The first thing you would do is prepare the substrate. Remove any loose crumbly substrate, make sure the substrate is clean, dry, free of any previous applied products, and foreign matter.
Prime the substrate with PermaFlex (240 sq ft per gal).
Patch and profile any joints, seams, cracks, holes and rough areas using LRB/TAV mixture (Liquid Rubber Base, Thickening Activator). 2-parts LRB is mixed with 1-part TAV. A 1” diameter bead of LRB/TAV mixture is used at wall/floor junctions and around most fixtures. A 3/4" diameter bead is typically used to seal open seams.
LRB/TAV ‘Round Caulk Bead Calculator’
Topcoat using 1 coat of PermaFlex (240 sq ft per gal).
If parapet walls are also covered with old materials simply strip them to expose an area of vertical substrate below the wall cap which is not contaminated with tar or cover all contaminated vertical surfaces with plywood. Terminate the vertical plywood at a point on the vertical wall which is not contaminated with tar or other sealers. Prime the area and apply LRB/TAV mixture to completely bond and seal this point.
It is easy to attach the bottom of the new vertical plywood to the new horizontal plywood by installing securely nailed 2"X4" "L Corners" continuously under the plywood at all parapet wall perimeters. This provides a nice raise in grade to direct water from this crucial potential leaking point seam at the base of the parapet wall. It is not necessary that the vertical terminations be attached securely with nails or other mechanical fasteners. Often only a few fasteners, clamps or even weights are required at this point to temporarily hold the plywood tight to the wall until the LRB/TAV mixture is installed and cured. The LRB/TAV mixtures will secure the top of this new plywood wall completely and more securely than nails due to the extremely strong adhesive properties of the products.
To prevent water damage to the new plywood substrate all that is required is that Permaflex primer and LRB/TAV mixtures, (used on joints and seams), are applied at least 45 minutes before a rain to prevent droplet damage to the newly applied products.