Since the dawn of the Plastering Industry, the minimum thickness of a Portland Cement Plaster panel was 7/8″. 3/8″ scratch coat, 3/8″ brown coat, and 1/8″ finish stucco coat adds up to a 7/8″ panel. Window casings, trims, base screeds reflect this basic panel depth.
I’d like to ask why not consider adding an additional 1/8″ to our future Plaster Panels and make every panel ONE INCH THICK.
Plaster used to be applied leisurely during an extended construction schedule. 28 days was normal turnaround time. Now application is so accelerated that almost every job is to be complete in half that time. I regularly consult on jobs where the General Contractor wants the plaster done in less than 14 days. And I have written specifications for short term projects where the plastering was done over the weekend. That’s framing to finish coat and paint coat in 2 days!
ASTM C-926, the Standard that is current (March 2017) Building Code, provides for a minimum application period of 48 hours (2 days) for curing between the scratch and the brown, 7 days between brown and finish for a total turnaround time of nine days to completion. Regularly there are environmental conditions that can lengthen those minimum periods.
Why 28 days? The story from my recollection of a conversation with Khalil Bennett, a contractor during that period, was that Tony Schirro, a Director of the Sacramento Valley Bureau for Lath and Plaster understood that the longer plaster cured, shrunk and cracked in the basecoat; and the longer building framing was allowed to squat and settle prior to the application of the lime based finish coat; the better the job for the end user. Tony was so respected that every Architectural Firm specification in Northern California required 28 day cures. And it became Industry Practice over time.
Currently, the pace of building is accelerating. Our industry has responded with specialty cements that have yet to prove themselves with 3-day turnarounds and have poor records of performance. I would suggest that maybe it’s time we increase the depth of the basic Portland Cement Plaster Panel a 1/8″; which will help minimize cracking (there have been studies proving this) and provide a full thickness wall.
Originally we installed paper and wire and set a 7/8″ edge and we had a 7/8″ wall. Now we integrate two 40 mil thicknesses of flashing at the windows, a liquid membrane, two layers of paper, an air barrier, a lath butterfly, and a crack control membrane that itself adds 1/8″ of thickness to the wall and shrink the cement material that is supposed to resist cracking by 1/4″. If we add 1/8″ to our trims and window casings, we can accommodate the best crack control lamina and have enough plaster left so we can actually call it PLASTER.
If you’ve a question about curing or need to talk to someone about curing your plaster project, or other situations, call or email: