PLASTER ON SLOPING WALL OR ROOF SURFACES
A sloping exterior wall or extension of an exterior wall into a sloping roof line may be plastered successfully. The intent of this recommendation is not to promote the practice, but to define good procedure so that the job can be successfully completed.
A good grade of exterior plywood should be installed over the framework of the building in those areas that have slope. (Note that the use of plywood under plaster is suggested ONLY IN SPECIAL CASES such as this, and it is required that plywood be protected well.)
Normally, plywood under plaster may create problems unless the wood is adequately insulated from moisture, because wood expands as water is absorbed into the fibers.
Drip screeds and weep screeds must be installed in cases described here. If the entire wall surface is designed with slope, a weep screed should be installed at the base of the wall.
If the sloping portion is a bent continuation of a vertical wall, that is, a pitched roof, install a drip screed at the line of bend from vertical to slope. The purpose of the drip screed is to divert any water that might penetrate the plaster membrane.
If the walls are designed with a parapet, the top of the wall must be flashed to seal against water entry. All windows, doors and other openings must be properly flashed around their perimeters.
Control joints should be installed to reduce transmission of stress from the framework to the plaster membrane and to isolate stresses within the plaster.
If the wall is two or more stories high, a drip screed or control joint should be installed horizontally at the plate line. The plate line is a line of weakness in the framework of a structure, therefore adequate protection must be provided.
An asphalt saturated felt strip should be placed under all control joints and drip screeds, and over the plywood, to protect the plywood. The strips of asphalt felt are attached to plywood to serve as a water barrier behind the metal. The full sheets of felt underlayment are applied after control joints, drip screeds and weep screeds have been installed.
The saturated felt underlayment is applied after control joints and drip screeds have been installed. Felt will overlap the metal base of all control joints and weep screeds to provide proper flashing.
One layer of 30 lb. or 45 lb. asphalt felt or two layers of 15 lb. felt should be laid over the plywood in weatherboard fashion. Felt can be bonded to the plywood with a hot-mopped asphalt coating, or a coat of asphalt emulsion may be applied with a notched trowel or a brush.
The lower edge of the full sheet of felt should be carried down and over the upper protective edge of the drip screed.
There are two choices of procedure that may be followed next. Self-furred, paperbacked stucco netting may be applied as a second layer, or another layer of 15 Ib. asphalt felt is laid over the base using care in application. The stucco netting is secured to the deck over the 15 lb. felt.
Proper care must be observed in securing stucco netting to the waterproofing membranes to avoid tearing or other damage. Nails, staples or speed nut applicators may be used as fasteners. If nails or staples are used, they should be driven through neoprene washers. Drive the nails or staples straight into the wall or deck to affect clean, undisturbed penetration of the felt.
After stucco netting has been applied in good order, proceed with the plastering operation. Observe pertinent precautions, such as being certain that the scratch coat is applied as heavily as possible, with as little mix water as can be used, and that scoring is shallow.
Cure each of the two base coats well before applying the third coat. There isn't any need to incorporate stearates or other waterproofing compounds in the third or color coat. It is much more Important that the waterproofing underlayment be installed properly than to depend on water-resistive additives in the final coat.