Permeable Paving in Somerville: How It Works, How It Earns Green Score, and Why the Install Decides Everything

Somerville is one of the densest cities in New England — and it is paved within an inch of its life. That paving is exactly why the city now writes stormwater rules into its zoning ordinance, why every new driveway needs a mitigation plan, and why a properly installed permeable paving system is one of the highest-leverage moves a property owner can make. Done right, it solves drainage, earns Green Score, and lasts decades. Done wrong, it clogs in two seasons and gets revised-and-resubmitted by the city. This guide walks through both sides. Want us to just handle it? See Somerville Green Score help — we design, score, document, permit, and build it.

What a permeable paving system actually is

A permeable paving system is not a surface — it is a vertical assembly. Water passes through the visible paving layer, drops into an engineered stone reservoir below, and then either infiltrates into the native soil or is conveyed away through an underdrain. From top to bottom, a typical system looks like this:

• Wear surface — concrete pavers with open joints, porous concrete unit pavers, porous asphalt, or pervious concrete. (Grass pavers and stone dust are not classified as pervious paving under Somerville's Green Score — they're treated as groundcovers unless a stone sub-base is engineered in.)
• Bedding course and joint stone — clean, washed, angular aggregate (ASTM C-33 No. 8, 89, 9, or 57). This is the layer most often built wrong by general paving crews. Sand will not work here — it clogs.
• Sub-base reservoir — open-graded #57 stone, sized for the storm event the lot needs to detain. This is where the system does its real job.
• Geotextile and subgrade — compacted properly so water moves through it without the lot settling under the paving.

The Somerville Zoning Ordinance is specific about the minimums: at least 5 inches of approved aggregate, a fresh-install surface infiltration rate of 100 inches per hour, an in-service rate of at least 10 inches per hour, and a maximum slope of 5% for any area you want to count toward Green Score. Below those thresholds, you have decorative pavers — not a code-compliant stormwater system.

Why Somerville requires permeable surfaces in the first place

Under City Ordinance Sec. 11-165(b), no one is allowed to send roof, surface, or drainage water into the public sewer or stormwater system, directly or indirectly. The Engineering Division has concluded that infiltrating 3/4″ of runoff from a property's impervious area satisfies that requirement. Translated into a driveway permit, that means:

• Provide 1 square foot of permeable surface for every 5 square feet of paved area.
• The permeable section must sit at the lowest point of the driveway, so water actually reaches it.
• New or expanded paved areas require twice the mitigation above — or a sealed engineering analysis to justify less.

For most properties, the cleanest way to satisfy this rule is to make the whole driveway, or the apron and turnaround, permeable. It's also the only path that earns Green Score credit at the same time.

How permeable paving earns Green Score

Green Score is Somerville's landscape performance standard. Any new principal building or substantial renovation has to hit a minimum score that varies by zoning district:

Pervious paving earns credit under the Soils category of the scorecard — at a higher multiplier when the combined depth of stone reservoir and free-draining sub-soil exceeds 24 inches. On a tight Somerville lot where you can't carve out room for a 200-square-foot rain garden, this is often the most points-per-square-foot you can buy. The de-paved lot area bonus stacks on top: every square foot of existing impervious asphalt or concrete you tear out and replace with pervious paving earns an additional bonus multiplier — points the city wants to see, because it directly shrinks the impervious footprint of the neighborhood.

The Green Score scorecard: how points are earned

The city scores your whole lot by multiplying the area of each landscape element by a set “multiplier,” adding them up, and dividing by your total lot size. Some elements earn far more than others:

• Permeable pavers — credited under the paving/soils category (roughly a 0.2 multiplier for a standard 6–24″ stone sub-base, higher for a deeper reservoir). On a tight lot with no room for a rain garden, this is often the most points per square foot you can buy.
• Landscaped soil — shallow beds earn about 0.3; deeper soil (24″+) earns more.
• Trees — among the highest-value elements. A newly planted small tree counts as roughly 150 sq ft of credited area and a large tree as about 450 sq ft — a big jump from a single plant.
• Rain gardens & stormwater planters — the top-weighted elements when there's room for them.

Bonus credits stack on top (+0.1 each, and they're cumulative): native species, publicly-visible frontage plantings, approved high-value shade trees (Red Maple, River Birch, White Oak, elm), de-paving old asphalt or concrete, and harvesting rainwater for at least 50% of your irrigation. A native, high-value, street-facing tree can earn all three at once.

Multiplier values are set by the City and can change — we always confirm the current figures against Somerville's official Green Scorecard before we submit.

What a passing 0.35 scorecard actually looks like

Abstract multipliers are hard to picture, so here’s a real, buildable example. Say you have a 1,500 sq ft lot — the house covers about 1,000 sq ft, leaving a tight 500 sq ft back yard plus a small street-facing front strip. To pass, your landscape has to add up to 525 weighted square feet (1,500 × 0.35). Here’s a layout that clears it without giving up your patio:

Divide the total by the lot size — 545 ÷ 1,500 = 0.363 — and you’ve cleared the 0.35 minimum with room to spare.

Notice what did the work: one shade tree earned 405 of the 545 points — about three-quarters of the whole score. On a tight Somerville lot, the two highest-leverage moves are almost always one approved large, native, street-facing shade tree, and making the patio permeable over torn-out asphalt. Everything else is supporting cast.

And because that tree sits in the street-facing frontage, it doesn’t eat into your 500 sq ft of usable back yard — you keep the full patio and still pass. That’s the difference between scrambling at permit time and designing to score from day one. Bring us your rough dimensions and we’ll run the scorecard for your specific lot before you commit to a layout.

The Gator Base advantage on tight lots

Traditional permeable base means excavating 6+ inches of stone by hand down a narrow Somerville side yard — slow, heavy, and hard on the soil. On the right project we use Alliance Gator Base: a lightweight structural panel that replaces roughly 288 lbs of crushed stone and cuts excavation to about 2 inches. Less digging, no heavy equipment squeezing through a gate, less soil compaction, and a built-in thermal layer that helps the patio resist freeze-thaw buckling. Paired with our build standards — a full open-graded stone base at the depth the use calls for, Gator Maxx polymeric jointing, and premium Techo-Bloc or Unilock pavers — you get a surface that drains, scores, and lasts.

The Green Score permit process, step by step

1. Site evaluation & layout — we measure the lot and design a layout that hits your district's minimum score.
2. Stamped drawings — MA-registered landscape architect plans (existing conditions, grading, materials, and a planting schedule).
3. Scorecard calculation — we complete the city's official Green Scorecard, balancing permeable paving, plantings, trees, and de-paving bonuses.
4. Submission & review — we handle the submittal to Planning & Zoning or Inspectional Services with the supporting documentation the city requires.
5. Approval notice — the city issues one of four decisions (approved, approved with minor changes, revise-and-resubmit, or denied). We aim for the first one by getting the paperwork right up front.

Why the install decides whether it survives

Permeable paving has a deserved reputation in some neighborhoods for failing — clogging, settling, heaving in February. In every failure I have inspected in Greater Boston, the system itself wasn't the problem. The install was. The most common mistakes:

• Sand instead of #8/#9 angular stone in the joints. Sand seals the surface in one season. The system stops infiltrating, the city stops counting it, and the homeowner has spent permeable paving money for impermeable performance.
• Subgrade compacted to the wrong density. Too loose and the lot settles. Too tight and the water can't infiltrate. The target is 95–98% Standard Proctor on the prepared subgrade — measured, not guessed.
• No edge restraints. New England's freeze-thaw cycle pushes pavers outward every winter. Without engineered edge restraints, the field walks itself apart in three to five seasons.
• No sediment management. Permeable systems clog from the top. A maintenance plan — annual vacuum sweep, no winter sand applied as deicer — is the difference between a 25-year system and a 5-year one.

“A permeable paver job is half hardscape, half drainage engineering. The crews that treat it like the first half are the crews whose work fails.”

Why design + build matters on a Somerville lot

Most Somerville lots are small, sloped, and surrounded by neighbors whose basements you do not want to flood. That changes the design conversation. Where does the water actually go after it leaves your reservoir? Is the native soil tight clay or sandy fill from an old construction era? Where can a 5% slope cap actually be held without re-grading the whole yard? Those questions don't get answered well when design and installation are split across two companies — the design comes back, the installer says “we can't build that,” and the homeowner pays twice.

Our practice handles design, grading, drainage planning, permeable system specification, Green Score documentation, the city permit submittal, and installation as one job. That means the lot gets surveyed by the people who will pour the base, the Green Scorecard gets filled out by someone who has actually answered Engineering's revise-and-resubmit comments before, and the homeowner has one phone number to call when the apron meets the new sidewalk wrong.

About Allan Spires & American Elm Landscape

Allan Spires founded American Elm Landscape in Somerville and has spent his career installing landscapes across Somerville, Cambridge, Medford, Arlington, and Greater Boston. The practice specializes in residential design + build with a particular focus on stormwater-smart hardscape — permeable driveways, patios, walkways, rain gardens, and Green Score-compliant landscape installations.

Because the same team handles the design, the permitting, and the install, projects move through Somerville's Engineering and Planning reviews with fewer revision cycles, and the system that gets built is the system that was actually specified.

Published May 18, 2026 · American Elm Landscape LLC, Somerville, MA. Sources referenced: City of Somerville Green Score Guide, Somerville Driveway & Pavement Permit Brochure, City of Somerville Stormwater Management, Somerville Zoning Ordinance Section 10.4.