If you do any reading about natural building you will undoubtedly hear the phrase “build a good hat and boots”. If you haven’t heard that before, congratulations, you are about to find out one of the most crucial aspects of any building whether using natural materials or not. The “hat and boots” refer to the roof and foundation of a building. These are the most crucial elements of a house that will prevent moisture damage, especially on vulnerable wall systems such as cob and strawbale.
To adequately protect your home’s walls, large roof overhangs--eaves--are used to shed water far away from the structure and prevent water to splashing back onto the sides of a house. Gutter systems and rain chains are still excellent ways to further prevent damage to wall systems and can be made to collect valuable rainwater for gardening.
What kind of roof can you use on a cob or strawbale house? Homes built using natural materials can have as conventional or unconventional of a roof as you desire. Our first home has an organically shaped living roof that drains rainwater to the front of the house. Our second house, a balecob structure, uses a metal roof and is described in detail below.
A key feature in many homes is the attic space above ceilings. This space is important in moving air to prevent condensation from occurring under the roof. Our house is an “unvented roof”, also called a “hot roof”. Rather than relying on air carrying moisture away, an unvented roof uses thick amounts closed cell insulation to prevent temperature differences on the sides of roof's sheathing. As can be seen in the layers of our roof, the roof decking has a thick layer of insulation directly above it to keep both sides of the wood warm and prevent a condensation point. For a more thorough understanding of how this type of system works, visit this link.
Both of our houses are load-bearing structures--the walls support the weight of the roof rather than using a skeletal structure used in most conventional homes. The rafters span the width of the living space and extend a further 2-3 feet beyond the walls.
Rafters can be secured into cob walls using “deadmen”--extra pieces of wood secured to the sides of each rafter creating an anchor into the mass wall. Where wood meets wood, hurricane ties are best for securing rafters to wooden top plates that help distribute roof loads to the entire wall. Top plates are needed for strawbale walls, but not necessary for cob walls that are monolithic structures.
There are many ways to create a layer of sheathing above your rafters. Our first house used readily available slab wood--the wood trimmed off of trees to square them for milling boards--but for the balecob house we chose to use tongue and groove boards that would function to support our roof while also being aesthetically pleasing as a ceiling.
We chose to use a thick layer of polyiso insulation--http://firestonebpco.com/roofing/insulation/. Why polyiso? This closed cell insulation provides the best R-value (insulation value) per inch of material and increases its insulative ability as additional thickness is added. Since our roof is relying on this insulation to prevent dew points from forming, it was especially important to add enough insulation to the roof structure. Polyiso board is also one of the most expensive insulation boards you can purchase, however it is readily available from demolition companies who frequently pull old boards from commercial buildings and resell at a fraction of the price.
The metal panels were purchased from Epic Steel in Missoula and were a quick way to protect our house from the oncoming snow. Having never worked with metal panels before, it was an intimidating process of cutting and screwing down the metal, but we were able to fasten them all down with only a few mistakes, and just in time for it to snow.
We have found that metal and the epdm pond liner used in living roofs cost roughly the same amount. Both need adequate insulation and decking; however, metal roofs are considerably lighter in comparison to living roofs and cost less for the amount of support needed in handling the weight load. We ALWAYS recommend anyone building to consult a span table to determine exactly how much support they need for their roof load--weight. Remember, it is better to over-engineer than to under-engineer and have huge problems a few years down the road.
All in all we are extremely happy with our roof choice and build. While not a particularly clean resource, metal does have an extremely long life and ultimately extends the life of the building it is protecting.