Box Gutters Behind a Parapet: What to Check in St. Louis

A box gutter is a trough built into the roof line rather than hung from the fascia. On many older St. Louis brick houses, and on some flat or low-slope additions, roof water runs along a channel formed between the roof deck and a parapet wall, then leaves through a downspout or a scupper set into the wall. You will not see it from the street. You may only notice it when water shows on a ceiling below the wall, or when a brick face stays dark after every rain. If your house has one, the trough is part of the roof and part of the gutter, so it needs its own line in any quote.

What a box gutter is made of

The trough is usually lined with sheet metal, often aluminum, galvanized steel or copper, bent to fit the channel and sealed at the seams with solder, sealant or a membrane. The lining runs up the back of the parapet and tucks under the counterflashing, and up the front under the roof edge. Each of those upturns is a place water can get behind the metal if the lining is nailed through instead of cleated, or if the counterflashing has been cut too short to cover it.

Because the box is formed on site, nobody sells it as a catalog part. What you get depends on who built it. Ask the crew to name the metal, its gauge and the seam method in writing. A quote that says only "box gutter, repair" leaves those details open, and those details decide how long the trough lasts.

Where the water leaves

Every box gutter needs an outlet. The usual outlets are a round or rectangular opening through the parapet, often called a scupper, that drops water into a downspout, or a short run of pipe through the wall to a downspout on the face of the house. Check that the outlet sits at the low point of the trough, not at the high end. A trough that slopes away from its outlet and drains into a dead corner will hold water until the next rain adds more.

Look at the downspout that receives the outlet. If it is undersized, or if it has a crimp or a sharp elbow near the bottom, the box fills before the water can leave it. In St. Louis, a heavy spring storm can drop more water in twenty minutes than a single downspout carries cleanly, so the outlet and the pipe have to be sized as a pair.

The outlet and the liner at the wall

The outlet is where many box gutter failures start. Water sits against the sheet metal at the scupper edge, the solder or sealant ages, and a small gap opens. Behind the wall, the water finds the mortar joints and runs down the inside of the brick. You may not see staining on the outside for months. Inside, the first sign is often a damp patch on the top floor ceiling near the exterior wall.

Check the sealant or solder at the outlet by hand, from a safe position, and look for cracks in the coating and white powder on the metal. White powder on aluminum means oxidation at a cut or scratch. On copper, a green stain running down the brick below a joint means water has been sitting on the metal for a long time.

Overflow and the back wall

A box gutter with no overflow will spill over the front lip or into the wall cavity when the outlet clogs. Some older boxes have no overflow at all. Ask whether the design includes one, where it discharges and whether it sits above the top of the lining. An overflow that exits at the same height as the trough is not really an overflow. It is a second clog point.

Leaves and grit collect in the low end of a box gutter. They do not wash out the way they do from a hung gutter, because the trough is flat or nearly flat and the debris sits. Plan on clearing the box by hand each spring and fall, and ask the roofer how the crew will reach it. A box that can only be cleaned from a ladder set over the roof edge will not be cleaned often.

Ice and the winter freeze

St. Louis freeze and thaw cycles hit a box gutter harder than a hung gutter, because water cannot drain out of the trough quickly during a thaw. Snow that slides off the upper slope lands in the box. When the temperature drops after the thaw, the water in the box freezes at the outlet first. The ice then backs water up under the roof edge. That is how an ice dam starts at a parapet wall even when the eave looks clear.

Heat cable can help at the outlet, but it is a stopgap, and it does not fix a short outlet or a wrong slope. If the box holds ice every winter, the fix is to correct the slope or the outlet size. Ask the roofer whether the design can take a heat cable at the outlet and whether the power supply sits on a circuit you can reach.

What to ask before the crew arrives

• What metal the trough is made of, and what gauge.
• How the lining is sealed at the outlet and at the wall, and whether the seal is solder, sealant or a membrane.
• Where the water leaves, and how the outlet is sized to the downspout below it.
• Whether the box has an overflow, where it discharges and how high it sits.
• Whether the counterflashing is cut and reset, or only caulked over.
• How the crew will reach the box for cleaning once the job is done.

Put the answers in the written scope. A line that says box gutter is included is not enough. The scope should state the metal, the gauge, the seam method, the outlet location and the downspout size, and it should say whether the counterflashing is replaced or left in place.

What Keys documents on a box gutter

When Keys inspects a box gutter, the crew photographs the trough from the roof and from the ground, the outlet and the downspout connection, and the wall above the box where the counterflashing sits. The written scope lists the measurements and the photos, and it goes into your hands. Questions about the box can go to the Keys office at (314) 220-2333. The crew attends the job as the contractor while you deal with your own insurer. Missouri law (RSMo 407.725) forbids a roofing contractor from representing or negotiating a homeowner's insurance claim, so the crew will not handle that part for you.