A masonry planter combines two challenges at once: keeping the soil moist for the plants and, at the same time, preventing that constant moisture from seeping into the structure it is built on.
Waterproofing the base and sides
The whole trough of the planter (base and side walls) must be waterproofed with a system resistant to constant, not just occasional, moisture, since the substrate retains water for a long time after each watering. The waterproofing must rise up the side walls to a point above the maximum expected substrate level.
Drainage layer: essential, not optional
Without a drainage layer at the bottom (gravel, drainage sheet or expanded clay) and an overflow drain, irrigation water accumulates at the bottom of the planter, drowns the plants' roots and increases water pressure against the waterproofing, shortening its life. This drain must be connected to a discharge point, not simply left to run loose.
Root protection
As on a planted roof, if the planter is going to hold deep-rooted plants (shrubs, small trees), it is wise to add an anti-root protection layer over the waterproofing, since some roots are able to perforate conventional membranes over time.
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