Maintenance-Free for How Long? Debris Matting and Bypass Growth on Guarded Gutters

Leaf guards are sold with a phrase no engineer would sign: maintenance-free. This paper tests the phrase by modelling how a guard's effective open area declines as debris mats over it, and when the screened capacity falls below what a design storm delivers. With stated accumulation rates, a guard on a roof under trees first bypasses more than ten percent of a 2.0 L/s design inflow at month 4, and by month 12 it is bypassing 62.0 percent of the inflow over its face; by month 24 the bypass reaches 80.9 percent. On an open roof without overhanging trees the same failure threshold arrives at month 9 and month-12 bypass is 24.8 percent. The guard does useful work — it keeps the gutter channel itself clear and moves the maintenance task from the gutter to the guard's surface — but the claim that the task disappears is disproved for both settings: what changes is where the debris waits, not whether someone must remove it. A cleaning interval rule is derived for each setting. All results are simulated, not measured.