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Green Roof Cools Home Addition in Takoma Park

Note: Alan Abrams designed this project on behalf of Heritage Building & Renovation, Inc.

The decision to employ a green roof strategy on this project located near Washington DC evolved naturally from the objectives put to us by the client.  Those were:

  1. To create “a meaningful place” for the client’s string quartet to practice and perform.
  2. To design and build an addition that would maintain a zero net increase in energy usage to heat, cool, and illuminate
  3. To preserve the lush back yard garden that included many carefully selected native plant species and a mature maple tree close to the house.
interior of green roof addition

A green roof above and a calm, quiet room below–ideal for listening

Designing A “Meaningful” Place

The first step in the design process was to listen.  We borrowed favorite pieces of music from the client’s collection and listened as we put the first lines on paper.

The listening phase produced a plan for a room addition that is offset from the main axis of the house at precisely 33.69 degrees.  That angle relates to the rectangular shape of the room whose sides have a ratio of 3:2.  That numeric embodies the musical concept of the circle of fifths.  In this way we achieved our client’s first objective: to design a “meaningful place” to play music.  Musicality is built into it.

Toward A Net Zero Energy Addition

There is also a practical reason for the unusual angle of the addition.  It allowed us to introduce passive solar design into the project.  The angle of the addition presents a large wall of south facing windows. During winter months, sunlight streams through the windows and falls upon the travertine floor where it is stored.  The floor radiates that stored warmth throughout the evening.  Roof overhangs are carefully calibrated to shade out the sun during the summer months but not to interfere with the sun’s rays during the cold of winter.

The passive solar design is a key component of the sustainable design strategy that helped us fulfill the client’s second objective:  to design and build a net zero energy addition.  Also, it demonstrates the aesthetic potential and beauty of energy efficient design.

Besides providing a no cost heat source for the room, sunlight unlocks the beauty of the interior finish materials.  Wood in the exposed beams, the travertine in the floor, and exposed brick from the original structure of the house all come to life in the changes in temperature and angle of light throughout the day.  As demonstrated, the pursuit of practicality and energy efficiency in green design does not preclude beauty and musicality in the architecture.

tall windows light interior of green roof addtion

Tall south facing windows light the interior and capture radiant heat from the winter sun

Green Roof—The Finishing Touch

The addition is covered with a vegetated roof, lovingly constructed by the owners.  This green roof also honors the part of their lush yard that was sacrificed to make way for the structure.  In a sense, we simply relocated a portion of the yard.  We moved it up on top of the house.

The living roof is a green building strategy that has many inter-related benefits.  A green roof provides practical and aesthetic contributions to quality of life in the home.  It also has positive environmental effects beyond the property where this house is sited.

green roof viewed from above

View of our living roof from the bedroom window

How the Green Roof Works

The working parts are few and simple, as follows:

  • Roof membrane
  • Root barrier
  • Water retention layer
  • Four inches of soil
  • Plants
  • Drainage system for over flow

Soil composition is 60% coarse lightweight mineral content so sediments won’t slip through the filter layer.  The plantings are primarily succulents.  The plant species and varieties are selected for continuous flowering throughout the season.  As one variety finishes flowering, another begins. Water that isn’t absorbed flows through gutters that empty into sediment ponds (rain gardens) in the yard where it naturally percolates into the ground.

green roof and deep overhangs help keep interior of addition cool in summer

The living roof and shade from the deep overhangs keep the interior cool in summer

Benefits of A Green Roof

  1. Energy Efficiency.  A green roof reduces heat gain because plants do not absorb heat from the sun, thereby helping keep the indoor space cooler in the summer.
  2. Reduced Environmental Impact.  The photosynthesis process of plants deflects infra-red rays—a normal roofing material (metal, concrete fiber, asphalt) will absorb heat and radiate it back into the air resulting the “urban heat island” effect in densely populated areas
  3. Durability.  A living roof protects the roof membrane from thermal shock.  Thermal shock occurs when the roofing material expands unevenly in response to temperature extremes and UV radiation that cause strain that can result in cracks and deterioration of the roof.
  4. Storm Water Management.  The action of plants drawing water off the rooftop helps protect the watershed.  Living roof material absorbs and retains water during rainstorms.  This in turn slows the speed of run-off, allowing rainwater to percolate naturally into the ground.  It would otherwise race along the surface into storm drains gaining destructive velocity and massive volume that scours out creeks and degrades rivers.
  5. Green Space Preservation.  This up in the air garden provides enchanting views from the bedroom windows while providing habitat for birds and insects
close up of a green roof

The portion of yard taken by the foundation now resides on the roof

Sustainable Design Imitates Nature

Like many core principles of sustainable design and green building, a green roof system simply mimics what happens in nature.  Green roofs can be constructed as a whole component or as modular units in 2×3 planting troughs.  The obvious advantage to a modular approach is that plantings may be removed without being destroyed should there be a need to repair the roof.  Since our roof area was relatively small, we took whole roof planting approach.