The dull subsystem that prevents the expensive call.

What it is

FNDN-1 is a name we made up for the set of moves that keep storm water away from the building envelope while still putting it to use on the property. It is not a single device or a single permit; it's the integration layer between the roof, the hardscape, and everything else we install. Most of FNDN-1 is grading work — moving 1/4″ to 1/2″ per foot of slope away from the slab for at least five feet, then onward to a basin, a swale, or a tank — but it also includes things like downspout extensions, French drains, foundation-side root barriers when we plant a thirsty tree closer than 12 feet from a slab, and stucco-mounted splashguards.

We carved this out as a separate subsystem in 2022 after our second year of getting called back to fix water-damage problems on lots where we had installed clean cisterns and good basins but failed to detail the building edge. The work was always small, always fast, and always cheaper to do up front than to retrofit later. So now we do it on the front end, and it has its own line item.

Why we run it

Tucson has roughly 27% of its annual rainfall fall in the months of July, August, and September — and most of that arrives in storms that exceed 0.5 inches per hour. A flat-roof Tucson house with 2,400 sf of catchment delivers about 1,500 gallons of water through its downspouts in a 1″ storm, and most of that arrives in fifteen minutes. If those downspouts dump within 4 feet of the slab — which the original 1948 builder almost certainly designed them to do, because there was nowhere else to put them — the water will saturate the soil under the footing, and on a clay-rich Tucson soil that swelling can crack interior partition walls within five years.

The simpler alternative is to extend the downspout four feet out, or to tie it into a tank, and to grade away. Three of our most common service calls are a downspout that came loose from its extension during the dry season and resumed dumping next to the slab. We retrofit those with stainless screws and a 1/8″ bend.

Schematic

The four moves

FNDN-1 is four moves in some combination on every lot.

1. Slope away from the slab. The International Residential Code R401.3 requires 6 inches of fall in the first 10 feet from the foundation. Most pre-1980 Tucson homes don't meet this requirement, particularly on the north and east elevations where the soil has sometimes been built up over the slab edge by a generation of leaves and old mulch. We re-grade.

2. Extend the downspouts. A 4-foot downspout extension is roughly $14 in materials and the difference between water sitting next to the slab and water reaching a basin. On about 80% of the lots we walk, this single move would solve the highest-priority problem. We do this on every lot we touch, even on cistern installs where the downspout already routes to a tank — we install a backup overflow extension to a basin past the perimeter.

3. Perimeter swale. A 6-inch-deep, 18-inch-wide depression running along the slab edge and tapering to an outlet at one corner. Captures wall-runoff during big storms; converts a row of blank stucco into a useful drainage element.

4. French drain (rare). Where conditions force water to ponding next to the slab and re-grading isn't possible — a tight side yard between two close-set houses, a slab tilted toward an HOA-required hedge — we run a perforated 4″ HDPE pipe in a gravel-and-fabric trench and outlet it past the building. We do four to six French drains a year. The rest of the time we don't need to.

Bill of materials, typical four-elevation re-grade + extensions