Providing Thru-Wall Flashing Support System (TWFSS) parts consisting of straight runs and splines used to bridge the cavity in cavity wall construction details. Made of PVC (free from plasticizers), TWFSS is installed under flexible thru-wall flashing membranes, providing installers a proper surface to complete seams and preventing flexible membranes from sagging, allowing water to migrate back to the exterior of the building, eliminating nuisance leaks.