Using lime on an asphalt shingle roof, affects on pH

Somewhere else on these forums, there is a suggestion of using a thin layer of lime on an asphalt shingle roof. This keeps asphalt gunk out of harvested water, cools the house, and is cheap.

My problem is: Rainwater is generally acid. I am in Colorado, and don't want to make my soil more alkaline. Would the lime just bring the rainwater pH to 7? After all, it will only be on the roof a minute or so. That would not be a problem. But I don't want alkaline water in my garden.

Will I get this result? If I do, is there any way to lower the pH once I collect the water?

Hi Gilbert:

It's my understanding from taking Geoff Lawton's PDC that truly "acidic" rain is localized to certain areas due to several factors including localized pollution and where the water is coming from. I believe I remember him saying that the most acidic rain ever measured was in Vermont.

I did find this website that explains some of this in more detail http://acmg.seas.harvard.edu/people/faculty/djj/book/bookchap13.html

13.3 EFFECTS OF ACID RAIN

Acid rain falling over most of the world has little environmental effect on the biosphere because it is rapidly neutralized after it falls. In particular, acid rain falling over the oceans is rapidly neutralized by the large supply of CO32- ions (chapter 6). Acid rain falling over regions with alkaline soils or rocks is quickly neutralized by reactions such as (R9) taking place once the acid has deposited to the surface. Only in continental areas with little acid-neutralizing capacity is the biosphere sensitive to acid rain. Over North America these areas include New England, eastern Canada, and mountainous regions, which have granitic bedrock and thin soils ( Figure 13-2 ).

As a person living in an arid region like you with alkaline soils, I think adding lime would exacerbate more than it would help. I guess the big question is what are you using the water from the asphalt shingle roof for? If for watering plants, no big deal - rainwater falls on roofs and flows into the soils all the time (and off asphalt roads, etc, etc). Healthy soils with abundant organic material will help to bind up toxins from the roof.

Now if you want potable water from an asphalt roof, I know of people who have painted a product called "elastomeric" onto their roofs - you can find food grade elastomeric. Here is a wonderful article Brad Lancaster, author of "Rainwater Harvesting for Drylands and Beyond Vol 1-2" (really - all westerners should own these books) pulled together about different types of roofs and the quality of rainwater, etc. http://www.harvestingrainwater.com/rainwater-harvesting-inforesources/suppliers/roof-catchment-and-cistern-system-materials-and-supplier/.

Another book that would probably be most interesting to you is "Toolbox for Sustainable City Living: a Do-It-Ourselves Guide" by Scott Kellog and Stacey Pettigrew. (the link is to the entire book online). I had the pleasure of hosting these folks when they came through Phoenix on their book tour - their work is IMPRESSIVE. Essentially they rehabilitated a brownfield into a productive landscape in Austin. There's a ton of stuff on bio-remediation in the book.

Sounds like a suggestion from someone with lime to sell. Lime is soluble in water. Like "most of it will wash off with the first good rain" soluble. When you dissolve lime in water, you get what chemists call "lime water", a useful reagent for neutralizing acidic gases like CO2 and SO2. Depending on how much lime you dump into the water, the pH can get quite high, up above pH=12.

You're right, you don't want water that alkaline in your garden. But the effect is transient -- being soluble, the lime will eventually wash away, and there are plenty of football fields with green grass growing very well even though they have lines of powdered lime on top of them.

Hey John! Thanks for swinging by and adding your thoughts on this thread - I love it when you "talk chemistry".

"Like "most of it will wash off with the first good rain" soluble"
I think that is correct with crushed barn lime however whitewash takes in the years to wash off At least as reported at builditsolar from those who have coated their roof with hydrated lime coatings have lasted 2 years without much loss of coating. The issue with elastometric on asphalt shingles is you have to work aginst the uneven surface and go from a breathing water barior to a monolithic non breathing surface and if you fail in one spot you have roof system failures because water gets trapped and rots boards. If I was to do elastometric coating I'd add a thin plywood over the shingles and then paint with elastometric because going right over the asphalt is going to cost a lot when it comes to the thickness one needs to achive to avoid system failure. Also a second raised roof performs very well in hot humid climates and I'm in georgia so yeah if money isn't a thing raiss the roof by 2 inches and some plywood and coat it with elastometric might work pretty well and for a long time but longer if you go with EPDM