Commercial gutters are a different animal from residential. The roof areas are larger, the rooflines are flatter, and the water has to move in volume or it backs up against a parapet and finds the deck membrane. We design, fabricate, and install commercial gutter systems across all four states we serve, and most of the calls come down to three things: box gutters, flat-roof drainage, and getting the size right.
The sizing part is where commercial jobs go wrong most often. A default 5-inch residential spec on a warehouse eave is a system designed to fail in its first hard rain. We work from the roof's drainage area, local rainfall intensity, and pitch, and we size the outlets and downspouts first, because they are the bottleneck, not the trough.
We work on retail, warehouse, office, agricultural, and multi-unit buildings, and we schedule around business hours and occupied space.
Box Gutters
A box gutter is a rectangular, high-capacity channel, usually run along the eave of a low-slope roof or built behind a parapet. It carries far more water than a residential K-style, and it is what most retail, warehouse, and multi-unit buildings actually need. We fabricate box gutters in heavy-gauge aluminum or steel, seal and mechanically fasten the corners, and size the outlets to the roof area feeding them. The failure point on box gutters is almost always an undersized or badly placed outlet, so that is where the design time goes.
Flat and Low-Slope Roof Drainage
Flat roofs do not shed water on their own. They drain to points, and the gutter or scupper at each point has to take everything the membrane sends it. On a low-slope roof we look at how the membrane drains, where water concentrates, and whether the building uses interior drains, scuppers through the parapet, or an eave-line box gutter. Then we build the collection and downspout system to match the volume those points produce in a hard rain or a fast snowmelt. Get this wrong and water ponds against the parapet. Get it right and the roof clears fast every time.
Sizing Is a Runoff Calculation, Not a Guess
We work from drainage area, rainfall intensity, and roof pitch to size the trough and, more importantly, the outlets and downspouts. The outlets are almost always the bottleneck, so we oversize those before the trough. A 6-inch K-style is often the smallest profile we will put on a commercial building, and most jobs go larger, to a box profile with oversized downspouts. We will show you the math behind the size we spec.
What Goes Into a Commercial System
Every component gets specced for load and volume, because commercial eaves carry snow slide and ice that would strip residential hardware off the wall.
- Oversized K-style or box gutters in heavy-gauge aluminum or steel
- Seamless runs to cut leak points on long eaves
- Outlets and downspouts sized to the roof area, not a default
- Reinforced hangers and brackets rated for snow slide and ice load
- Slope set for fast drainage, with tie-ins that carry water clear of entries and parking
- Guard options for systems that cannot afford a clog, installed by the only Alu-Rex Certified Installer in your region
What You Get
- Sized from a runoff calculation, shown to you
- Box gutter fabrication in aluminum or steel
- Flat and low-slope drainage design
- Hardware rated for snow slide and ice load
- Scheduled around business hours and tenants
- Water-tested at every outlet before handoff
How the Job Runs
- 01 Walk and measure
We get on the roof, find where water concentrates, and measure the drainage areas. No one-size spec, no quote off a satellite photo.
- 02 Fabricate on site
Seamless runs formed at the building to cut joints; box sections fabricated to the parapet and outlet plan.
- 03 Install and water test
Commercial hangers and fasteners, correct slope, then we run water and confirm every outlet carries before we leave.
Common Questions
01 What size do commercial gutters need to be?
It depends on roof drainage area and rainfall intensity, not the building type. We size the downspouts and outlets first, since those are the bottleneck. Most commercial jobs run a 6-inch-plus K-style or a box profile with oversized downspouts, and we show you the calculation behind the spec.
02 Do you do box gutters and flat-roof systems?
Yes. Box gutters and low-slope collection systems are most of our commercial work, including scupper and parapet drainage details.
03 Can you work on an occupied building?
Yes. We schedule around your hours, tenants, and customer traffic, and we keep entries and walkways clear while crews are working overhead.
04 Do commercial gutters need guards or maintenance?
One or the other. A plugged box gutter outlet can put a lot of water somewhere expensive fast, so commercial systems need either scheduled cleanings or guard protection at the outlets and troughs. We install Alu-Rex systems as the only Alu-Rex Certified Installer in your region and can spec either approach for your building.
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