Do the double

by Rhiannon James

Photo by Joe Lee

There’s no better place to grow herbs indoors than a sunny kitchen windowsill. The plants get all the light they need and you have a constant supply of deliciously fresh flavours on hand when you cook. There’s just one problem. To grow a good range of herbs is a bit of a squeeze on your average-sized sill. But of course, there is another way – build our double-decker planter and you’ll have twice as much room to grow the herbs that you want.  Just follow the steps below.

Stuff to get

  1. 3 x 1.8 metre lengths of 145mm x 18mm PSE (Planed Square Edge) softwood planks
  2. Around 40 zinc-plated screws around 1.5 or 2 inches long (#8 or similar)
  3. Tough plastic to line the window boxes (optional)
  4. A 400mm long plastic window box.  If your window box is larger than 130 mm (height), 170 mm (width) and 400 mm (length) you may have to adjust either the size of the timber you use (either wider or narrower) or adjust the cut sizes mentioned below to accommodate the box
  5. 3 x 10cm plastic plant pots
  6. Horticultural grit, compost and a selection of herbs.

Tools you’ll need

  1. Electric hand drill
  2. Screwdrivers, both electric and manual
  3. Handsaw for wood
  4. Pencil and set-square
  5. Small drill bit (for drilling screw holes)
  6. Countersink bit (for countersinking screw holes)
  7. Tape measure
  8. Sandpaper or sanding block
  9. Staple gun or drawing pins


Step 1:  Mark out and cut the timber panels

At this stage, you need to mark out and cut the following panels from the planks:

  1. 1 x front panel (for the lower box) at 440 mm long
  2. 1 x back panel (for the upper box)at 440 mm long
  3. 2 x upper box side panels at 200mm long
  4. 2 x lower box side panels at 145mm long
  5. 2 x base panels at 405 mm long
  6. 3 x vertical panels at 235mm long

Use a set square to get an accurate right-angle to cut along.

Remember that when you make each cut with the saw, this will remove a few millimetres from the length of the plank.  One way to make sure you take account of this is to wait until you have cut one panel before you measure out the next, rather than measuring out all the pieces at the start. Once you have cut all your panels, take your sandpaper and use this to clear off any rough edges.  A quick sanding to remove splinters and smooth the end grain a little will be fine.


Step 2: Drill and countersink screw holes in the front, back and side panels

Now you need to create holes in the panels to screw through.  The best way to start is to mark out all the holes with a pencil.

At this stage, it’s useful to understand how the panels fit together. The back panel, the two upper box side panels and one base panel will form the upper box. The front panel, the two lower box side panels and the other base panel will form the lower box. The three vertical panels will connect the two boxes and form the front of the upper box and the back of the lower box. Side panels are therefore attached at the front of the back panel for the upper box and behind the front panel on the lower box.

You will need to make six holes in the front panel and six holes in the back panel (two to attach each side panel and two to attach the base).

Lay both the front and the back panels flat on your work surface. To help you position your screws, hold each side panel in position against the front or back panel, but on the opposite side to where it will attach, and draw a pencil outline to show the inner edge. Repeat with the base panels. Then draw crosses to show exactly where you want to centre your drill holes. Aim for the centre of the side or base panel edge you are screwing into (this will be around 9mm from the sides of your front or back panel for the side pieces and 9mm from the bottom of your front or back panel for each base).  For the screws into the side panels, offset the holes around 25mm from the top and the bottom of the front and back panels. For the two screws into the base, offset around 120mm from each side.  Now drill each of these holes with the small drill bit.

Next, countersink each of these holes with the countersink bit .

You will then need to make one hole along the bottom edge of each lower box side panel to attach it to the base and two holes in each upper box side panel. Mark out a line as above to show the inner edge.  Then mark, drill and countersink holes as above, remembering to offset around 9mm from the bottom edge. For the lower box side panels, centre each hole about 70mm away from each side. For the upper box side panels, make two holes in each piece, one 25mm from the edge attaching to the back panel and one 115mm away.

Step 3:  Initial assembly of the boxes

Now you’ve drilled and countersunk your holes, it’s time to start assembling the boxes.  Please note, the base panel for the upper box is not deep enough to complete this box at this point.  Insert the screws, through the back panel into the two upper box side panels and through the front panel into the two lower box side panels.  Take care that you get each panel in place as accurately as possible.


To finish off the lower box, put the base panel into place and push it up flush against the front panel. Once everything is properly aligned, insert the screws through the front and side panels to hold the base in place.

Repeat for the upper box, pushing the base up flush against the back panel.

You will see that the base panel is not deep enough to complete the upper box

so you will now need to cut an additional strip of plank to fit the gap.  Start by measuring the gap.  Once you have this measurement (it should be 405mm x about 55mm), you will need to cut a strip of this size.

Now, insert the strip into the base of the upper box.  Once aligned, drill and countersink one hole in each side panel (about 25mm from the open front) and insert the two screws.  The top box should now look like this .


Step 4:  Drill and countersink holes in the vertical panels and assemble

Take the three vertical panels and lay them out on your worktable in front of you.  Mark out, in pencil, where the vertical pieces will meet the top box.  This will be a horizontal band and two vertical strips.  Drill and countersink two holes in both vertical strips (to connect to the side panels) and six holes in the horizontal band (two for each vertical panel), Each hole in the horizontal band should be offset around 40mm from the edges of each vertical panel. The following picture shows the layout of the holes you need to create.

Now, turn the vertical planks over so the countersunk holes are facing down and mark out again where these panels will meet the lower box.  Repeat the drill and countersink procedure above in the same locations as above, except that you need only use three screws in the horizontal band (one for each vertical panel) Centre the screws in each horizontal band about 70mm from both sides of each panel.  The following picture shows the layout of the holes to drill to connect the bottom box.

Next, assemble the top and bottom boxes with the vertical panels and screw together.

Step 5:  Place liner in boxes (optional)

Take the plastic liner, cut it to size and place it in both window boxes.  Using either the staple gun or drawing pins, fasten the liner inside each box ensuring that it comes up at least 25mm from the bottom.  This liner will allow you to use pots with drainage holes inside the boxes because it will catch any excess water.  As an alternative, you could use drainage trays.

Step 6:  Plant up your boxes

The upper box is designed to fit a standard window box. Plant this up with a selection of herbs which like similar conditions – chives and basil, for example, both like moist compost. The lower box will take three standard 10cm pots so you can plant these up with any herbs you choose.


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