6 Step Raised Bed Build with Western Red Cedar Boards Over a 3-Metre Run
A 3-metre raised bed in western red cedar runs to roughly six 38mm boards and a Saturday afternoon, assuming you cut the corners square. The wood costs more than treated softwood and lasts longer without it. Here is the build, plus what goes wrong when the run is long enough to bow in the middle.
Western red cedar weathers to a silver-grey within two seasons and holds together at the joints far longer than the treated softwood most builders reach for first. The reason is the natural oils in the heartwood, thujaplicins, which resist rot without any tanalising. A 3-metre run is where the material earns its price. Treated boards over that length tend to cup and pull their screws within four or five winters. Cedar at 38mm thickness stays flatter.
The length also creates the one structural problem worth solving before you start. Anything past about 1.8 metres wants a mid-span brace, or the soil pressure pushes the long sides into a slow belly. Skip that and the bed looks fine in spring and embarrassing by August.
What three metres actually costs you in timber
For a bed 3m long, 1.2m wide and two boards high, you are buying six lengths of 150mm by 38mm cedar at 3m, and four at 1.2m for the ends, plus offcuts for corner posts. At UK and Irish merchant prices that has been running well north of treated softwood, often two to three times the rate per metre. The temptation is to drop to 22mm boards to save money. Over 3m, 22mm flexes badly and the screw heads work loose as the timber moves.
Four corner posts of 50mm by 50mm cedar, cut to the finished height plus 150mm for driving into the ground, do most of the holding. The boards screw to the posts, not to each other. End grain to face grain is a weak joint and splits when the soil swells after rain. A fifth post at the 1.5m midpoint on each long side is the brace that stops the belly. That is ten posts for a single bed, which surprises people pricing it for the first time.
Use stainless or silicon-bronze screws. Cedar’s tannins react with ordinary zinc plating and leave black streaks down the boards inside a year. A box of 100 stainless 5mm by 60mm screws covers the whole build with spares.
The six steps, corner posts first
Start by marking the rectangle with string and four pegs, then check the diagonals match. A 3m by 1.2m rectangle has diagonals of roughly 3.23m. If those two measurements differ by more than a centimetre, the corners are not square and every board after will fight you.
Step two: drive the four corner posts so 150mm sits below grade. A post rammer or a heavy maul does it. Step three: screw the bottom long boards to the posts, two screws per post, keeping the top edge level with a spirit level laid along the full 3m. Step four: add the bottom end boards, then the midpoint braces, sinking those 150mm as well. Step five: the second course of boards, staggered so the joints do not line up with the course below. Step six: cap the top edge if you want somewhere to sit, with 100mm cedar laid flat across the posts.
The order matters because the braces go in after the bottom course but before the second. Try to add them last and you cannot get the rammer in.
Filling it without buying three tonnes of topsoil
A bed this size holds close to 0.9 cubic metres at full depth, which is a lot of bagged compost. The cheaper route layers material the way a hugelkultur mound works. Coarse woody debris and twiggy prunings go in the bottom 200mm, then a layer of turf flipped grass-side down, then half-finished compost, then the top 150mm of screened topsoil or a soil-and-compost blend where the roots actually live.
The woody base breaks down over two or three seasons and the bed settles. Expect to top up 50 to 80mm of compost each spring. That settling is normal and not a fault in the fill. Cane fruit such as autumn raspberries do well in a bed like this, and they want support. Run two strands of galvanised wire between the end posts at 600mm and 1.2m, tension them with a straining bolt, and tie the canes in loosely with soft jute. Galvanised wire over cedar is fine here because the wire never touches the tannin-bearing wet timber directly.
Blueberries are the exception that does not belong in this bed at all. They need ericaceous conditions around pH 4.5 to 5.5, and a general-purpose fill sits far too high. Grow those in a separate container of ericaceous compost where you control the whole root zone, and water with rainwater, since hard tap water creeps the pH back up over a season.
A note on mowing around it
A 3m bed creates four metres of new edge to trim. A Bosch Rotak electric mower will not reach tight to the boards, so leave a 100mm gravel or paving strip at the base to mow over, or accept the strimming.
Sweet peas up the end posts
The corner and brace posts give you ready-made supports for a climbing crop, and Spencer-type sweet peas are the classic choice for cut flowers with long stems and the strong scent the older grandiflora types kept. Varieties like Gwendoline and Mrs Bernard Jones are Spencer selections bred for show-bench stem length.
Sow them in deep root-trainers or cardboard tubes in autumn or late winter, two seeds per cell, in a cool greenhouse or cold frame. Pinch the tips out above the second or third pair of leaves once they reach 100mm, which forces side shoots and a bushier plant with more flowering stems. Sweet peas resent root disturbance, so the deep narrow cell that slides out intact matters more than the compost brand.
Hardening off is where autumn-sown sweet peas get killed by impatience. Move the trays from the cold frame to a sheltered outdoor spot for a week, closing the frame light at night for the first few days, then leaving it propped, then off entirely. Plant out at the foot of the posts when the soil has lost its winter chill, usually once daytime temperatures hold in double figures. Net or twine spirals up the 50mm posts give the tendrils something to grip, since they cannot climb a smooth square edge on their own.
When the long side bows anyway
Suppose you built it without the midpoint braces, because the bed looked solid empty and the extra posts felt like overkill. By the second wet spring the long boards have pushed out 30 to 40mm at the centre. The fix does not require dismantling anything. Drive a single post hard against the outside of the bulge, then run a length of galvanised wire across the bed at soil level to the matching post opposite, and tension it with a straining ratchet until the boards pull back to straight.
The wire sits buried in the fill and does the job the brace post should have done from the start. It is uglier and you will catch it with a trowel occasionally. The lesson holds for any bed past 1.8m: the soil does not care how good the corners look.
What the cedar buys you is time before any of this maintenance compounds. The open question is whether the price premium pays back for a gardener who moves house every few years, or only for the one who expects the same bed to be standing in fifteen.
