Living Roofs for the Pacific Northwest

Living roofs for buildings have been a goal of mine since my first sod-roofed structure in Minnesota more than 25 years ago. The idea of "giving back" to our ecological community, to create oxygen, slow storm water runoff, and lessen the ecological impact of our buildings adds a dimension of rightness to our structures that are sorely lacking today. Cost, weight, and technical problems have been major impediments to the development and application of living roofs. A recent survey exploring available technologies for a living roof for the Bank of Astoria in Manzanita, Oregon showed that considerable progress has been made, but most alternatives remain costly, heavy, and technologically complex.

It dawned on me suddenly in my explorations that a relatively unexplored alternative ­ moss roofs ­ might hold immense potential for the Maritime Pacific Northwest. We spend millions of dollars a year preventing moss growth on roofs in this region, so it can't be too hard to grow in those conditions! Is it really a hazard to the integrity of a roof, or merely an aesthetic aversion to the patina of time and life? Could we take the most common, least expensive roof construction in the region ­ composition shingles ­ and turn them into a beautiful moss-shielded living roof?

I contacted Malarkey Roofing in Portland, a major regional roofing manufacturer. My initial query brought an incredulous silence, then, "I'll have to get back to you on this." After talking with the field warranty manager, who examines all the failing roofs, and the firm's technical director, the tech rep gave me an amazed reply. "We can't really see any problems. Moss roots go horizontally, not deep into the shingles, and we can find no record of real damage! Particularly if you use laminated double-thickness shingles, it probably would work!"

Some mosses do have burrowing roots, but not any common to NW. Their roots are shallow, not even growing into the fiberglass mats of the shingles. Many shingle manufacturers produce two different lines of shingles. Malarkey's "Hurricane" line, for example, contains an algae block, which would harm moss growth. Their Alaskan line does NOT have algae block pellets on it, and would probably work okay. Their "Legacy 35" architectural laminated shingle is available with or without algae block. As suggested, it might be a good choice because of its greater thickness of laminations. The SBS (in Alaskan shingles) is softer than standard asphalt, and may be easier for root penetration. There might be a need to "weed" out tree seedlings, etc. that might start in the moss and burrow deeper.

So I started digging deeper regarding mosses. George Schenk's Moss Gardening, Timber Press '97, indicated that mosses grow better on rough rocks rather than smooth; need acidic rather than basic conditions ­ pH5.5 ideal; and should be kept away from any calcium, which inhibits or kills the mosses (changes pH?).

The famous Saihoji Garden in Kyoto has Leucobryum (jade colors) and Polytrichum (dark green) as most predominant mosses - both of which are abundant natives here, the latter being most widespread. Polytrichum (hairy cap mosses) spore in spring and early summer. They are easily transplanted; easily kept healthy; enjoy sun, half shade, or four-fifths shade; and any acidic soil ­ but when transplanted from fertile place to poorer ground can have problems. The Bloedel Reserve, on Bainbridge Island, started with Irish moss, which gave way to Eurhynichium praelongum in dryish ground and Brachythecium frigidum in damp ground. Later both were elbowed out by Rhytidiadelphus loreus (soft yellow-green)which took over most areas. The greenish-gray Leucobryum glaucum carpets the Savill Garden in Great Britain.

Another planting was done in the region using rock-based moss in sun, using Polytrichum piliferum, Dicranum, and Racomitrium hetrostichum. Polytrichum became dominant, on a site hot and dry in the summer and early autumn. Schenk indicates that sunny ground that dries in summer will attract Polytrichum, Racomitrium, and Homalothecium. Soil that is sunny and perpetually damp attracts Calliergonella or Aulacomnium. Shady and damp may attract Brachythecium, or a variety of others. Someone warned us that tree-based and rock-based mosses are frequently different and won't transfer to the alternate medium. Schenk indicates that some grow well both places.

Schenk claims mosses and lichen do not cause roof leaks, and removing them will not prevent or delay the eventual leaking of a roof retained past its years of soundness. Edge heaving of asphalt shingles may be caused by moss, but is highly unusual even over decades. The photo of a moss roof with this article is from Schenk's book, showing Racomitrium ­ a rich golden green moss and seeker of sunny mineral surfaces ­ in full cover on asphalt shingle roof for forty years in Seattle area with no leaks. Racomitrium is a rock dweller, and performs best in open, breezy location, on sloping ground, or on rocks. Response to irrigation is uncertain...safer not to water in hot dry weather.

All this brought me to the next step ­ figuring out how to grow it ­ and to grow it in a way which did not take an extended unsightly period before coverage was achieved. Water and nutrients were an easy issue ­ a soaker hose set on the ridge with small sandbags, and a mini-pump from a tank collecting and recycling greywater from the building. Actual propagation has proven more of a problem. Several people said propagation through crumbled fragments is okay for those that do it in the wild... Leucobryum, Racomitrium, Dicranoweisia. I could find nobody who seems to "grow" moss commercially - many greenhouses just clean it off from around other plants and stuff it in pots! I got lots of confident recommendations from academia ­ that work in tempered greenhouse conditions but not in the field, and many home stories of "smoothies" made from moss and fish meal or sour milk, etc. My success in the field with test samples has been non-existent.

Suggestions have included possible dusting of powdered sulfur over soil to increase acidity - about 2.5 lbs. over 100 sq.ft. is said to help encourage arrival of moss. Or dust with skimmed milk powder, aluminum sulfate, rhody fertilizer; or using rock powder, but not calcium-based (granite is acidic, etc.) Fertilizer applied to mosses beefs them up like steroid athletes. Liquid, not dry fertilizer is recommended ­ any composed primarily of nitrogen, phosphorus and potassium can be sprayed or sprinkled on. Trace elements are okay, but, again, no lime. Liquid fertilizer on about 3-week intervals from spring to autumn is suggested ­ 5-10-10 or 5-15-10 or other in moderately potent dilution.

Watering as frequently as wanted is said to be okay, except for Racomitrium, which prefers to sleep dry in droughty weather. It never needs watering, not even when newly transplanted. Watering, especially in dry weather, can actually cause fungus infection. The use of water-absorbent polymer crystals has been suggested (and tried in my field tests). They come in three grades ­ large, medium, and small. Medium seems to work, the manufacturer says the gel remains effective up to 5 years, after which it biodegrades.

The missing element in implementing living moss roofs in the Pacific Northwest at this time is a process to initiate timely and uniform moss growth on new roofs. The "ideal" process might be a spray that becomes a relatively clear adhesive film on the roof (sort of like Shoe Goo?), holding gel, nutrients, spores and/or chopped moss on the roof without washing off, until the moss can become established. Covering the roof area temporarily with a raised tent of polyethylene to raise humidity and temp (moderately) might assist. There might be a need to avoid or minimize the ceramic color coatings given to many granules used on comp shingles.

Discussion on this item at the Northwest EcoBuilding Guild Retreat brought forth suggestion of using raw linseed oil sprayed on the roof, which sounds like an excellent avenue to explore. Dries, but sticky if thick enough, acts naturally as a nutrient and is normally not used outside in our climate because of its ability to attract fungal growth, etc. This has the potential also of being sprayed on the shingles (with moss/spores) by the manufacturer and sold directly (in waterproof wrapping!) all prepped. Also could be retro-sprayed on all the existing comp. shingle roofs west of the Cascades! Comp. shingles now have warranty lives of up to 40 years, are recyclable, and would likely have use-life extension through use of the moss overlay.

So, anyone with ideas, information, experience on how to get moss to grow on roofs will be embraced in somewhat shaggy and wet, moss-covered arms! Let me know any leads you might have.

Tom Bender
© 6 Dec. '00