Building a Simple Timber Planter Box for the Patio or Doorstep

A timber planter box is one of the most satisfying first outdoor projects. It uses straight cuts and simple screwed joints, it needs only a handful of tools and the finished result is useful the very same day. This design makes a box roughly 600 millimetres long, 300 millimetres wide and about 300 millimetres tall, which is a good size for herbs, flowers or a few small shrubs. You can adjust the dimensions to suit your space, as long as you keep the same method.

Choosing your timber

Outdoor timber needs to cope with damp, so choose something suited to the job. Pressure-treated softwood is a common, affordable choice, and some naturally durable timbers also work well. Whichever you buy, look for straight boards without large knots or splits, and check the supplier's advice about outdoor use. If you plan to grow edible plants, ask the supplier whether the timber is suitable, or line the inside of the box with a barrier. Our guide to choosing wood for your first woodworking project has useful tips for spotting good boards.

What you will need

• Boards about 90 to 100 millimetres wide and 19 millimetres thick for the sides: three boards on each of the four sides
• Four corner posts, around 38 by 38 millimetres, cut to the height of three boards plus the small gaps between them
• Slats for the base
• Exterior-grade screws, around 40 millimetres long for the sides and slightly shorter for the base
• A tape measure, a pencil and a try square
• A hand saw or a circular saw
• A cordless drill with a drill bit and a driver bit, plus a bit for drainage holes
• Sandpaper
• An outdoor-suitable wood finish and a brush
• Eye protection and a dust mask

Safety first

When cutting or sanding treated timber, wear a dust mask and eye protection, and work outdoors or in a well-ventilated space. Wash your hands afterwards, and never burn offcuts of treated timber. If you are not confident using a saw, ask the timber merchant to cut the boards to length for you.

Step 1: Measure and cut

The plan is simple. Each short end is made from three boards cut to 300 millimetres, which cover the full width of the box. Each long side is made from three boards cut to 562 millimetres, which fit between the two end pieces (600 millimetres minus the 19 millimetre thickness of each end). The four corner posts all match the height of three boards plus gaps, which comes to roughly 290 to 300 millimetres. Measure twice, mark with a square and cut on the waste side of the line. Lightly sand the cut ends to remove splinters.

Step 2: Build the ends, then join the sides

Lay two corner posts on the bench so that their outer edges are 300 millimetres apart. Pre-drill pilot holes about 10 millimetres in from each end of an end board, so that the screws do not split the timber, then screw the board across the two posts, flush with the top. Add the other two boards below it, leaving a gap of a few millimetres between each for drainage and so that the wood can move. If you are curious about why timber needs that room, our article on why wood moves with the seasons explains it clearly. Build both ends this way.

Stand the two ends upright, then screw the long boards to the sides of the posts, one board at a time, again with small gaps. Check that the frame is square by measuring across the diagonals. Equal diagonals mean the frame is square.

Step 3: Add the base

Turn the box over and fix the base slats across the bottom, leaving gaps of a centimetre or so between them so that water can drain. Add four small feet or a couple of battens underneath, so that the box sits slightly off the ground. This keeps the base from sitting in puddles and helps it last longer.

Step 4: Drill drainage holes

Even with gaps, a few drainage holes are a good idea. Drill several holes of around 10 millimetres through the base in the places where water might pool. If you are not sure how to avoid problems when drilling, read our summary of common cordless drill mistakes.

Step 5: Finish and fill

Sand any rough edges, then brush on an outdoor finish, following the instructions on the tin. Let it dry fully. A liner of a breathable material, such as landscape fabric, stops soil washing out through the gaps. Place the planter on a flat, level surface, ideally in the spot it will stay in, because it becomes heavy once filled with compost and damp soil.

Frequently asked questions

How long will a timber planter last? That depends on the timber, the finish and the conditions. Good drainage, raised feet and a fresh coat of finish every year or two help it last.

Should I line the inside? A breathable liner reduces soil loss and protects the wood, but make sure water can still drain freely.

Can I make it taller? Yes, but taller boxes hold much more soil and weigh a lot more, so build it where it will stay.