standing seams, soldered on copper, sealed and riveted on steel | Built-in gutters on older homes, visible cornices, long service life |
| Membrane liner | TPO, PVC or modified bitumen over a clean substrate | Heat-welded or torched laps | Commercial box gutters under single-ply or modified roofs |
Why do box gutters need expansion joints?
Because a 100-foot run of steel grows and shrinks close to an inch between a January freeze and an August afternoon. Short runs can move at the outlets. Longer runs get a purpose-built expansion joint, typically every 40 to 50 feet and at inside corners, where two gutter ends are capped and a cover plate bridges the gap. A liner that cannot move tears at its weakest solder joint, and that joint is usually right over the wall you least want wet.
Should a box gutter be repaired or replaced?
Repair the liner when the wood or structure behind it is sound and the trough is big enough. Replace the gutter when the substrate has rotted, the profile cannot carry the roof above it, or the system has been patched so many times that no two sections match.
- Repair: resoldering seams, replacing a rusted section of liner, adding a missing expansion joint, enlarging outlets, or fitting a new conductor head and downspout.
- Reline: stripping the old liner, replacing rotted wood, and installing a new continuous metal or membrane liner with proper fall and outlets. The most common job on 1920s to 1950s homes.
- Replace: a new fabricated box gutter, usually on commercial buildings where the old gutter was undersized or has pulled away from the eave.
Every option comes with a written scope. Storm damage to gutters and conductor heads is commonly part of a hail claim; we document it for your adjuster and, as Texas law requires, never waive or absorb deductibles. The cornice and fascia carpentry that goes with a reline is covered under our soffit and fascia service.
How do box gutters differ from seamless K-style gutters?
Capacity, placement and how they are made. Seamless K-style gutters are roll-formed on site from coil in 5- and 6-inch sizes and hung on the fascia. Box gutters are fabricated in the shop in larger sections and either built into the roof or hung from a commercial eave.
| Factor | Seamless K-style | Box gutter |
|---|---|---|
| Where it sits | Hung on the fascia below the roof edge | Built into the eave, or hung behind a fascia at a commercial roof edge |
| Size | 5 or 6 inches | Typically 6 to 12 inches wide or more, sized to the roof |
| How it is made | Roll-formed on site in one continuous run | Shop-fabricated in sections with splice and expansion joints |
| Outlets | 2×3 or 3×4 downspouts | Scuppers, conductor heads, 4-inch and larger downspouts |
| Typical use | Most DFW homes | Pre-1950 homes with built-in gutters, metal buildings, commercial roofs |
If your home has ordinary eaves and fascia, start with our gutter sizing guide and the seamless gutter installation page. If you have a hidden gutter behind a cornice or a commercial eave with scuppers, you are in the right place, and our post on how box gutters work and when a DFW building needs them goes deeper. Box gutters are fabricated alongside metal coping and metal flashing in our Forney sheet metal shop, and the same crews install commercial gutters with our commercial metal roofing.
Box Gutter FAQ
Box gutter questions from DFW owners and managers
What is a box gutter?
A rectangular gutter that is either built into the roof edge behind a cornice, common on homes built before about 1950, or fabricated as a large hung trough on commercial buildings. Box gutters are lined with metal or membrane and carry more water than a standard K-style gutter.
How much does it cost to reline a box gutter in Dallas?
It depends on the length, the liner metal, how much rotted wood is behind the old liner, and access height, so we price it per linear foot after inspecting the gutter. Your written quote separates the liner, carpentry, outlets and downspouts so nothing is open-ended.
Can a built-in gutter be replaced with regular gutters?
Sometimes, by closing the trough and hanging K-style gutters on a new fascia, but it changes the look of a historic cornice and the roof edge detail. On most older homes relining the built-in gutter keeps the architecture and solves the leak with less disruption.
How big should the downspouts be on a box gutter?
As a starting point allow about 1 square inch of downspout opening per 100 square feet of roof, then check the run length and rain intensity against industry sizing tables. Commercial box gutters usually need 4-inch round or 4×5-inch rectangular downspouts with conductor heads and overflow scuppers.
Why does my box gutter leak at the seams?
Thermal movement. A long metal liner expands in Texas heat and contracts in a freeze, and without an expansion joint the stress lands on the solder or sealant at a seam. Rust at the low point of the trough, where water stands because the fall is wrong, is the other common cause.
Do you make box gutters for metal buildings?
Yes. We fabricate commercial box gutters, conductor heads, scupper boxes and downspouts in 24- and 22-gauge steel or .040 and .050 aluminum, color-matched to the roof and wall panels, and install them or deliver them to your contractor anywhere in Dallas-Fort Worth.
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Sources
Rainfall figures on this page come from this reference (checked October 2026):
- iWeatherNet: DFW rainfall normals and year-to-date totals – the roughly 36-inch annual normal, about 80 rain days and May as the wettest month
Get the hidden gutter fixed before the cornice goes
Book an inspection of your built-in or commercial box gutter and get a written scope for repair, reline or replacement. Call (469) 569-3956 or request your estimate online.