Planters are often considered the area where waterproofing is most problematic.
Green roofs are really extended planters and are becoming increasingly popular.
We have two situations to consider.
New Planter Box Construction
From a waterproofing point of view, concrete walls are much to be preferred over porous blockwork which unfortunately is so frequently used nowadays. Because planters are exposed to rain and the soil is usually wet, it is best to isolate them from the living areas of the building rather than risk having a common wall.
For most planters, the base is concrete and seldom drains properly. Since it is always safer to waterproof the structural elements, it is preferable to lay a screed to falls on top of the membrane so there is no stagnant water to become a breeding ground for creepy crawlies. Care must be taken however that when spreading the screed, the membrane is not cut. Always water test to ensure everything is OK.
Provided the planter is wide enough to torch apply a sheet membrane typically minimum 600mm, the fully bonded torch applied Wrmco SBS sheet membrane is safer to use than a liquid applied membrane. For narrower planters, the best membrane is a solvent based urethane rubber as none of the less expensive water based materials will last for many years in the wet environment.
The normal Wrimco SBS modified bitumen is ideal for garden roofs and most planters but where plants with aggressive root systems such as bamboo are planted, it would be wise to choose the Wrimco Anti-Root SBS. Single layer with double detailing is normally sufficient but two layers on the base or even two layers for base and sides are frequently specified.
Since the membrane on the sides of the planter are vulnerable to damage from tilling the soil, it is essential that the membrane be properly protected.
Membrane can be protected by: Cuspated protection board, XPS Polystryene board, and Corflute board.
No useful purpose results from having drainage cell at the sides but is certainly needed for the base, with 1mm filter fabric on top and a 50mm layer of coarse double washed river sand before the planter mix is loaded (Double washed to remove the fine particles).
Repair Leaking Planter Box
First check that water is not bypassing the membrane system. This can most readily occur with blockwork so the tops, down inside to the membrane and the outside need to be well waterproofed. Take particular attention to irrigation pipes and electrical conduits.
Fortunately planters can be fairly easily emptied, cleaned and re-waterproofed same as for new construction. If an APP or PVC membrane had been used, it is preferable to remove them but for liquid applied membranes that are still well adhered, the Wrimco SBS can be torched over them provided the bulk of the heat from the gas torch is directed onto the SBS. When applying solvent based urethane, first prime with WBE (Water Based Epoxy).