Commercial Gutters in NJ: How Flat and Low-Slope Buildings Drain, and When the Gutters Are the Problem

On a house, gutters are easy to picture. On a commercial building with a flat or nearly flat roof, it is less obvious where the water goes. Yet a commercial roof collects a huge amount of rain. When that water doesn't leave the roof quickly, you get leaks, stained walls, icy sidewalks, and a roof that wears out early.

This guide is for New Jersey building owners and property managers. It explains in plain English how water leaves a flat or low-slope roof, where gutters fit in, and the signs that your gutter system is the weak link.

How water leaves a flat or low-slope roof

"Flat" commercial roofs are not truly flat. They have a slight slope that moves water toward the places it can leave the roof. Most buildings use one or more of these:

1. Interior drains. Drains set into the roof surface that carry water down through pipes inside the building.
2. Scuppers. Openings through the low wall around the edge of the roof (called a parapet). Water flows out through the opening, often into a box on the outside wall that feeds a downspout.
3. Gutters. Used where the roof edge has no parapet wall. Water runs off the edge into a gutter, then down the downspouts.

Many buildings use a mix. Gutters are common on low-slope roofs, strip centers, warehouses, offices, and mixed-use buildings where the roof edge is open.

Why drainage matters so much on a flat roof

On a sloped roof, gravity does most of the work. On a low-slope roof, water only leaves as fast as the slope and the drainage system allow. When water sits, problems start.

The roofing industry's rule of thumb is simple: a properly draining low-slope roof should not have water still sitting on it 48 hours after the rain stops (in weather that allows it to dry). Water that sits longer than that is called ponding. Ponding can:

• Add weight. One inch of water weighs about 5 pounds per square foot. Across a large roof, that adds up fast.
• Wear out the roof faster. Standing water is hard on the waterproof sheet that covers the roof, and on its seams, over time.
• Collect dirt and debris that clogs drains and gutter outlets, making the next storm worse.

That is why building code sets a minimum slope for new flat roofs. For the flat roof systems WDM installs (TPO, EPDM, and modified bitumen, which are the plastic, rubber, and asphalt-based sheets that cover most flat roofs), the minimum is 1/4 inch of slope for every 12 inches of roof. It is also why the gutters at the roof edge need to be sized to keep up.

How much rain are we talking about?

New Jersey gets a lot of rain, and it can come fast. Most of New Jersey gets about 43 to 47 inches of rain and melted snow a year, and parts of north central New Jersey get up to about 51 inches, according to the Office of the New Jersey State Climatologist at Rutgers. Big storms can drop far more in a short time. During the remains of Hurricane Ida on September 1 and 2, 2021, parts of Somerset County got more than 9 inches of rain, and Newark Airport got more than 3 inches of rain in a single hour.

Gutters on a large commercial roof have to handle that water from a big area all at once. That is where many systems fall short.

Signs your commercial gutters are the problem

Here are the most common warning signs:

• Water pouring over the front of the gutter in heavy rain, even when the gutter is clear. The gutter or its downspouts are too small for the roof area feeding them.
• Stains, streaks, or peeling paint on the walls below the roof edge.
• Sagging gutters or gutters pulling away from the building. The hangers (the brackets that hold the gutter to the building) may be too far apart or failing, especially after snow and ice.
• Leaking seams and joints. Older gutters pieced together from short sections, with many joints, tend to leak at those joints.
• Standing water in the gutter after the rain stops, which usually means the gutter isn't sloped correctly toward the downspouts.
• Ice on sidewalks and entrances in winter, right below a downspout.
• Water pooling at the foundation or washing out landscaping near downspouts.

What a properly built commercial gutter system looks like

1. It is sized for the roof, not guessed

Gutter and downspout sizes should be based on how much roof area drains to each section and how hard it rains in your area. That is the method the sheet metal industry uses. A small roof over an entrance and a large warehouse roof need very different systems.

WDM installs seamless 5-inch and 6-inch gutter systems, and we size them based on the roof and local rainfall. WDM also offers custom gutter solutions, and we can talk through your building during the free estimate. Our post on 5-inch vs. 6-inch gutters explains the capacity difference in more detail.

2. It has enough downspouts, in the right places

On a big roof, adding downspouts is often just as important as the gutter size. An undersized or badly placed downspout can bottleneck a perfectly good gutter. Downspouts should also send water away from the building, and away from walkways and entrances.

3. It is seamless

Seamless gutters are made on site from a single piece of metal for each run, so there are no joints along the length of the gutter. That removes the most common place for leaks. WDM installs seamless 5-inch and 6-inch gutter systems.

4. It is pitched correctly

A gutter needs a slight, steady slope toward each downspout so water keeps moving and doesn't sit. Proper pitch is part of every WDM gutter install.

5. It is hung to handle winter

Snow and ice add heavy loads to gutters. Hangers spaced closer together help a gutter hold its shape and stay attached through winter.

Don't forget winter safety

In New Jersey, courts have held for decades that commercial property owners must keep the public sidewalk next to their property reasonably safe, including clearing snow and ice within a reasonable time after a storm ends. A downspout that dumps water across a walkway, where it freezes overnight, creates exactly the kind of hazard you don't want. Pointing downspouts away from walkways and entrances is a simple fix that helps.

Gutters and the roof: look at both

Gutter problems and roof problems often show up together. Water that can't get off the roof fast enough puts more stress on the roof itself. If your building's roof is aging, it makes sense to look at both at the same time. WDM installs commercial flat and low-slope roofing, including TPO, EPDM, and modified bitumen, along with gutters, under one team. Our post on what a commercial roofer checks during a flat-roof inspection covers the roof side.

Frequently asked questions

Do flat commercial roofs need gutters?Many do. Where the roof edge has no parapet wall, water runs off the edge and gutters with downspouts carry it away. Other roofs use interior drains or scuppers, and some use a mix.

How long can water sit on a flat roof?The roofing industry's guideline is that water should not remain on a low-slope roof 48 hours after rain, in conditions that allow drying. Longer is called ponding.

Why do commercial gutters overflow?Most often because the gutters or downspouts are too small for the roof area feeding them, there are too few downspouts, or the gutter isn't pitched toward its outlets.

The bottom line

A commercial roof is only as good as the way it sheds water. If your gutters overflow, sag, leak at the seams, or create ice at your entrances, the system is probably undersized, short on downspouts, or poorly pitched. A properly sized, seamless system fixes the problem at the source.

WDM Contracting is based in Monroe, NJ and installs commercial and residential gutter systems and commercial roofing across New Jersey. We come out, look at the building in person, and give you a written proposal with the full scope, materials, and timeline.

Request your free estimate or call 848-360-6121.