Spec'ing Gutters for a New ADU in Rocklin: Sizing, Setbacks & Where the Water Has to Go

Do I Need Gutters on an ADU? What the Code Says

No section of the California Residential Code says the word “gutter” in a way that mandates one on every house. What the code does require is control of the water, and on a Rocklin ADU that control is nearly impossible without gutters.

Two provisions of the 2025 California Residential Code, in effect since January 1, 2026, do the work here:

• - CRC R801.3, Roof drainage. Where expansive or collapsible soils are known to exist, every dwelling must have a controlled method of water disposal from the roof that collects and discharges roof drainage to the ground surface not less than 5 feet from foundation walls, or to an approved drainage system. An ADU is a dwelling. Whether R801.3 is triggered on your lot depends on the soils report, but Rocklin's clay-heavy soils make it the safe assumption. Our clay soil foundation drainage guide covers why those soils punish uncontrolled roof water.
• - CRC R401.3, Drainage. Surface water must be diverted to a storm sewer or another approved point of collection that does not create a hazard, and lots must be graded so the ground falls at least 6 inches within the first 10 feet from the foundation. The exception matters more than the rule for ADUs: where lot lines, walls, slopes, or other physical barriers prohibit that fall, drains or swales must be constructed to ensure drainage away from the structure.

Rocklin does not publish an ADU gutter handout, and neither does Placer County. What the Building Division reviews is the site plan and its grading and drainage notes. A plan set that shows roof water dripping off an eave 2 feet from the neighbor's fence, onto ground that can't slope away from the building, is a plan set that comes back with comments.

There is also the neighbor. California follows the reasonable-use rule from Keys v. Romley: you cannot concentrate roof runoff onto the lot next door. A 1,200-square-foot roof that used to be lawn now sheds roughly 17,000 gallons of new ADU roof runoff a year in a 23-inch rain year, and on a 4-foot setback a lot of it would land on the other side of the fence. See our guide to neighbor gutter drainage and California property-line law for how those disputes play out.

The 4-Foot Setback Problem: Where the Water Can't Go

California Government Code Section 66314(d)(7) caps the side and rear setback a city can demand for a new detached ADU at 4 feet. (The ADU statutes were renumbered from former Section 65852.2 by SB 477, effective March 25, 2024.) Rocklin adopted the same 4-foot side and rear setback in Municipal Code Chapter 17.67, and the city's own ADU development standards add a warning worth reading twice: in some cases a setback may need to be larger than the minimum because of easements for utilities, drainage, or access.

Most ADU designers use every inch of that allowance. That is where the drainage plan breaks, because three different distances collide in the 4 feet between the ADU wall and the fence.

Collision 1: The eave can't reach the fence

Rocklin's ADU FAQ says eaves and HVAC equipment may project 3 feet into a required setback under RMC 17.08. Read alone, that suggests a 3-foot eave leaving the gutter a foot from the fence. The building code does not allow it. CRC Table R302.1(1) prohibits projections less than 2 feet from the lot line, and between 2 and 5 feet the underside of the projection needs 1-hour fire-resistance-rated construction, heavy timber, or fire-retardant-treated wood. The more restrictive rule governs, so on the setback side you get a 2-foot eave at most, with a rated soffit, and the gutter's outer face lands about 2 feet from the property line. Most plan checkers measure to the outermost edge of the projection, gutter included, so a 12- to 18-inch eave on those elevations keeps you clear.

Collision 2: R801.3 says 5 feet, the lot gives you 4

If your soils report flags expansive clay, R801.3 wants roof drainage discharged at least 5 feet from the foundation wall or into an approved drainage system. Five feet from the ADU wall on the setback side is 1 foot into the neighbor's yard. The only way to satisfy the section on that elevation is the second option: an approved drainage system, which in Rocklin means a solid pipe.

Collision 3: R401.3 needs 10 feet of fall you don't have

The 6-inches-in-10-feet grading rule assumes open ground. With 4 feet to the fence, the exception applies automatically: drains or swales must be built to ensure drainage away from the structure. A shallow swale along the side yard running toward the front or rear can work on a lot with real fall; on a flat Rocklin tract lot the answer is usually a trench drain or a piped downspout system. Either way, it has to be drawn on the site plan and it has to be built before the side-yard concrete goes in.

Eave depth, fascia height, and downspout drop locations are set at framing. Changing any of them after stucco costs more than the gutters themselves.

What Size Gutters for an ADU: The Rocklin Rainfall Math

Gutter sizing for an accessory dwelling unit uses the same method as any roof: multiply the plan area of the roof feeding each gutter run by a pitch factor to get effective area, then compare that against the gutter's capacity at the local design rainfall intensity. The design intensity is the number most online calculators get wrong for Rocklin, so start there.

NOAA Atlas 14, Volume 6, gives point precipitation frequency estimates for Rocklin (38.79 N, 121.24 W). The sheet-metal industry sizes gutters on the 100-year, 5-minute burst, and for Rocklin that is 0.307 inches in 5 minutes, or 3.7 inches per hour. The 10-year, 5-minute storm is 2.3 inches per hour. Over a full hour the 100-year storm averages only 1.0 inch per hour, which is why the 5-minute figure controls: gutters overflow in bursts, not averages.

Step 1: Effective roof area per gutter run

Measure the roof plan area draining to each gutter, including the eave overhang, and multiply by the pitch factor used in the standard sizing tables: 1.0 for roofs up to 3:12, 1.05 for 4:12 to 5:12, 1.1 for 6:12 to 8:12, 1.2 for 9:12 to 11:12, and 1.3 for 12:12 and steeper. Our roof pitch to gutter size guide walks through the factor table in more detail.

An 800-square-foot ADU with a 20-by-40 footprint, 2-foot eaves, and a 5:12 gable has a roof plan of about 24 by 44 feet, or 1,056 square feet. Each eave gutter takes half: 528 square feet, times 1.05, is about 550 square feet of effective area. A 1,200-square-foot hip-roof ADU at 4:12 puts roughly 1,500 square feet of roof on four runs, and even if one run carried half of it the effective area would be under 800 square feet.

Step 2: Gutter capacity at 3.7 inches per hour

The widely published gutter capacity table (the same one This Old House and most manufacturers reproduce from SMACNA-derived data) rates a 5-inch K-style gutter at 5,520 square feet of roof at 1 inch per hour and a 6-inch K-style at 7,960. Half-round profiles carry less: 2,500 and 3,840 square feet. Divide each by Rocklin's 3.7 inches per hour and you get the real capacities below.

The conclusion is blunt: a 5-inch K-style gutter has almost three times the capacity an ADU eave run needs, even in a 100-year burst. Half-round gutters, popular on cottage-style ADU elevations, still clear the load on an 800-square-foot unit but lose most of their margin on a 1,200-square-foot one. If the primary house already has 6-inch gutters, matching them on the ADU is a cosmetic and debris decision, not a hydraulic one. Our 5-inch vs 6-inch sizing guide covers the tradeoffs.

Step 3: Downspouts are the real constraint

Sheet-metal practice sizes downspouts at roughly 1 square inch of cross-section per 100 square feet of roof, which gives a 2x3-inch downspout (6 square inches) about 600 square feet and a 3x4-inch (12 square inches) about 1,200 square feet. That rule is deliberately conservative compared with plumbing-code leader tables because it assumes some debris in the outlet, which is realistic under Rocklin oaks.

• - 800 sq ft ADU, ~550 sq ft per run: one 2x3 downspout per run is at the limit. One 3x4 per run, or two 2x3s, is the right spec.
• - 1,200 sq ft ADU, up to ~790 sq ft on the worst run: a single 2x3 is undersized by the rule. Use a 3x4 or split the run with two outlets.
• - Run length: keep each downspout serving no more than about 35 to 40 feet of gutter so the slope (1/16 to 1/8 inch per foot) stays hidden behind the fascia.
• - Debris: a 3x4 outlet bridges with leaves far less often than a 2x3. On ADUs under oak or pine canopy we pair 6-inch gutters, 3x4 downspouts, and micro-mesh guards. See the best gutter guards for oak trees.

The count matters more than the size on a setback lot, because every downspout is a point where the water leaves the roof and has to go somewhere legal. Two downspouts on the fence side of an ADU means two underground drops, not one.

Where ADU Rainwater Drains: Five Discharge Options, Ranked

ADU downspout drainage in Placer County comes down to five endpoints, and every downspout on the unit needs one of them on the site plan. They are ranked by how often they pass plan check in Rocklin without comment and how well they hold up in an atmospheric-river winter.

Rocklin vs Placer County vs Roseville: What Each Jurisdiction Reviews

State ADU law sets the floor everywhere in Placer County, but the stormwater rules that shape the downspout plan differ by jurisdiction. Here is what is actually on the books as of September 2026, and where each agency publishes nothing and defaults to standard practice.

The practical takeaway on thresholds: an ADU roof alone is 800 to 1,500 square feet of new impervious surface. Add a 400-square-foot patio and a widened driveway and a Granite Bay or Roseville project crosses 2,500 square feet and picks up a site design measure requirement, which is usually satisfied by routing downspouts to landscaping or a cistern. Rocklin, as its own Phase II permittee, applies the state permit's post-construction thresholds through its own review, and the safest move is to show the disconnection or storage on the plan before the Engineering Division asks for it.

For the statewide permit and code layer that sits under all of this, read our California gutter building codes and permits guide. If the ADU is attached or shares a roofline with a patio cover, the tie-in rules in our patio cover and ADU gutter guide apply instead of the detached-unit setback math above.

The ADU Gutter Spec Sheet to Hand Your Builder

Most ADU plan sets say “gutters and downspouts per code” and nothing else, which is how a $2,000 line item turns into a $6,000 change order after the side yard is poured. Put these items in writing on the plans or in the builder's scope.

Rocklin's oak ordinance protects the trees, so the ADU gets built under the canopy. Spec the gutters for the leaf load, not just the rain.

Worked Example: A 1,000 sq ft ADU on a Rocklin Lot

Here is how the numbers land on a typical project, the kind we see across Stanford Ranch and the older Rocklin neighborhoods off Sunset Boulevard: a 25-by-40-foot detached ADU in the rear yard of a 7,000-square-foot tract lot, long side parallel to the rear fence, set at the 4-foot rear and 4-foot east side setbacks, with a 5:12 gable running the long way and 18-inch eaves.

• - Roof plan: 28 by 43 feet, 1,204 square feet. Each eave run carries 602 square feet, times 1.05, is 632 square feet effective.
• - Gutter: 5-inch K-style clears 1,490 square feet at 3.7 inches per hour, so 5-inch is fine hydraulically. The lot has two valley oaks over the rear fence that are protected under Rocklin's oak tree ordinance, so the spec goes to 6-inch with micro-mesh guards.
• - Downspouts: 632 square feet per run exceeds the 600-square-foot allowance for a single 2x3, so each 43-foot run gets one 3x4 downspout at the end away from the setback corner, plus a second 3x4 mid-run on the fence side to keep the slope hidden.
• - Rear (setback) run: two downspouts drop into 4-inch solid PVC, run west along the rear setback, then north up the open west side yard 55 feet to a pop-up emitter in the front lawn, 12 feet from the house foundation. Total pipe is about 95 feet, which at 1 percent needs roughly a foot of fall from the rear corner to the front lawn, and a rear-to-front graded tract pad usually has it.
• - Front (courtyard) run: one 3x4 to a 6-foot surface extension and splash block onto the courtyard lawn, which slopes 6 inches in 10 feet toward a rear-yard area drain the builder is installing anyway.
• - Eaves: 18 inches on the rear and east elevations leaves the gutter face 2.5 feet from the fence; the soffit on those two sides is specified as 1-hour rated per Table R302.1(1).

Put on the plans before permit, that drainage design is a single line item in the ADU bid. Discovered after the side-yard concrete is poured, the same 95 feet of pipe means saw-cutting, and the rear downspouts end up on splash blocks 2 feet from the neighbor's fence until someone complains.

What ADU Gutters and Drainage Cost in Rocklin (2026)

Rocklin Gutter Guard's 2026 ranges for detached ADU work, installed during construction:

• - Seamless aluminum gutters and downspouts, 60 to 120 linear feet: $1,800 to $4,500. Five-inch runs $8 to $15 per foot and 6-inch $10 to $17 per foot; the upgrade adds roughly $2 to $4 per linear foot.
• - Underground drain line to a pop-up emitter or curb core: $800 to $2,500 depending on length, bedrock, and whether a curb core and encroachment permit are involved.
• - Micro-mesh gutter guards: priced per linear foot on top of the gutter; worth it on any ADU under canopy because nobody climbs a ladder to clean a rental unit's gutters on schedule.
• - Post-construction retrofit penalty: 20 to 30 percent more for the gutters, and far more for drainage once hardscape is in.

The cheapest version of this project is the one where the gutter contractor sees the plan set before framing. For a new ADU in Rocklin, Roseville, Lincoln, Loomis, or Granite Bay, we review the drainage sheet at no charge and mark up downspout locations and discharge paths before you submit. Start with our Rocklin gutter installation, downspout installation, and underground downspout pages, or the Granite Bay gutter installation page for unincorporated lots. Building on a production-builder lot? Our new construction gutter guide covers the primary home side of the same job.

FAQ: ADU Gutters and Drainage