Questions about Roofing Options
with replies by Thomas J. Elpel
Questions:
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Is a living roof worth the extra cost?
Dear Tom,
I recently purchased your book "Living Homes" after finding your website several weeks ago. I'm really into it. My wife and I have been, on and off, designing our home for the last couple of years, and the knowledge in your book is a great addition. We now plan to use slipform techniques to make a passive solar house, and eventually live off the grid.
I do have one question. We researched the whole concept of underground homes and living roofs some time ago and determined that the extra cost in reinforcement for the roof would probably be beyond our means when the time came to start slinging cement. Recently I have sort of rekindled my interest in the living roof concept and have read some sources which describe a living roof which is not as great in depth as those described in the books published in the early 80s.
I'm still not sure if its a good idea, however. One source quotes that one inch of insulation equals about 25 inches of soil in terms of R value. If I were to construct a living roof I would probably layer insulation under the dirt anyway, so that isn't really an issue. My main question is, 'Are there any real benefits to such a roof?' I have emailed several sources and read up on it, but it seems that I really just find answers from those who would rather promote this mode of building instead of those with a no nonsense delivery. From reading your book, I gather that you may fall into the "no nonsense" category.
So, in your building travels, have you seen any living roofs that where worth the extra cost? I also am interested in learning if such a roof is really feasible in an area with high snow fall. I live in the shadow of the Tug Hill in Northern New York, where we are purported to have the highest snowfall in the east (while shoveling my driveway, I would tend to agree). Some sources may contend that a living roof's thermal mass may tend to lessen the overall snow load on a roof. But I'm not sure if I buy it. Any thoughts on the matter?
Sincerely,
Adam
Adam,
I think that living roofs are awesome and often aesthetically very pleasing, but I don't yet see much practical or economical justification of them. Here in the arid West a living roof would require frequent watering, at least to get some growth established, and thicker soil depth to keep from drying out so fast. A living roof can also be a fire hazard here in this part of the country, because it would exist as a dry field of grass on top of the house for nine or ten months of the year. The greatest advantage to a living roof out here would be to help make a house visually disappear where it would otherwise be an eyesore on the open plains.
Obviously your situation is much different in the Northeast. There is enough moisture to keep a living roof green throughout the growing season, and you don't need a lot of soil to hold the water. I understand that there are thin living roof mats that can be purchased and rolled out into place. (See www.roofmeadow.com for more information.) All you need is water to make them grow. In a high rainfall area it may even be advantageous to use a living roof to utilize more of the rain water, rather than having it flood off the house, when there is already more than enough on the ground. But economically, a living roof is certainly no less expensive than a conventional roof and probably would cost more.
As far as insulation goes, there would be a slight gain in R-value from the dirt and organic matter, but probably not enough to justify the extra expense. However, the added insulation value of snow should be considered. Most houses, including mine, have fiberglass or cellulose insulation in the roof, with air circulating over the insulation to dry it out. Snow would have a tremendous insulative value on top of an already-insulated building, but the value is lost because of the air gap. In other words, you could have a free, extra layer of insulation on the house every winter when you need it most, just by building a roof with some type of rigid-board insulation that needs no venting. This is the same type of roof system that would usually be used as a base for a living roof anyway.
On the other hand, I would definitely be concerned about the snow load in your area, and either build your house strong enough to support the heaviest possible snow load in, or build with a steep roof pitch and slick metal roofing to get the weight off the house as quickly as possible, directing it away from the driveway and sidewalks, of course.
As for the advantage of using the thermal mass of a living roof to lessen the snowload, there is none. Thermal mass is the ability to absorb heat from the air when the air is warm and to release it when the air is cooler. It would have no impact on the snow load.
I hope this helps!
Sincerely,
Thomas J. Elpel
DirtCheapBuilder.com
