An upper roof can send water through one gutter and onto a lower roof served by another. Both runs need to account for that water, but adding their volumes would count some of the same rainfall twice. When comparing a Tampa Bay gutter estimate, first ask whether its area figure describes a separate catchment or everything feeding a downstream run.
CamoKrew’s roof-runoff worksheet, a locally published resource, introduces zone-by-zone planning and upper-roof contributions. LSL extends that idea below with a hypothetical accounting example. It is not a correction to the company’s arithmetic or a recommendation of its services.
Two roof areas, one transfer
Assume an upper catchment A is 400 square feet and a lower catchment B is 800 square feet, measured horizontally. These are separate areas receiving rain directly: B does not include the footprint already counted in A. For this example only, all water from A reaches the lower run serving B, and there are no losses.
At one inch of rainfall, the calculation is:
| Location or total | Calculation | Theoretical volume |
|---|---|---|
| Upper run, serving A | 400 × 1 × 0.623 | 249.2 gallons |
| Rain landing directly on B | 800 × 1 × 0.623 | 498.4 gallons |
| Lower run, receiving A and B | (400 + 800) × 1 × 0.623 | 747.6 gallons |
The unique rainfall volume is 747.6 gallons, not 996.8 gallons. That larger number comes from adding the upper run’s 249.2 to the lower run’s 747.6; the lower figure already contains the upper contribution. Likewise, a proposal listing a 400-square-foot upper catchment and a 1,200-square-foot downstream contributing area does not establish 1,600 square feet of separate roof.
This distinction does not mean the upper run can be ignored. It means unique roof area and water passing each run answer different questions. If some upper water follows another route, this assumed transfer no longer describes the property. Ask the estimator to identify the route and verify the contributing areas. The example does not approve draining an upper roof onto a lower one.
Check the volume scale without turning it into a gutter size
Use horizontal catchment area, not the sloped roofing surface:
horizontal area × rainfall depth × 0.623 = theoretical gallons
The EPA’s NEWR rainwater-collection equation identifies 0.623 as a unit conversion. Its separate 75% collection-efficiency assumption belongs to its monthly harvesting screen; LSL does not apply that assumption to a gutter or reproduce the full NEWR model here. The USGS roof example gives about 1,743 gallons for one inch on 2,800 square feet: 1,743 ÷ 2,800 = 0.6225, consistent at the stated rounded precision.
| Horizontal zone | 1 inch | 2 inches | 3 inches |
|---|---|---|---|
| 400 sq. ft. | 249 gal. | 498 gal. | 748 gal. |
| 800 sq. ft. | 498 gal. | 997 gal. | 1,495 gal. |
| 1,200 sq. ft. | 748 gal. | 1,495 gal. | 2,243 gal. |
These nine values are LSL’s independently reproduced comparison calculations, rounded to the nearest gallon. The one-, two- and three-inch depths are scenarios, not Tampa Bay design storms or recorded local rainfall. A one-inch depth without a duration supplies no rainfall rate, and the figures predict neither actual capture nor a component’s capacity.
Put the ambiguity back to the estimator
Use a ground-level sketch or an existing plan; do not climb onto a roof to complete this exercise. Label distinct catchments A, B and so on, and draw any known transfer separately. Mark uncertain dimensions or routes unverified for the estimator to resolve. A ground photograph does not establish hidden drainage or an accurate area.
For an unclear proposal, an original LSL question prompt is:
Does the area stated for this lower run already include the upper roof? Please identify the separate catchments, the transfer between them, who verified the dimensions, and the final discharge point included in this price.
Keep the answer with the revised proposal. If the work ends at the downspout but grading or an underground connection remains unresolved, name that exclusion rather than treating the volume calculation as a drainage solution.
Where Hillsborough County is the permitting authority, this scope difference matters. Its ordinary-repair instructions include gutters and downspouts when structural components are not altered or installed, but distinguish alteration, replacement or relocation of drain leaders and drainage piping as work that is not a minor repair. Two quotes serving the same 1,200-square-foot catchment are therefore not equivalent if one only repairs a gutter and the other changes downstream drainage. Ask the County to classify the complete proposed work; do not infer an exemption from equal gallons, the product name or the price.
Our separate Hillsborough gutter and soffit scope guide follows that authority question in more detail. The transfer calculation establishes neither permit eligibility nor a lawful discharge route.
Sources were reviewed September 12, 2026. The company source is a public planning article, not a documented expert submission. LSL created the transfer example, checked the arithmetic and clarified source roles; we did not measure a roof, inspect a property, test a gutter or review a real estimate.