So, our roof sprung a leak.
I realize some might question my sharing this small bit of personal woe, but really, what homeowners don’t have an issue now any then? In fact, I’m told that roofs need to be replaced routinely–it’s an expected hassle, especially for some of the shorter-lived roof types. That means you, too, might be in the market for a new roof, and if you’re not now, (if you’re lucky enough to own a home) you will be someday in the future.
So what kind of a roof do we want?
The thing is that roof-shopping is an opportunity, a chance to make decisions about some small corner of the built environment instead of simply accepting it as a given. What sort of roof do I want? Can I have one that’s different? That’s especially mine? That better suits my tastes and values and generally makes me happy to live beneath?
Can, in other words, a roof be a response to the challenge and crisis of climate change?
Yes, it can.
First, let’s take a look at how roofs are related to climate change. In my reading, I’ve identified three broad categories of relevance: roofs have carbon footprints, so it’s possible to choose a roofing type with a small one; roofs have an impact on the energy use of the house; and some roofs have additional tricks, such as generating renewable electricity.
All of these together, plus such practical matters as cost, become part of the picture for making a final choice.
The Roof’s Footprint
Roofs, like everything else, have a carbon footprint. One way for a homeowner to respond to the climate crisis is to get a roof with a smaller footprint. I was able to find a study that did compare the carbon footprints of various roofing types, at least in Australia, but unfortunately it did not include asphalt shingle, which is the simplest and least-expensive in our area.
The study compared typical residential roofs of sheet metal, clay tile, and concrete tile, including in each case the wooden frame beneath (though not, apparently, insulation). The analysis looked at both greenhouse emissions and embodied energy for each from cradle to grave and found out that the footprint of metal roofing can differ radically depending on whether it is recycled afterwards. The abstract of the paper (reading the full text would require money I don’t have) did not include all of the relevant numbers, so I had to do some math. “CO2e-” means carbon dioxide equivalent, recognizing that there are other greenhouse gasses and their warming potential varies. CO2e- is a way to quantify and express warming potential as a single figure regardless of which greenhouse gasses in what relative quantities are involved.
It’s also not clear from the abstract how much of each roofing material was involved–obviously, larger roofs would have bigger numbers–but the roof in the analysis was the same size in each case. “T” presumably stands for metric tonne, which is somewhat larger than the American ton.
- Clay tile has a carbon footprint of 4.4 t of CO2e-.
- Sheet-metal roofing that is not recycled has a carbon footprint of 9.85 t of CO2e-.
- The carbon footprint of concrete tile wasn’t given, but is intermediate.
- Sheet metal roofing that is recycled “can obtain significant carbon and embodied energy saving benefits (i.e. 71–73%) compared to clay tile or concrete roof covers,” a grammatically ambiguous statement that seems to suggest the following:
- Metal roofs, if recycled, have a carbon footprint of 1.27 t of CO2e-
- Concrete tile has a carbon footprint of 4.7 t CO2e-
- Metal, if later recycled = 1.27
- Clay = 4.4
- Concrete = 4.7
- Metal, if not later recycled, = 9.85
Curiously, concrete tile, not never-recycled metal, has the highest embodied energy.
So, what about other materials? Unfortunately, figures from different analyses aren’t directly comparable, since analyzing carbon footprints requires making a lot of arbitrary decisions about what to include and what not to include–and each person who does such an analysis ends up making those decisions differently. So I can’t combine results from multiple studies. Anyway, I wasn’t able to find life-cycle analyses of other roofing materials.
I did find an American analysis of the carbon footprints of disposing of various construction materials, including asphalt shingles, and the article did comment on the footprint of construction. Curiously, shingles made with a core of fiberglass felt have a lower footprint than those made of more natural-seeming paper felt, since paper absorbs water and must be dried, a process that takes energy. The fiberglass variety are already far more common.
Another surprise was that incinerating the shingles lowers the footprint of disposal because they can replace other fuels for energy generation, including fuels that have more greenhouse gas emission for the same amount of energy–shingles can’t be incinerated in most facilities because of “impurities” (I’m guessing this means air-quality concerns?) but are accepted as fuel for cement kilns in Europe. Asphalt shingles can also be melted down and added to the asphalt mixes used in roadways, which reduces the total amount of asphalt used and thus lowers emissions related to sourcing the material. Asphalt shingles can, in theory, be recycled into new asphalt shingles, but it’s technically difficult and nobody is doing it. The shingles can’t be composted. None of that is very useful for a homeowner, though, especially as I don’t have access to a European cement kiln.
And I wasn’t able to compare the footprint of asphalt shingles to that of metal roofing.
The Roof’s Role in the House’s Footprint
What type of roof a house has can alter the energy use (and thus the carbon footprint) of the house as a whole. For example, a white roof reflects heat and keeps the house cooler, reducing the temptation to use the air conditioner. A black roof absorbs heat and keeps the house warmer, reducing the need for heating in winter. Which one is better depends on the local climate where the house sits and whether the occupants tolerate cold or heat better. There may, in the future, be coatings available that will darken or lighten in response to temperature, but as yet we must pick a color.
Different materials also vary in their ability to conduct heat into the house, so asphalt shingles will warm a house more than a metal roof of the same color–both because asphalt absorbs a lot of heat and because asphalt roofs are designed to transfer as much heat as possible from the shingles to the house beneath, since otherwise the shingles get too hot and are damaged by heat.
A roof with excellent insulating capacity will keep the house temperature from varying as much, an advantage in both hot and cold weather. The R-value (insulating ability) of a whole roof typically depends largely on a layer of insulation, because the roofing surface tends to have a low R-value no matter what it’s made of, but they do vary, so a material with a higher R-value is better, all else being equal. Asphalt shingle is .44 and wooden shingle is .97. I wasn’t able to get a figure for metal roofing, but it is similar to that of asphalt.
Roofs with Benefits
There are roofs that do more than simply cover the top of the house.
Green roofs, that is roofs designed to support living plants, reduce local air pollution, reduce storm-water run-off, provide animal habitat (depending on what’s planted up there), and can even grow food (if the roof is accessible enough to harvest). They also sequester carbon, although the chance of that carbon remaining sequestered very long is slim–most have to be replaced after about 40 years.
Solar roofs incorporate solar panels and generate electricity.
And, while it’s a bit off-topic for us here, flat roofs surfaced in gravel provide nesting habitat for certain birds (nighthawks, for example).
It’s no good just asking what roofing material is “better for the climate” in a vague way. To make a decision, we have to know what kinds of benefits we’re talking about. Now, we do know, so we can get on with exploring specific materials.
Asphalt shingles are made out of sheets of felt (either paper-based or, more commonly, fiberglass) that have been soaked in a thick type of asphalt and sprinkled with coarse sand. They are popular because they are cheap and, in the short term, sturdy (though quality can vary a lot). Unfortunately, their R-value is low, they conduct a lot of heat into the house, and they don’t last very long. Most must be replaced about every 20 years, meaning that however large their carbon footprint is, a sixty-year-old house roofed in asphalt has three such footprints, not one.
Metal roofs are moderate in cost (higher than asphalt but lower than some other options) and relatively long-lasting, on the order of 60 years. Their R-value is low, but they conduct very little heat into the house, especially if given a heat-reflective coating. If the metal will be recycled at the end of their service, their total carbon footprint is quite low. They resist most kinds of damage very well, don’t burn (important, as climate change makes wildfires more frequent!), and don’t support the growth of moss or algae, though they can be dented by hail. Many different styles are available–metal roofs can be made to look like several other materials–but do have one aesthetic disadvantage; rain falling on them is very noisy.
Wooden shingles are carbon neutral, or close to it (processing and transportation surely involve some emissions), can be made with reclaimed wood from other buildings, and unlike almost all other options, they can be composted upon retirement. The price is only a little higher than asphalt shingle. While at first consideration they seem excellent, I have noticed that even buildings with wood shingle siding almost never have wood shingle roofs. I’m not sure I’ve ever seen a wood shingle roof, come to think of it. Why not?
Wood shingle is actually not recommended from an environmental perspective, at least not by people who don’t sell wood shingle. The problem is two-fold. First, while they can last 30 years under good conditions, wood shingles are vulnerable to rot, and many don’t last even as long as asphalt shingles do–that makes them less economical and increases their environmental impact. Perhaps more importantly, they are relatively dark in color, so they absorb heat. I assume they could be painted, but at the cost of much of their aesthetic value. They are also less fire-resistant than most other roofing types.
Green roofs have all the advantages of a garden, plus they’re great at keeping the house cool. Unfortunately, the weight of the soil and water means that not all buildings can support these roofs. Installation is also expensive, though not necessarily more so than higher-end forms of traditional roofing.
Green roof designs are categorized by soil depth (and therefore what kinds of plants can grow on them) and by how much maintenance, including irrigation, they need. The categories are labeled “intensive,” “semi-intensive,” and “extensive.” It is possible to have a roof that is only partially covered by a garden and is otherwise more traditional.
Solar roofs involve tiles that are each little solar panels. I’m not sure what their virtues as roofing are, and they are more expensive that traditional solar panels; their primary advantage is that they don’t look like solar panels. It’s a moot point for my husband and I anyway, as we live in a forest.
Passive solar energy, in which water is heated in the roof, might also count as a “solar roof,” but is again rendered moot for us by trees.
There are other roofing options, such as slate, rubber (made to look like slate), concrete tiles, clay tiles, and good-old-fashioned thatch. I love the idea of thatch, and I’ve heard it performs very well. If gathered locally, its carbon footprint could be virtually zero. However, dry thatch likes to catch on fire, and the chance of our finding a qualified thatcher in Maryland is just about nil. The other options are either expensive or hard to find or both, and also easily damaged by hail or other impacts (perhaps not the best thing as climate change makes extreme weather more likely).
That being said, slate has a very low carbon footprint and is reputed to be environmentally excellent, according to multiple sources, most of which sell slate. Clay and concrete have moderate carbon footprints, as already noted. I have not found figures for the other “others.”
The View From (or of) the Roof
Since it doesn’t look like I can have thatch or a green roof, I’ll be pushing for metal. If we end up priced out of that, white-painted asphalt shingle will do, though paint doesn’t stick to shingle very well. Metal is much easier to make white, and I like the fact that it lasts much longer.
Our area has hot summers and cool, but not cold, winters. We are also prone to wind–in the decade or so I’ve been here, we’ve had to cope with hurricanes, nor’easters, a derecho, a tornado, and frequent blustery days (it’s too windy to bike for several days in any typical week). We therefore need a roof that resists wind, rain, and stuff falling on the roof (such as tree branches) and that can keep our house as cool as possible in the summer. We do not need help keeping the house warm in the winter, especially since we heat with sustainably harvested wood, not with fossil fuel. Given our forested lot, solar shingles don’t make sense, especially since we buy renewably-sourced electricity anyway. And cost is a consideration because are not independently wealthy.
Your considerations, and thus your conclusions, may differ.