Hi Stef,
Just addressing the question about how much water you need for plastering activities. You’ll need about one gallon of water per square foot of plaster at 1” thick. Estimate the total square feet of exterior and interior wall surface on your
project, multiply by one gallon....
Here’s the math. Think of plaster as a four-part mixture: 1 part binder (clay or lime) and 3 parts aggregate (sharp sand). (Note that mix ratios vary—it might be 1:2 or 1:2.5 or 1:3.5 depending on binder and aggregate, but 1:3 is common and also makes it easy to explain the calculations.) The binder almost completely disappears into the voids between the sand particles, so if you add four units of these materials (1 unit binder plus 3 units aggregate) you’ll still end up with 3 units of plaster. Although the water amount will vary with both the type of binder and how wet the sand is, a rough rule is that you need as much water as you have binder in any given batch. In my
experience, a little more water with lime, and little less with clay, but again, much depends on binder type and the sand’s moisture content. The water also mostly “disappears” into the mix and doesn’t count towards an increased volume.
So, if you put 1 cubic foot (7.5 gallons) of binder (clay or lime) into a mixer with 3 cubic feet of sand (7.5 gallons x 3, or 22.5 gallons), you'll need about one cubic foot (7.5 gallons) of water to yield a workable 3 cubic foot plaster mix. (The order you add these materials also depends on the mixer and binder type).
How much wall surface does that cover? Imagine a cube that is 12” on all sides—a cubic foot. Slice it into layers 1/2" thick, which is a typical clay or lime plaster thickness for base coats over straw bale walls, although 3/8” is also common. You’ll have twenty-four of these 12” squares, so a cubic foot of plaster will cover 24 square feet of wall at ½” thick.
The 3 cubic foot mix you have will cover around 72 square feet of wall surface at ½” thickness, and require about 7.5 gallons of water to make. Just 1 cubic foot of plaster will need 1/3 of this water, or 2.5 gallons (7.5/3) for 24 square feet, which works out to be about .83 pints per square foot (2.5 gallons x 8 pints per gallon = 20 pints/24 square feet = .83 pints). I’d round up to 1 pint for simplicity. If you have two base coats, each ½” thick, you’ll need 2 pints per square foot. Add some water for the much thinner finish plaster mix and any damp curing for lime plasters—might as well round up to about another pint or two (½ gallon) per square foot of wall surface just for the plaster mix.
Then there’s tool and equipment clean up after each plaster coat—it’s amazing how much water gets used to wash out mixers, hose off ladders and scaffolding, and clean off tools. Unless you’re really miserly with water, this can easily add another pint or two per square foot. Now we’re talking about somewhere between ¾ and 1 gallon per square foot of wall surface for all three plaster coats, including clean up. Because there’s going to be some waste—people accidentally forget to turn hoses off and let buckets overflow—I’d use the higher number. You really don’t want to run out of water midway through plastering a wall!
If you have a building with 1000 square feet total of interior and exterior wall surface you’ll need at least 1000 gallons of water on hand—preferable clean water—not muddy or full of algae or other
organic material. Also, be cautious about mixing lime plaster with warm water (that has been sitting in a tank in the hot sun) because the plaster will set up more quickly, significantly reducing the working time. I have noticed this effect more with natural hydraulic and artificially hydraulic limes than with Type-S or quick lime, but when it comes to mixing plasters I expect that cool water is better all around unless you’re working in cold weather conditions.
Good Luck!
Jim Reiland
Many Hands Builders
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