With the second level walls framed, plumbed and braced it is time to frame the roof or actually roofs. On top of the two different height walls are two separate roofs, framed with different materials. The lower roof will be the planted roof, built up with multiple layers of waterproofing and drainage material, topped with 12″ of soil. The upper roof is the roof that supports the dormer that covers the roof access stairs, with the remaining area, about 8′ x 14′ serving as a patio. The different uses and associated loads of each roof dictated the size of the framing material needed. In the case of the planted roof, covered in saturated soil, the loads required a 16″ I joist, for the other are, just serving as a patio, only a 12″ I joist was needed. The deeper the joist, the more insulation that can be packed in, which is a good thing, and I would have prefered to use 16″ joists on both roofs, but that additional 4″ would have put the rear section of the house beyond the maximum allowable building height. So we have to be satisfied with an R value of about 50 including the foam tapers on the top of the deck for that roof, where the planted roof will be more like R70.
More Cantilevers
Since the bay windows project from the building without bearing directly on the foundation, their weight being carried by cantilevered flitch plates at their base, they are not able to carry the loads from the planted roof. So to support the load from the portion of the roof that sits above the bay window we employed double 16″ LVL’s cantilevered over the bay. The double LVL’s served as the rim along the entire front and southeast side of the roof.
The beams join at a corner of the building that is suggested by the rest of the walls, but not realized until the roof comes together over the bay window. This creates an interesting play of triangles: the two corners of the window projecting from the main building and the roof projecting over the center of the window like a ship’s prow. This will become clearer when the second outer wall increases all three projecting points and even more so when the sheathing solidifies the mass of the building.
We also needed a set of double LVL’s across the transition between the lower roof and the upper roof. Because of the orientation of the dormer for the roof stairs, the joists for the upper roof had to run back to front. This necessitated a beam across the building at the roof transition. The lower roof joists don’t bear on this beam, they run side to side with both ends landing on a wall, with the exception of the joists over the bay that hang from the previously mentioned cantilevered beam.
Planted Roofs
It is easy to write about the process of building the roofs, placing framing members and revealing the puzzle pieces that will go together to make the whole. In the end this is what building a house is, a series of steps and layers that culminate in a place where people live. It is also easy to get lost in those steps, to substitute process for purpose. In our case I think it is worthwhile to discuss the purpose for the planted roof.
Not everyone loves green roofs, the value that they add to a sustainable design is debated. Engineers tend to point out that the benefits they provide are available through other means. Their contribution to reducing urban heat islands can be accomplished with high reflectivity roof materials. Their mitigation of storm water run off can be handled through infiltration systems on the ground. Because dirt is not a very good insulator, planted roofs don’t really add to the energy efficiency of buildings, except that the plants on the roof, supplied with sufficient water, will provide evaporative cooling through transpiration. Better and cheaper to increase the insulation on the roof to improve the efficiency of the building, and insulation will perform during both the cooling and heating seasons. And using potable water to irrigate plants on the ground is wasteful enough without adding irrigation to the roof.
I agree with all that I just wrote, and there are good studies to back it up, but there are a couple of reasons, general to building and specific to this project, why we decided a planted roof was purposeful and included it in this house.
Water
It is hard to assess how much of building is directed toward managing water, water from the sky, water from the ground, water in the air. Maybe it is just in the Southeast, but I don’t think so and here we are mostly spared the problem of frozen water, thank God. Anyway it is a lot. The conventional wisdom and vernacular architecture in the South embraces steeply pitched roofs to shed rain water quickly off the house. I have found, though, over many years of repairing houses it seems more damage is done by the rain water after it leaves the roof than from the time it spends on the roof. Overflowing gutters, splash from the eaves, flooding foundations, all lead to problems for houses. When I built my first flat-roofed house I never noticed the water that left the roof, it was collected behind a parapet, drained into a downspout, then through underground pipes and finally flowed onto the ground forty feet from the house. The roofing material was a heat sealed membrane that was user-friendly and durable. The one drawback was that the leaves from nearby trees tended to collect at the screens for the roof drains, but at least when you have to clean them you are standing on a (nearly) level surface. So the only thing better: a flat roof where the leaves don’t matter. A filter of soil and the other layers of the planted roof negate the ability of accumulated leaves to clog the outflow for the roof. As opposed to a traditional roof that encourages the rain water to flow off the roof as quickly and directly as possible, dragging with it any debris that has landed on the roof, the planted roof slows the rain, absorbs and retains some of it, and filters it through a broad area before releasing it slowly to the roofing to convey it away from the building. It may seem counter-intuitive, but part of my no maintenance scheme included the planted roof. I was going to garden somewhere, why not accomplish roof maintenance at the same time I was planting or weeding.
The Garden
My musings on water management made the idea attractive, but the reason for the planted roof is the desire for garden space on a piece of land so small its hard to escape our own shadow. We have always had gardens, frequently unruly, sometimes very productive, sometimes less so, a garden is part of who we are and I have nearly arrived at the place that I believe if land isn’t going to be left to its own and return to what here would be a subtropical deciduous jungle, it ought to grow food. The house dominates the lot, add a parking space, walkways, space for our aging dog, consider the north side lost in the shade of the house, bamboo and a struggling pecan tree on the adjacent property shade some of the narrow strip to the south; even so we will plant the front hill with blueberries and strawberries, we will put some blackberries on the fence line. But the roof offered an unobstructed view of the sun, and though small, plenty of space for a generous kitchen garden. The roof is where we will have our cooking herbs and spring lettuce. Every garden we have ever had has been an evolutionary project and this will be as well. I see some years with sweet potato vines cascading down the walls, while beans grow on a trellis above them, maybe our virgin roof top soil will even produce a zucchini before the squash borers kill the vine.
Watering the Roof
One drawback to planted roofs is that they almost certainly need irrigation. Maybe not always and not as much with the carpet of sedums that often make up green roofs, but with plants like tomatoes that are thirsty in the ground, watering will be a necessity on the roof. I didn’t want to take an idea that is borderline sustainable and throw it clearly on the wrong side by watering our roof with potable water. To avoid this we needed to get our irrigation water from somewhere other than the tap. My first choice is stored rainwater, and we have some roof area that will not be planted that can be collected during rain storms and used during the hottest months. Several hundred gallons of stored water can last a few weeks without rain as long as the irrigation is efficient and there is plenty of mulch. We will have a 300 gallon storage tank under the porch with a pump to lift the water to the roof, and a subsurface irrigation system to put the water where it is most needed. The water keeps the plants alive and makes the garden viable, but during the hottest months it also provides the one energy benefit of a planted roof, evaporative cooling.
We did get the roof framing done and moved on to building the outside wall, things are wet now, but next week looks clear. A good week’s work and we’ll be close to the end of the framing
small house small lot 