How To Build A Planter Box For A Deck

Decking boards are usually made from pressure-treated wood, cedar or redwood, all tough woods that will last for many years. If you’ve finished a decking project and have some spare deck boards, why not make your own planter box.

It’s easy to do and is a great way to save money and create a useful item for your garden. You can also buy damaged decking at hugely reduced prices from DIY stores, they’ll be happy to sell it on and it’ll be perfect for a planter.

What you’ll need

1. Sketch out your plans

It’s a good idea to sketch out the planter you wish to build and work out exactly how many pieces of wood you’ll need to create it. Remember that you can use pressure-treated wood for the base.

2. Measure and cut

Mark and cut the lengths of decking to the sizes you require. You’ll need to drill pilot holes roughly 2.5 cm from the edge of each plank so that the wood won’t split when you screw the planks together.

3. Create the frame

Begin by creating the frame of your planter, take the 4 edge pieces and screw them together.

If you’re creating a planter box that’s more than one decking plank deep, you’ll need to cut 4 lengths of your pressure-treated wood to act as corner supports to screw the planks into.

4. Add another layer

Add the next layer of decking boards in the same way, screwing the planks together. Keep adding the boards until you are happy with the height of your planter.

5. Make the base

To create the base of your planter, you can staple a cut to size weed membrane to the bottom of the box frame, followed by a strong wire mesh panel. This can also be stapled to the bottom of the planks. It’ll provide a strong base and you won’t need to worry about rotting wood. There’s a great video explaining how to make a planter box base here.

Step 1: What You Will Need:

Tools We Used:

  • hammer, for showing nails who’s boss.
  • A screwdriver.
  • chop saw – Hassan Chop!
  • jigsaw, for the cut outs.
  • A circular saw for the base slats.
  • A couple of ‘G’ clamps for holding it together.
  • staple gun for attaching the plastic lining.

Materials We Used:

  • Decking planks (2 x 4800mm long and 1 x 3600mm long).
  • 2 lengths of treated timber (19mm x 32mm x 2400mm).
  • 6 lengths of timber offcuts (30mm x 50mm x 500mm).
  • 10 decking plank offcuts (at least 630mm in length).
  • 48 self tapping decking screws.
  • 20 clout nails.
  • Polythene or pond-liner or some other plastic (for lining the inside of the planter).
  • 350 litres of multi-purpose compost.
  • Broken garden pots or stones to layer underneath the compost, to aid drainage.

We managed to get most of our stuff at discount prices from Wickes – we bought damaged decking planks for about half price and damaged compost for £1.99 a bag. It came repackaged in strong polythene, which we then used for the lining. Most garden centres or DIY shops will have some goods damaged by careless forklift drivers which they’ll be selling cheap – have a look around at the back or ask someone. It’s definitely worth it – our planter only cost £32 (not counting the hardware and offcuts we already had). That includes filling it with compost and, thanks to the BBC, the seeds we’ll be planting.

Step 2: Cutting List

Make a cutting list of the parts you need to make the planter – then you can work out what you need to buy and what you can use from the shed.

  • 6 side pieces (1400mm lengths of the decking plank).
  • 6 end pieces (630mm lengths of the decking plank).
  • 6 legs (30mm x 50mm x 500mm).
  • 10 slats for the base.
  • 6 lengths of timber to make a baton to hold the base slats (19mm x 32mm x Various).

 Measure and cut a ‘master’ piece, then use it as a template to cut the rest of the pieces. That way, even if it’s a little bit out, then all the pieces will still turn out the same. It’s more important that it has it’s own internal integrity rather than being mathematically accurate.

Step 3: Making It Fancy

With a tiny bit of extra effort in a couple of places you can make the planter look more professional. Before constructing it we chamfered the edges of the longer side pieces, this was done by setting the chop saw at a 45 degree angle and starting the cut 12mm from the edge (our decks were 30mm).

Chamfer the bottoms of the legs to stop the wood splitting. I had a go at this on an offcut and made it look like a stake that could kill a vampire.

Step 4: Main Construction

Now you’ve got your pieces cut, you’ve made the edges look fancy and it’s all lying there ready for construction…

A. Get your clamps ready, then lay out the first corner and leg. Take a long side, a short side and a leg and place them together as shown in the image (1). This is upside down to the way it will finally sit. The small off-cut under the leg is to set the leg slightly below the top of the planter so you can’t see it from the side. See image 1.

B. Clamp the parts together so that you can still get two screws into both decking parts. See image 2 and 3.

C. Screw the corners of the decking to the legs. There should be a screw in each corner of the decking planks. See image 4 for approximate screw positions.

D. Move on to the next corner – don’t forget to offset the leg with the off-cut. See Image 5.

E. Finish all four corners in the same manner – see Image 6.

F. Screw down all the side pieces – fit them up first to see which order they should go depending on any warped pieces. See Image 7 for approximate screw positions and Image 8 showing all side pieces attached and the planter turned right side up.

G. Turn the planter upside down again – legs in the air. Measure the space between the corner legs on the long side, in our case this was 1240 mm. Half this number and measure and mark on the bottom of the bottom side piece on both sides. This is the centre point at which you want to attach the middle legs. Remember to offset these using the off-cut and attach using six screws. See image 9 for a shot of the planter with middle legs attached.

H. Cut the thin pieces of timber to make the slat supports by measuring the distance between each of the legs. You’ll need two end pieces and then four side pieces for between a corner leg, the middle leg and the other corner leg; on each side. Much simpler than that sentence makes it sound! See Images 10-14 for shots of the slat supports in place. We had a piece left over which was long enough to span the planter, so we added an extra support across the middle – it’s not necessary and serves no structural purpose, but you might as well use the wood if it’s left over.

Step 5: The Base Slats – Construction Phase Two

I. Cut 10 x 63cm slats from the offcut decking planks. Cut one measured piece and then use it as a template for the others. Test fit each one as you go along. See images 2 and 3.

J. Cut out the corners on two slats, so that they fit around the legs in the corners. We drew around an offcut of the legs and made two cuts for each corner with a jigsaw. See Image 4 for a view of the finished cutout.

K. One slat will need two cutouts around the middle legs (we were really lucky and the cutout we needed was exactly the same as the corners, but it may end up in the middle of the slat and need a three side cut with jigsaw). See Image 5 for our finished cutout and Image 6 for a view of the half-size slat.

L. Unless you are really lucky (or work out the initial dimensions based on the slat width) there will be one slat that needs to be narrower than the others to fill in the gap. We cut one slat in half with the circular saw to plug our gap.

M. Place all the slats into the base.

N. Bask in the glory of completing the woodwork part of construction.

Step 6: The Final Steps

Now we’ve got the wooden construction finished we need to drill drainage holes, line the inside and fill it with compost.

O. Drill some holes in the base of the planter. You want to hold some water, but it needs to be able to drain away as well – worst case scenario your vegetables could rot.

P. The damaged compost bags we bought came repackaged in larger sacks (Image 2).

Q. We slit the sides and stapled the plastic to the inside of the planter. Roll the top of the plastic over and staple it about 2cm below the top of the planter. Take extra care around the legs.

R. Place broken pots or large stone in the base for drainage (see Image 4).

S. And now for a bit of maths to find the volume of the planter, and therefore how much compost we need… We can find the volume (v) by multiplying the length (l), the width (w) and the height (h).

lwh = v

So, in centimetres:

(140 x 63) x 42 = 370440

1000 cm3 is equivalent to 1 litre in volume.
So 370440 cm3 is equivalent to approximately 370.5 litres.
Our bags hold 75 litres, so we needed five to fill the planter.

T. And that’s all folks! It’s ready for planting.

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