Drains Fine on Hose Test But Not in Heavy Rain: Bypass vs Normal Decision Tree

Why this matters

A hose test is the standard diagnostic and it has a real limitation: garden hose flow is a small fraction of heavy rainfall over a roof. A gutter that passes hose flow cleanly can fail catastrophically in a downpour, and a tech who relies only on the hose result will sign off on a system that comes back as an overflow callback. The fix is not to throw out the hose test but to interpret it correctly and to add the additional checks that catch the conditions only heavy rain reveals. This tree walks the gap between bypass-flow conditions and normal-flow conditions.

Section 1: Why the hose test passes when rain fails

A garden hose delivers roughly five to eight gallons per minute. A 1,000 sq ft roof in a 2-inch-per-hour rainfall delivers more than 100 gpm. Thunderstorm bursts can be two to three times higher.

The hose test verifies:

• The gutter has no full blockage.
• Water can reach the downspout from the test entry point.
• The downspout is not fully clogged.
• The pitch is sufficient to move water at low flow.

The hose test does not verify:

• That the gutter has enough capacity for peak rainfall.
• That the outlet sizing matches the spout.
• That a roof valley delivery point does not overshoot.
• That a partial blockage further down the run does not back up under higher flow.
• That the downspout outlet does not constrict under hydraulic head.
• That the underground drain line has the throughput for sustained heavy flow.
• That seams and end caps do not leak under standing-water depth.

The hose passes a low bar. Heavy rain tests the system.

Section 2: The capacity branch

When the hose passes but rain overflows, the most common cause is undersized capacity for the roof tributary area or for the rainfall intensity at the location.

Sort by:

• Gutter profile (5-inch vs 6-inch, K-style vs half-round).
• Downspout size (2x3 vs 3x4 vs 4x4) and count.
• Tributary roof area served by each downspout.
• Local rainfall intensity per regional climate data.
• Roof slope (steeper roofs need higher capacity because runoff is delivered faster).

SMACNA sizing tables published in the Architectural Sheet Metal Manual give the standard reference. If the existing system reads under the table threshold for the local rainfall and the actual tributary area, the answer is capacity upgrade, not diagnostic deeper.

Section 3: The partial-blockage branch

A run with a partial blockage handles hose flow at the low rate but backs up at higher flow.

Common partial blockages:

• Granule silt in the trough that raises the floor and reduces peak-flow depth.
• A small twig at the outlet that hose flow squeezes past but rain backs against.
• A downspout elbow with debris packed inside the curve.
• A guard or strainer at the outlet that the hose dislodges but rain rebuilds.

Inspect with the gutter dry. Look for granule sediment, examine outlet strainers under good light, and probe downspout elbows by feel or with a flexible camera.

Section 4: The downspout-bottleneck branch

The downspout passes a hose but does not pass rain.

Conditions that fit:

• Downspout outlet hole undersized relative to the spout (a 3x4 spout dropped into a 2x3 outlet hole flows at the smaller dimension).
• Outlet drop is a sharp 90 instead of a high-flow gradual transition.
• Spout has a kinked, dented, or crushed section that throttles flow under head pressure but passes a hose by gravity.
• Spout discharges into an underground drain line that backs up under heavy sustained flow.

Test for this by running the hose at the highest flow the spigot delivers for several minutes while watching the gutter water level over the outlet. If the water level rises during the test even though water is moving, the outlet or downstream system is the throttle.

Section 5: The valley and overshoot branch

A roof valley dumps a concentrated load that hose-from-a-different-point does not replicate.

Signs:

• Hose test from one end of the run drains clean.
• Overflow in actual rain is at a specific point that is downstream of a roof valley.
• The rest of the run is dry in the same rain event.

The hose entered at a point with low local delivery. The rain delivers at a high-concentration point. The fix is at the valley delivery: splash guard, diverter, or local capacity upsize.

Velocity overshoot on steep roofs is the related case. Hose flow drops gently into the trough. Roof runoff in heavy rain shoots across the gutter front edge entirely. A high-back gutter profile or inboard relocation addresses this.

Section 6: The underground-drain branch

The visible system passes a hose but rain backs up at the spout outlet.

This is hard to catch on a short hose test because the underground line has reserve capacity for short bursts. Run the hose for at least 10 minutes continuously to load the downstream line. Watch for water rising at the spout boot.

Other signs:

• Spout discharges into a tile, dry well, or curb drain with no visible flow at grade.
• Older clay-tile drain lines with root intrusion or collapse history.
• Dry well that has filled with silt or has saturated soil around it.
• Drain line discharging to a low point that backs up in heavy rain regardless of the line capacity.

The fix is downstream of the gutter and is a separate scope from gutter repair.

Section 7: The augmented hose test

When a standard hose test is inconclusive, run a heavier test: multiple hoses simultaneously, extended duration (15 to 30 minutes) to load downstream lines, hose entered at the actual problem-delivery point (under the valley, at the steepest section), or inspection during an actual rain event.

Section 8: Communicating the call

The customer who watched water drain fine on the hose wants to know why a service call is still needed. The honest answer is that the hose test passes a low bar and rain tests a higher one, and the gap between them is where the problem lives. Walk through the specific bypass condition (capacity, partial blockage, outlet bottleneck, valley delivery, underground drain) and tie to an observable.

References

• SMACNA Architectural Sheet Metal Manual, sizing and capacity tables for gutters and downspouts.
• IRC R903, Roof Drainage.
• ASTM E2128, Standard Guide for Evaluating Water Leakage of Building Walls.
• NRCA Roofing Manual, roof drainage capacity guidance.