Thursday, October 29, 2009

Roofing and Soil Quality

Lately we have been pondering the potential impacts, real or imagined, of various roofing materials and potential impacts to garden soil quality.

Because about 80% of our watering supply is from roof-fed rainbarrels into a very intensive small-sized vegetable plot, the roofing material we choose to replace our battered asphalt shingles could theoretically have some long-term impact on our garden soil quality. However, there does not seem to be much readily available data on the topic (or at least not that I've yet found).

I have started to compile some info but thought I'd throw this post out in case some passer-by could recommend some info related to this topic. We are leaning toward a metallic roofing material, for several reasons. Metal roofing generally seems to be the preference for rain water collecting, provided it does not use large amounts of lead in the coating.

More to follow on this subject.


The Blog Fodder said...

I doubt the grit and gravel off the asphalt shingles will do your garden any harm. Find out what glue is used to hold it onto the shingle. Might give you some clue as to potential toxicity problems.

Not sure what metal roofing sounds like in a rain storm. It is increasingly popular here as a replacement for the old grey asbestos slabs that have been used since Soviet times.

Middle Earth Garden said...

It is not the grit and gravel that is in question, but the hydrocarbons that leach out of an asphalt roof over time. Certain hydrocarbons have high levels of toxicity and are very persistent in the environment.

Some of my coworkers have spent a lot of time at work dealing with removal of hydrocarbons from soil at contaminated sites (not surprising, there is a lot of hydrocarbons in Alberta soil). Many of the sites are not even agricultural land. It feels like wasted time when the contamination could have easily been prevented.

Makes me question why we would willingly put a hydrocarbon saturated asphalt roof on our home that will leach hydrocarbons (known toxins) into our vegetable garden, and then eat veggies which inevitably will have some soil attached to them, albeit in very small amounts. Are the amounts of toxins too small to have much affect? Possibly. But in light of uncertainty, if possible health hazards can be avoided (within reason), it seems prudent to avoid.

Just one factor of many as we weigh pros and cons for the various choices available.

Dave said...

Although in some places, orangy clay tile is used, in most of Australia, galvanized steel or steel coated with other materials (eg zinc-aluminium resins)is the typical roofing material. Steel roofs are very loud in a rainstorm and very creaky as they heat up in the sun. But you get used to it. I miss the thudding of raindrops in this bitumen shingle land.

Australia is also a place where rainwater is extensively used both for human consumption and for gardens. Steele roofs, rainwater collection, and steel tanks go together well.

I can't remember ever seeing the typical North American bitumen-gravel roofs in Australia. Can't say I especially like them and the gravel in the gutters and rainbarrels is annoying. I don't think that bitumen is water soluable, though, so I'm not sure why there would be any worry about using water that runs off the roof. I'd be more worried about any additional binding agents that might be added.

Middle Earth Garden said...

Dave: Asphalt shingles are likely popular in Canada mostly due to low cost since we are so close to petroleum extraction and refineries. Asphalt shingles contain about 20-40% asphalt by weight according to:

A straight-seam metal roof (with hidden screws) for our house is almost double the cost of premium 50-year asphalt shingles (materials & installation). Fancier metal roofs are even more.

The asphalt in the shingles most certainly leaches out over time, as described below by a major asphalt shingle manufacturer we are considering:

"The life expectancy of asphalt shingles is based on the performance of three components, and their ability to resist weathering. Made from petroleum, asphalt contains oils that make asphalt shingles easy to work with and effective at protecting your roof and home. As time goes on, these oils come to the surface and are weathered away by the elements. It’s this weathering process that ages a shingle."

The question is not "does the asphalt leach out" but rather "is there any potential long-term negative health effects to asphalt leaching into the garden soil via rainwater collection". Probably no cause for concern, but would be interesting to know if someone conducted some studies on the topic. It's always nice to make a truly informed decision, though rarely possible.

Dave said...

Hi Middle Earth Garden:

I did an electronic literature search for research on any environmental effects of asphalt shingles, but came up with near zero. But Water Research Volume 35, Issue 17, December 2001pp. 4200-4207 has a review by Brandt & de Groot (Aqueous leaching of polycyclic aromatic hydrocarbons from bitumen and asphalt) that mentions roofs. To quote them, "Few papers about aqueous leaching of PAHs from bitumen into water exist". In Brandt & de Groot's tests they found "leach water from bitumens [9 commercial bitumens and one asphalt made from bitumen] stay well below the surface water limits that exist in several EEC-countries and are also more than an order of magnitude lower than the current EEC limits for potable water".

So maybe no concern, although not much reseach. There are some recent papers on PAHs (polycyclic aromatic hydrocarbons) in fumes from hot asphalt and their danger to road workers and asphalt plant workers in Eastern Europe. So, maybe a bigger worry is breathing in your garden on a hot day.

Middle Earth Garden said...

Dave: Thanks for the info. In my trolling of the internet, I came to a similar conclusion. It is likely something that should be studied more in the future, but for now based on the limited data available it seems the health risk is very low.

In the end we have decided to go with asphalt shingles, as the very low cost compared to alternatives is hard to beat. I had hoped that metal would work out to a lower life-cycle cost, but even though it is generally considered to outlast asphalt, the higher cost and other potential maintenance needs does not seem to justify the higher cost. There are other factors in the decision I hope to discuss in a future posting, beyond what I can squish into this comment box!

Ann said...

I don't really think that the water stays on the roofing long enough to collect any residue hydrocarbons from your asphalt shingles. Rain usually means cooler temperatures and that would also mean less hydrocarbon leaching. If you are really concerned, test the water in your rainbarrels.

Middle Earth Garden said...

Testing the water is a good idea Ann. The hydrocarbons definitely leach out over time, this is what causes the shingles to shrivel and curl. I have considered testing the water and soil, more for curiosity sake than concern. However, I'm fairly certain the amounts of hydrocarbons in the water or soil would be below the detection limits. The soil would be a better test for long-term cumulative effects.

My coworker said they have cleaned up sites where the hydrocarbons give off a strong odour but are still below environmental standards for acceptable levels in soil (just general soil, not necessarily agricultural soil). So that does say something, either the toxicity of hydrocarbons is relatively low or the standards are somewhat lenient (or both). I'm not losing sleep over it, it is more curiousity than anything else. Thanks for contributing to the discussion.

Ann said...

Just ran across this link regarding the run-off from the roof.

Middle Earth Garden said...


Thanks for that link. Looks like a good overall summary of the topic that confirms the conclusions of the various sources I researched on the net.

Anonymous said...

I have just conducted leachate tests on asphalt shingles in Sydney using cut tiles. The roof has just been installed. All the PAH tests were less than 0.001 mg/L however the formaldehyde varied from 0.2mg/L for the direct water from the roof and 0.5mg/L for the ambient leachate (both within the limit here in Australia). We also conducted a leachate at 50C and found no increase in PAHS but the formaldehyde increased to 1.4mg/L. The advice I have been given was leave the roof a little longer before harvesting as the formaldehyde dissipates after a few months. The water is considereed safe to dink and that is what I intend to do. As Ann points out when it rains the temperature is much lower and the tiles are not leaching. I intend to conduct regular tests on the water and look into water purification to increase safety but I wish I had not installed asphalt shingles in the first place to be safe...also looking at an eco-friendly reflective epoxy to reduce heat from the roof and "seal" the tiles but must still be able to drink the water- any ideas? Hope this helps the dialogue. Nick Halkyard

Tim Flynn said...

We're facing a similar situation with a small building going up in our garden space. Our research has been thin but I have to say common sense dictates that as an asphalt roof deteriorates (inevitable) - the elements that have broken down have to go somewhere. It only makes sense that the deterioration would go with the runoff.
If you plan not to change your shingles for twenty years - you need a twenty year test - not a one or two year test. As we're dealing with a small building we're likely going with untreated cedar. Eventually we'll do rain catchment off solar on the main house.