Green roof installation in Washington, DC

Weight is the first question. A thin extensive green roof with three to four inches of growing media weighs roughly 15 to 25 pounds per square foot when saturated, on top of the roofing itself. Many rowhouse joists from the early 1900s were sized for a lighter load, and some have been notched for plumbing or weakened by old leaks.

So we don't start with plants. We start by opening the ceiling or roof at a few points, measuring joist size, span and spacing, and noting any damage. On most projects a structural engineer then confirms capacity or specifies sistered joists, and we build to the engineer's numbers.

Alley garages behind rowhouses in Brookland and Capitol Hill, along with newer rear additions, are often easier candidates than the main roof, since their framing is younger and the spans are shorter.

Below the plants sits a complete roofing assembly. We use a root-resistant membrane, typically PVC, TPO with a root barrier, or a modified bitumen sheet with copper-foil or chemical root resistance. Every seam is welded or torched and tested before anything covers it, because finding a leak later means removing soil.

Above the membrane we lay a protection fabric, then a drainage and retention mat that holds some water for dry spells and drains the rest toward the outlet. A filter fabric keeps fine soil from clogging the drain layer. Engineered growing media, mostly expanded shale or a similar mineral blend with a little organic matter, goes on top.

Plants go in as plugs, cuttings or pre-grown sedum mats. Mats give instant coverage and resist wind scour through the first exposed summer.

• Root-resistant waterproofing membrane, flood tested
• Protection fabric and a drainage or retention mat
• Filter fabric to keep fines out of the drain layer
• Lightweight engineered growing media, about 3 to 6 inches deep
• Sedum mats, plugs or a mixed native planting
• Washed stone border at parapets, outlets and the hatch where nothing grows

DOEE's RiverSmart Rooftops program has offered rebates for green roofs on District properties as part of its stormwater work. Amounts, eligibility and paperwork have changed over the years, so check current terms with DOEE before you plan around a number. Pre-approval before installation has typically been required, and we set our start date around that step.

We provide the drawings, layer specs and photos that rebate applications usually ask for. What you receive and when is between you and the program, so we never fold an expected rebate into our price.

The city also bills properties for stormwater through a charge tied to hard, paved and roofed area. Green roofs reduce runoff, and DOEE has offered discounts connected to that; ask about current terms there too.

Parapets, scuppers and the roof hatch get a border of washed river rock where no plants grow, usually 12 to 18 inches wide. That border keeps roots away from flashing and lets you inspect the most leak-prone spots without disturbing anything.

Drains and scuppers get inspection chambers, so leaves and sediment can be cleared. On rowhouses draining to a rear scupper, we size the scupper and downspout for the slower but longer outflow a green roof produces.

Where the green roof meets a neighbor's roof at a party wall with no parapet, we keep a wider gravel strip and raise the membrane at the joint. Our piece on party-wall leaks explains why that seam deserves extra care.

Sedum roofs are low-maintenance, not no-maintenance. Expect to water during the first dry summer while plants root, pull tree seedlings a couple of times a year, and clear the drain chambers each fall.

We offer a spring and fall visit to weed, fill bare spots and check the flashing. For many owners the bigger change is the view from an upstairs window: a green surface where hot tar used to be.

Most residential green roofs in the District are extensive systems: shallow media, low plants and little upkeep. Semi-intensive roofs run deeper, often six to ten inches, and can carry grasses, small perennials and a wider mix of native plants. They hold more stormwater but weigh considerably more and need irrigation in dry summers.

Intensive roof gardens with trees and deep beds are a structural project rather than a roofing add-on, and they rarely fit on a rowhouse. If you want something between a sedum mat and a garden, we can combine an extensive area with a few raised planters set on pads near the parapet, where the framing is strongest.