The phrase reduce, reuse, recycle is purposely listed in order of sustainability. It is always better to reduce our consumption, then reuse products, and only then to recycle as much as we can.
My last two columns have been about the timbers and posts that hold up our roof. As our construction progresses, we can now talk about that living, green roof.
A green roof is one of the primary reasons to build an earth-sheltered house because this kind of roof is an insulator from winter’s cold and summer’s heat. The rest of the time, it can be a garden, a patio or a soccer field.
Unfortunately, a green roof isn’t just a case of piling dirt on your shingles. There are many considerations, including insulation, waterproofing, drainage, soil depth, slope and enough moisture retention to support the plants you want to grow. A green roof is a many-layered thing.
First, we needed an interior ceiling above the beams. We didn’t want a standard drywall and paint ceiling and were lucky enough to source the 4.5-cm (1 3/4-inch) pine tongue and grove decking that was the original floor and ceiling of the Ottawa airplane hangar that also provided our beams. Not only would this give us a wooden interior ceiling, but the beams would be reunited with their long-time companion, the decking.
Each plank had to be sanded and, in some cases, planed, but we decided to spend money on local labour rather than materials that have been shipped a long distance.
It is turning out that working with young, local labour (most of the very accomplished crew is under 30 years of age) has been a highly rewarding part of this project.
Next, we needed a strong, flat surface on top of our interior decking to form the base of the exterior roof. We considered many materials, including concrete, but finally settled on structurally integrated panels (SIPs) that are non-CFC polyurethane foam sandwiched between sheet steel. Ours were four feet wide, and varied from eight to 20 feet in length.
The rigid insulation core of the SIP performs as insulation, vapour barrier and air barrier. A well-built home using SIPs will have a tighter building envelope and higher insulating properties to reduce drafts and operating costs. Our six-inch thick SIPs were rated as R-45. Also, due to the standardized and all-in-one nature of SIPs, construction time can be reduced over building a frame home and requires fewer trades for system integration. The panels can be used as floor, wall or roof.
Once we decided on SIPs, the next stage was finding them. There are SIP distributors in major centres, just like other building supplies, but we found a Toronto supplier who had panels with cosmetic defects that were originally for the walls of big-box stores. We were able to buy enough in various colours for the house for $7,500, about half price and a significant savings.
The beauty of SIPs is that they provide a smooth, flat surface for the impermeable membrane that seals the roof from the soil on top of it. As you can imagine, the main villain in earth-sheltered construction is moisture, so a membrane that keeps moisture out is essential. Again, we researched many different materials but decided to go with proven, tested material used in flat roof applications called EPDM (ethylene propylene diene monomer synthetic rubber). It is also used for water resistance in high-voltage cable, as a pond liner, in RV roofs and chainmail applications. I guess modern knights want to be resistant to moisture, as well as battle-axes.
The main properties of EPDM are its outstanding heat, ozone and weather resistance. We had an EPDM roof on our underground generator house at the farm that was also a patio of sorts on the ground above. It was exposed to the sun and the elements for ten years and, other than a bad corner where I chewed it with the lawn mower, it looks the same as the day we put it on.
EPDM does not pollute runoff rainwater, which is of vital importance to us because we will collect this water in a rain barrel and use it to water plants. Many houses equipped with rainwater harvesting use EPDM roofing.
A second option would have been to spray a rubber coating on the roof. It sounds intriguing because it eliminates worry about sealing any seams, but we didn’t find out about this method until after our roof was completed. I was giving my Queen’s sustainability class a tour, and one of the students mentioned spray-on rubber roofing, but by then it was too late. We installed a bed of half-inch cellulose roofboard on top of the SIPs to eliminate any possibility of punctures, then we were ready for the membrane.
The 60-gauge (1.50-mm thick) rolls of EPDM that we used weighed more than a ton, so they had to be lifted to the roof by crane. It took a day to lay out the membrane and seal the middle seam, then reinforce the corners. Once the glue was dry, we dammed the edges and flooded the entire surface with 13,000 litres of water weighing 13 metric tonnes from the garden hose – a good test of the roof for both moisture and strength.
It passed, so we proceeded to the several green roof layers above the membrane, and I’ll give you the dirt on that next time.