A floating shelf looks effortless, but the illusion depends entirely on what you can't see. Get the batten and the fixings right and a modest length of oak will carry tins, cookbooks or a row of mugs without a single visible bracket. Get them wrong and it will sag, split, or part company with the wall altogether. This project suits a weekend afternoon, a 40mm-thick board of European oak, and a router or a sharp rebate plane.

Choosing the Oak and Settling on Proportions

Start with the board, because the dimensions dictate the hardware. A 40mm thick, 200mm deep shelf is the sweet spot for alcoves and hallways: chunky enough to look substantial, slim enough not to crowd a small space. For kitchens, go slightly deeper at 250mm so a dinner plate or a tin sits comfortably. Keep unsupported runs to 1,200mm maximum, and halve that if you are spanning between two alcove walls without any support in the middle.

Buy kiln-dried oak that has been in the house for at least a week before you cut it, so it has acclimatised and any movement happens before you fix it down. Check the board for cupping with a straight edge rather than trusting your eye. A gentle bow can be planed out; a twisted board should go back to the timber yard.

For the hidden batten you want a piece of planed timber roughly 60mm wide by 20mm thick, cut 50mm shorter than the shelf so it disappears into the wall's shadow when the shelf is fitted. If the wall is uneven, a slightly thicker batten gives you more material to scribe back.

Measuring, Marking and Levelling

A shelf that looks level but is fixed to a sloped ceiling will read as crooked forever, so take your datum from the room rather than the wall.

• Decide the height and mark it at both ends with a pencil, then strike a line with a long spirit level or a chalk line.
• Check the level in both directions — long level along the wall, short level front to back — before you commit to a single hole.
• Find the studs with a detector and mark them in faint pencil above your line.
• Measure the width of the alcove or wall at three heights: top, middle and bottom. Solid walls are rarely parallel.

If the walls are out of square, cut the shelf to the widest measurement and scribe the ends to fit with a plane. A 2mm gap is easier to hide than a shelf wedged in so tightly it splits.

Routing the Hidden Rebate

Clamp the shelf upside down on a pair of trestles, face side down on scrap battens so the underside stays clean. Mark a line 20mm in from the back edge, then set a router with a straight cutter to take out a channel 21mm deep and 61mm wide — just enough clearance for the batten to slide in without forcing.

Take two or three passes at increasing depth rather than one heavy cut, moving the router steadily and keeping the base flat. Square the ends of the rebate with a sharp chisel, working from the outside in. If you do not own a router, a rebate plane and a steady hand will do the same job in twenty minutes.

Test-fit the batten into the rebate before you touch the wall. It should slide on with light hand pressure and no rocking.

Fixing the Batten Securely

This is the part that determines whether the shelf holds. The batten must go into something structural, not just plasterboard.

• Masonry: use 8mm x 75mm countersunk screws with matching plugs, at least four along a 1,200mm batten, one every 250mm.
• Studwork: use 5mm x 80mm screws driven into the stud centres, and add a plasterboard anchor between studs only for stability, never for load.
• Dot-and-dab plasterboard: avoid relying on the board alone; find the blockwork behind or fix into the studs at the reveal.

Drill pilot holes through the batten, countersink them, and hold the batten on your marked line while you drill the first hole. Fix one screw at each end, check the level again, then add the remaining screws. If the wall is bowed, pack behind the batten with thin plastic shims rather than over-tightening and pulling it out of shape.

Fitting, Finishing and Keeping It True

Sand the shelf through 120, 180 and 240 grit, always along the grain, and ease the arrises with a couple of passes so the front edge feels soft in the hand. Apply three coats of Danish oil or a hardwax oil, wiping off the excess after twenty minutes and leaving eight hours between coats. Burnish the final coat lightly with 0000 wire wool for a satin sheen.

Push the shelf onto the batten, checking that it sits flat along its length. If you want the shelf to stay put, run a bead of clear silicone along the back edge of the rebate or drive two 30mm panel pins down through the top face into the batten, filled afterwards with a wax filler stick matched to the oak.

Finally, run a thin bead of decorator's caulk where the shelf meets an uneven wall and smooth it with a damp finger. Six months on, check the fixings are still snug — a quarter turn on each screw is all a well-built oak shelf should ever need.