Gutter & Downspout Sizing Calculator
Size PEMB-style box gutters from roof area, rainfall intensity, and downspout spacing. Check required depth and whether 4×4, 4×5, 5×5, or 6″ round outlets can carry the flow.
Gutter Design (Box Style)
1. Roof Parameters
2. Layout
Dist between downspouts
Total gutter run
3. Gutter Detail
Box Gutter Sizing for Metal Buildings
Industrial box gutters fail when downspouts are spaced too far or the trough is too shallow for the rainfall intensity. This tool sizes from tributary area per downspout — not the whole roof dumped into one outlet — which matches how PEMB eave gutters actually drain between outlets.
How to use this calculator
- Enter total roof area draining to the gutter run and the design rainfall (in/hr) from local criteria or the metal building manufacturer.
- Set downspout spacing and eave length so the tool can estimate outlet count and trib width × spacing.
- Choose gutter width and downspout size; review required depth and whether outlet capacity shows OK, WARNING, or OVERLOAD.
Flow and capacity notes
GPM per downspout ≈ trib area × intensity × 0.0104. Required cross-section uses a PEMB-calibrated rule of thumb, then depth = area / width (minimum about 3″ for a workable box). Typical capacities used here: 4×4 ≈ 180 GPM, 4×5 ≈ 225, 5×5 ≈ 280, 6″ round ≈ 320. Real gutters also need slope to the outlet, clean leaves/ice planning, and expansion joints on long runs — capacity alone does not replace manufacturer tables.
FAQ
Is roof area the plan area or slope area?
Use the horizontal projected drainage area that feeds the eave, consistent with most metal building drainage methods. Steep roofs may need manufacturer adjustments.
Where do I get rainfall intensity?
From the building code, ASCE rainfall maps, or the PEMB vendor’s design criteria for the site. Do not guess a coastal 4 in/hr value for an inland warehouse.
Why is minimum depth 3 inches?
Shallower boxes clog and lose freeboard. Even if the area formula returns less, keep a practical minimum for fabrication and maintenance.
Does this design hangers and expansion?
No — hydraulic sizing only. Follow the gutter manufacturer for hanger spacing, thermal expansion, and outlet detailing.