Watering potted plants is more efficient if they have trays under them, but the thirsty ones run out faster than the others. Putting them all in the same "tray" means they can share.
In our case, this project also helps hide the untidiness under the deck...
*We're also using our shower and bath water to irrigate a "compost biomass" garden (comfrey, sugarcane, culinary ginger, etc).
Step 1: Materials and tools
polythene (the thicker the better)
duct tape (gaffer, duck, etc)
hose fittings (see picture)
drill (preferably electric)
craft knife (although a decent pair of scissors would be better)
Step 2: Prepare deck and build frame
The deck need not be sanded smooth, but it is important to check for unseated nails, splinters* and rough spots. Naturally you should do this before building the frame, but hey.
*Cheapo deck timber, but has lasted over 20 years.