In the cliff dwellings of the Southwest, there are intact mud plasters that have lasted well over 700 years, mostly without maintenance! Earth plasters can be extremely durable.
There are different approaches to the durability question. Some say if you want plasters to last longer, make them thicker. It's true, that works. In the Southwest, vertical earth plasters on exterior surfaces erode an average of 1 inch every 20 years (per Adobe & Rammed Earth Building Design and Construction by Paul Graham McHenry Jr.), so a few inches should last decades.
Cedar Rose has found that house design, the plaster mix, and application techniques (particularly the need to key plasters in to the straw and any previous coats) all affect durability. I agree. If you want your plaster to last, pay attention to all of these. (This subject is probably an entire article in itself.)
More and more in my work I try to think of durability with sustainability. How hard is it going to be to repair? I try to keep my mixes as simple as possible, and leave the owners with several buckets of dry mix that can be wet down and slapped on a needy wall quickly and easily.
To check a plaster patch for durability, rub it hard with your thumb. Does it resist dusting and crumbling? Pinch it hard between thumb and finger. Is it hard to smush? Turn a hose on exterior plasters (after they have dried thoroughly). Does it eat into the plaster slowly or does it disintegrate quickly? These tests will help you determine what will hold up and what won't. If you get blasted with horizontal rain and hail often in your area, consider adding lime to the exterior mix or putting a finish coat of lime over your plasters.
Courtesy of Natural Building Resources
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