Introduction: Make a Pot Trough for Saving Water on Your Deck

In the great supercity of Auckland, NZ we now have to pay not only for the water that is reticulated into our property, but also for reticulating it out (the "wastewater charge" is based on how much comes in rather than how much goes out through the wastewater system, so even if you use most of your water on your garden, you still pay a wastewater fee). There is debate about the justice of this, but here's a way we're reducing costs by reducing the water coming in*. 

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)
hose/watering can/bucket

Step 2: Prepare Deck and Build Frame

For this project, we started small as determined by the length between two deck uprights - enough for eight medium pots. I used timber I had to hand, but obviously you could make this look even better.

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.

Step 3: Prepare Plastic Liner

Lay out your plastic, leaving plenty of overlap each end: remember that it has to fold up at each end of the trough. My plastic is 900mm wide, so is plenty wide enough for a double layer. If you have thicker plastic, you may only need one layer.

As my plastic used to be a painting dropcloth and had holes, I had to check it against the sunlight and stick bits of duct tape on the holes on the side that would not show.

Step 4: Lay Plastic in Trough and Check for Lowest End

You will need a lovely assistant. 
Fold the plastic in half lengthwise and lay it in the trough. Make the front edge sit about 1cm below the top of the front board and put in a staple 1cm  below the edge of the plastic, about 30cm from one end. Put in two or three more staples along that edge, but keep away from the ends (you will need room for cutting and folding).

Have a discussion with your gorgeous helper about which end is the lowest. Even though you're sure she's not right, loosely fit the plastic in place and pour some water in it. Gracefully admit you were mistaken, and lift the plastic to drain out the water. Use the dog's bone-eating-towel to dry the plastic - this towel will also be useful to kneel on*. This whole bit could be avoided if you use a spirit level beforehand.

* Three uses for the same towel. Points for the first one to make the obvious towel reference: extra points for the actual quote and book.

Step 5: Drill the Drainage Hole

Tuck the front edge of plastic well into the front board, then without moving that side, do the same with the rear board.
If you have two layers of plastic, carefully fold back the top layer, and stick on a 200-250mm length of duct tape folded back on itself so the sticky side is out. Press it on to the lower plastic layer above where the drainage hole is to go.

Carefully roll back the top layer and press it firmly onto the duct tape.

Stick two more short lengths of tape on the top plastic layer above the previous tape. This is all to make a firm base for the drainage fittings.

Use the holesaw guide bit to drill through all the tape and plastic layers into the deck. Do not let the saw teeth touch the duct tape.
Fold the plastic back and drill through the deck. Be cautious as you near the end of the cut as you could suddenly break through the deck and almost smack your head on the upper deck...

Step 6: Fit the Drainage Fittings

Centre the top nut over the small hole in the duct tape and mark around it.

Place a backing board behind the plastic and use the craft knife or scissors to cut a hole about 3mm smaller all around than the line you just marked.

Carefully force the hole over the top nut (so that the nut is poking through from the topside). If you are concerned about leakage, you may want to run a bead of hot glue or silicone sealer between the top nut and the upper duct tape before you screw everything together.

Firmly screw on the bottom nut and place the assembly into the deck hole.

Gently screw in the riser assembly. The riser can be made from anything with a thread that fits the top nut - in my case two adaptors gave me the approximately 50mm I needed. My top nut already had a rubber washer, but you will need to find one if your's doesn't. Or use thread tape.

Step 7: Finish Folding and Stapling the Plastic

Go around the trough folding and stapling the plastic 1cm below the top of the boards. I put a staple in every 200mm or so.

The corners can be tricky, and you may need to cut off some excess*. Remember, though, that the plastic will not be readily seen once the pots are in.

Remove any staples you may have used to temporarily hold the rear edge of the plastic.

* "Measure twice, cut once"!

Step 8: Place Pots in Trough and Water Them

Water the plants and note how much water you are now saving.

Put your tools away and tidy the workspace.

This trough allowed for eight pots with room to spare for wider spacing of the plants that need it - not planned, just serendipitous*. Taller plants, like the Bog Sage near the steps, would hide the under-deck mess better, but the parsley, sage and thyme** are easy to get at for cooking with fresh herbs.

* "The Three Princes of Serendip"
** "Scarborough Fair"