How to Fit Kitchen Cabinets and Carcasses with Wood Screws

A long spirit level (at least 600 mm) is the most important tool for kitchen fitting. It goes on top of every unit as you work. Without it, you have no way to know whether a unit is level until the worktop goes on and it is too late.

Use your spirit level to find the highest point on the floor where the cabinets will sit. Mark this height on the wall. This is your datum line — all base unit legs will be adjusted up or down to reach this same height.

Standard UK base units sit at 870 mm finished height to the top of the worktop (including the 40 mm worktop). Mark a level horizontal line along the full wall at the height of the top of the cabinet carcasses — usually around 830 mm from the high point of the floor. Every unit must reach this line, not the floor.

Select a drill bit at around 70–75% of the screw's core (inner thread) diameter. For a 4 mm woodscrew, a 2.5–3 mm pilot bit is correct. The hole clears a path for the shank and gives the thread something to bite into without forcing the fibres apart.

Drill the pilot hole through the first panel and continue into the receiving panel by at least the length of the thread engagement you need. For a 4 mm × 35 mm screw going through an 18 mm panel, the pilot should run 15–17 mm into the second panel.

Then use a countersink bit to cut the conical recess for the screw head. The head must sit flush or just below the panel face — never proud. A proud head either prevents the adjacent panel from sitting flat or shows through any applied finish.

Clear the floor or a large workbench. Lay the carcass panels flat and check each one against the assembly instructions — most flat-pack carcasses number the panels or colour-code the edges. Identify the top, bottom, left side, right side and back panel before you drill anything.

Check that the surface you are assembling on is flat. A twist in the surface transfers to the carcass. Even a 2 mm twist means a door gap that cannot be adjusted out later.

Mark the screw positions on the face of the first panel using a pencil. For a standard carcass, two fixings at the top and two at the bottom of each side panel are typical. Space fixings evenly. Keep them at least 20 mm from panel corners.

Drill the pilot hole and countersink through the face panel. Then hold or clamp it against the receiving panel and use the pilot hole as a guide to mark — or drill through — into the edge of the second panel. If the pilot continues into the edge, drill only to the depth you need, not all the way through.

A common mistake is driving screws to pull panels into alignment. Screws are not designed to drag one board flat against another. If the joint is not flush before you drive, the screw just locks in the misalignment permanently.

Use at least two G-clamps — one at the top and one at the bottom of each joint — to hold the panels flush and square. Check the corner is at 90 degrees with a square before tightening the clamps fully. Only then drive the screws.

Set the clutch on your drill-driver low. Test on a scrap piece of the same board first. The target is the screw head sitting flush or 0.5–1 mm below the panel face — not buried into it. Over-driving crushes the board fibres around the head, widens the recess, and significantly weakens the joint.

Drive slowly as the head approaches the surface. Impact drivers are generally not recommended for carcass screws in MDF or chipboard — the sudden torque bursts make it too easy to over-drive before you can react. A drill-driver with the clutch set correctly is far more controlled.

Place the first unit in the corner. Adjust the plastic legs until the top of the carcass meets your level line on the wall. Use your spirit level across the top — check both front-to-back and side-to-side. Adjust individual legs until the unit is perfectly level in both directions.

Do not fix the unit to the wall at this stage. Position all base units first, adjust each for level, then go back and fix them once the whole run is sitting correctly.

Most UK kitchen base units are fixed to the wall using two L-shaped metal brackets. Attach one bracket to each rear top corner of the unit. Hold the bracket against the wall and mark the screw positions with a pencil.

Move the unit back from the wall slightly to give you room to drill. Use your pipe and cable detector before drilling. Drill the wall holes using the correct bit for your wall type — masonry bit for brick or block, wood bit for timber stud. Insert appropriate wall plugs and reposition the unit. Screw the brackets to the wall through the plugs.

Hold the wall bracket up to the back of the cabinet. Measure the distance from the top of the cabinet to the base of the bracket's hook point. Measure from the side to the bracket's centre. Transfer these measurements accurately to the wall.

Mark both bracket positions for the first unit, then use your spirit level to draw a horizontal line connecting them. Both brackets must be at exactly the same height or the cabinet will hang at an angle.

Check for solid material behind the plaster. Use a stud detector to locate timber studs in a stud partition, or drill a small test hole to confirm masonry behind a plaster skim. For solid brick or block walls, use masonry anchors or frame fixings. For timber stud walls, drive screws directly into the stud — no plug needed.

Most wall brackets use a 4–5 mm screw through the bracket into the wall fixing. Use the screw specified by the bracket manufacturer. Do not substitute a shorter or narrower screw.

Lift the cabinet slightly above the brackets and lower it so the internal hooks drop onto the wall brackets. Most modern wall units have an adjuster block inside the top of the cabinet — turn it to raise or lower the cabinet until your spirit level reads perfectly level across the front and top face.

Do not move on to the next cabinet until the current one is level and hanging securely. One unit out of level throws off every unit that follows it in the run.

Place a G-clamp at the top and one at the bottom of the two adjacent unit sides. Tighten the clamps until the front faces of both units are exactly flush — run your finger across the joint to feel for any step. Do not drive any screws until the faces are flush.

Check both units are still level after clamping. The clamps can pull one unit fractionally up or down. Re-adjust the level before driving.

The neatest method for joining adjacent carcasses is to fix through the side panel between the hinge-plate positions. Remove the hinge plate from one of the adjacent units. Drill a pilot hole through the side panel of that unit and into the side panel of the next unit. Drive the screw flush. Replace the hinge plate to conceal the screwhead entirely.

You need one fixing near the top and one near the bottom of each joint. Use a 4 mm × 30–35 mm screw — the same size as for your original carcass joints.

A 4 mm × 50 mm screw through an 18 mm face panel into the edge of a second 18 mm panel will break through the far face of the second panel. That is an irreparable hole visible from outside the carcass. Always calculate the thread engagement before ordering screws.

Fix: For 18 mm face-to-edge joints, use 4 mm × 30–35 mm. The screw passes through 18 mm and bites 12–17 mm into the second panel — well short of the far face.

Chipboard and MDF edges have almost no lateral grip. Drive a screw into the edge without a pilot hole and the board splits. The joint fails immediately or within weeks under the load of cabinet contents.

Fix: Always pre-drill a 2.5–3 mm pilot hole into board edges before driving. Keep fixings at least 20 mm from corners and at least 20 mm from the nearest edge of the panel.

A screw buried 3–4 mm into chipboard has not gained extra grip — it has lost it. The crushed fibres around the over-driven head cannot hold the screw from pulling out under load. The recess is too large to re-drive and the hole must be filled before the joint can be remade.

Fix: Set the clutch on your drill-driver to the lowest setting that drives the screw flush. Test on a scrap board. Stop the moment the head is level with the panel face.

Impact drivers are excellent for structural timber work and long screws. For carcass assembly in chipboard and MDF, the sudden rotational impacts make over-driving very likely — you cannot react fast enough to the clutch because there is no clutch. One burst too many and the screw head is buried and the joint is weakened.

Fix: Use a drill-driver with an adjustable clutch for all carcass and panel joinery work. Reserve the impact driver for wall fixings and structural timber.

Plasterboard on its own is not strong enough to support a loaded kitchen wall unit. Fixings into plasterboard alone will pull out over time — sometimes immediately under the weight of crockery. This is a safety hazard.

Fix: Always locate the timber studs or masonry behind the plasterboard and fix into them. If there is no stud in the right position, fit a timber noggin between studs before boarding. For solid brick or block, use appropriate masonry anchors sized for the screw and load.