Comm. NT: 15.
Reading acchinna-mariyādaṃ with Vism-mhṭ, which says:
10. Abbhokāsikaṅgakathāvaṇṇanā
Without a drip-ledge cut (acchinna-mariyādaṃ)’ means without a drip-ledge (mariyāda) made above, which might come under the heading of a drip-ledge (mariyāda-saṅkhepena) made to prevent rain water from coming in.
But if the rain water comes under the overhang (pabbhāra) and is allowed to go in under it, then this comes under the heading of the open air (abbhokāsika-saṅkhepa)
This seems to refer to the widespread habit in ancient Sri Lanka of cutting a drip-ledge on overhanging rocks used for bhikkhus’ dwellings so that the rain that falls on top of the rock drips down in front of the space under the overhang instead of trickling down under the rock and wetting the back and floor. Pabbhāra in this context is “over hang” rather than “slope.”