How to Build a Planter Box Using Composite Decking

Planter boxes are a great way to add some style and flair to your garden. They can also be used as raised beds for growing vegetables and flowers.

Composite decking is an excellent material for building planters, because it is weather resistant and looks good in any setting.

In this article we will be walking through how to build a composite planter for a single tree or plant. However this design can be used to build planters of most sizes with some small adjustments.

We have included a list of materials and tools you will need with links to amazon to help you get started.



Step 1: Build the frame

First you will need to decide how big you want your planter to be. Consider the size of your garden and where you would like this to go. Its a good idea to have a picture of something similar designed.

The frame will be covered with composite decking at a later stage so make sure you account for the thickness of this when building the frame. For our purposes the composite decking used will be 20mm thick and 145mm wide (common measurements for composite decking).

Decide on the gaps that you want between the composite decking boards, we recommend 5mm as this looks best. Then decide how tall you want it to be and work out how many boards widths + gaps this will be.

In our example it is 4 boards widths tall (145mm x 4) + gaps (5mm x 3) coming to a total of 595mm. A small amount of additional wood is fine so we will make the frame 605mm tall overall.

Cut the 50 x 50mm pressure treated timber and screw the corners together using 80mm wood screws.

Step 2: Fit the bottom planks

Building a composite planter step 2: The base

Fit the composite decking boards and cut any notches needed to get around the frame. Start at the two outer edges and work in to the centre. Ensure the boards are fixed in place with 50mm wood screws with pilot holes first.

Step 3: fit the liner

Building a composite planter step 3: The liner

Its important that you use a liner to avoid soil spilling out between the gaps in the planks. We recommend using weed fabric as its will allow moisture to pass through without letting soil spill out.

Attach the liner with staples or tacks to the inside of the frame. Don’t pull it too tight as you want the outer planks and frame to take the weight of the soil. The liner will tear if pulled too tightly.

Step 4: Attached the edging boards

Building a composite planter step 4: The outer decking

Measure and cut your composite decking board. In our design we have make then the width of the frame plus the width of a decking board long. You can be a couple of millimetres off here as the ends of the boards will be covered.

It’s important to note that you will need solid composite boards. The hollow boards are good for decking but don’t hold up well to screws going through them.

Work out the lengths you need and cut the correct number of boards. If you have a chop saw and a bench you can easily set this up so that each board is exactly the same.

Start at the top of the frame, getting the top board flush. Attach it in place using two 50mm wood screws in each end. Make sure to drill pilot holes using this special tool so the screw heads are flush with the surface of the boards.

Using the 5mm spacers push the next board up so that it is tight against the spacers under the previous board. Hold it in place and attach it in the same way. Repeat this until all the boards are on.

If you are finding it difficult to hold the boards in place, consider getting hold of some clamps.

Step 5: Fit the picture framing boards

Building a composite planter step 5: The picture framing

The picture framing boards top off the look by creating a nice border. They are not essential to the design working but we think planters look much batter with them.

The picture framing boards are the hardest part of the whole build because you will need to get them just right first time or risk wasting some expensive composite decking.

Work out the overhang that you would like, we have gone for 30mm. Measure the very outer edge of the current planter then add your additional margin on. This is how wide you want the outer edge of the picture framing to be.

Measure each length out carefully then use your right-angle tool to draw a 45-degree angle which you will cut along. Try to be as accurate as possible here to avoid any wastage or ugly gaps.

Fit the picture framing down using two 50mm screws and clip a cap onto the top of each one to hide it from view.

Step 6: Fit the corner edging

Building a composite planter step 6: The corner trim

The cover the rough looking corners cut the corner edging pieces to the correct length, make sure then end flush with the bottom of the bottom decking planks.

Stick them on with some strong glue and leave to cure for the recommended time.

Step 7: Fill the planter

Building a composite planter step 7: The filler

The planter will be ready to fill when the glue has had a few hours to set. We recommend adding 50mm of gravel to the bottom of the planter to assist with drainage. Then it’s up to you to add whatever type of soil and plants you like.

In conclusion

Composite planters can be a great way to add a garden bed or grow some plants. They can be built quickly and are also weather resistant which is perfect for our varied UK climate.

We hope you enjoy your stylish new planter. Thanks for reading.