Plastering is hard enough, so don’t beat yourself up trying to figure out how much plaster you will need. The first option I HIGHLY recommend is to contact the management at the place from which you intend to order your material and ask them to calculate the order. You can give them your building’s dimensions and other details they request and they will give you a very accurate bag count and sand quantity. In fact, they will most likely be more accurate than you will with the formulas below.
There are two major players in the Natural Hydraulic Lime Plaster arena. The East Coast is covered by De Gruchy’s Lime Works located in Pennsylvania. They will happily help you estimate the material order and are also able to send out samples of materials and colors to help you choose the final coat material and color. They ship directly to you and if you use my vendor code (95501NHL) they will give you a discount on any purchase you make. Check out their website at http://www.limeworks.us/home.php. You can also purchase a cool plaster application gun that allows you to spray on your plaster, speeding the application process!
The West Coast is Handled by one main distributor: TransMineral USA, Inc. located in California. They supply many retail stores with the plaster and can also drop ship larger orders directly to you. The folks there are always happy to help clients get the order right, the first time. You can check out their website at www.limes.us. Let them know that I sent you and they may be willing to help you out a bit on the product pricing. I can’t make any promises right now, but it is possible.
So if you insist on doing your own calculations, give the following numbers a shot. Best of luck and don’t forget to plan for the necessary excess of mistakes and dropped mud. The following is based on an estimated wall surface of 900 Square Feet. I have also used the NHL 3.5 as the material as that is the norm for my climate. DO NOT assume that this is the right material, and therefore ratio, for you. Ask the folks above to help make that determination. Also, I have given you a couple different options for the sand/lime mixing ratio. The more lime, the stickier the plaster. The more sand, the less expensive the plaster.
SCRATCH & BROWN
Scratch Coat: 1 part NHL 3.5 to 1.5 parts sand (mixing ratio) at 3/4″ = 1440 lbs.
Brown Coat: 1 part NHL 3.5 to 2 parts sand (mixing ratio) at 1/2″ = 728 lbs
Total Scratch & Brown = 2,168 lbs / 55 lb bags = 40 bags
Scratch Coat: 1 part NHL 3.5 to 2 parts sand (mixing ratio) at 3/4″ = 1092 lbs.
Brown Coat: 1 part NHL 3.5 to 2.5 parts sand (mixing ratio) at 1/2″ = 560 lbs.
Total Scratch & Brown = 1,652 lbs / 55 lb bags = 31 bags
Alternative 1: 1 part NHL 2 to 2 parts sand (mixing ratio) at 1/8″ = 164 lbs or 3 bags
Alternative 2: Ecomortar F premix at 1/8″ = 14 bags (This is much easier to use than mixing with sand. I strongly recommend the pre mix bags.)