Fitting Under-Drawer Runners: Installing Concealed Slides for Wooden Drawers
An undermount runner is a concealed slide system fitted underneath the drawer base, keeping the side panels of the wooden drawer visually clear.
Precision in Concealed Areas: Undermount Installation
When a wooden drawer needs to show off its full visual appeal, visible metal runners on the sides spoil the look. The solution for high-end cabinet making is the undermount runner. This system is installed invisibly beneath the drawer base. It acts as the concealed backbone of the drawer, supporting the load whilst ensuring smooth, quiet running.
Fitting requires precise work and strict adherence to dimensional specifications. Unlike simple side-mounted runners, the drawer box must be constructed specifically for undermount fitting. The base must be recessed, and the side walls must extend downwards. This guide is aimed at fitters and experienced joiners looking to install concealed undermount drawer runners properly into a cabinet carcass.
The time required for initial installation, including measuring and adjustment, is around 30 to 45 minutes per drawer.
Required Tools and Materials
To accurately position and screw the runners into the cabinet and attach the locking clips to the drawer, prepare the following tools:
- Tape measure or folding rule
- Precision try square
- Pencil or marking awl
- Cordless screwdriver with matching bits (usually PZ or TX)
- Wood drill bits (Ø 6 mm for the rear hook hole, Ø 3 mm for pilot holes)
- Countersunk chipboard screws (typically Ø 3.5 x 16 mm or 4 x 16 mm)
- The runner system (left and right)
- Two locking clips (left and right) to secure the drawer onto the runners
Preparation: Checking the Wooden Drawer Construction Dimensions
Before screwing the runners into the carcass, ensure that the wooden drawer meets the mandatory construction dimensions required for undermount installation.
- Base Recess: The drawer base must not sit flush with the bottom edge of the side walls. It must be grooved in and set upwards by exactly 12 mm - 15 mm. This void accommodates the runner mechanism later.
- Internal Width: The drawer width is strictly tied to the internal width of the cabinet (the clear width). For a drawer side wall thickness of 16 mm, a deduction of 10 mm from the clear cabinet width is generally used to calculate the outer width of the drawer. For 19 mm drawer sides, this deduction changes according to the runner manufacturer's specifications.
- Depth: The nominal length of the runner must match the drawer depth. For a drawer with an outer length of 500 mm, you strictly require drawer runners 500 mm. If the cabinet is shallower, the drawer must be planned shorter, for example using drawer runners 400 mm.
Step-by-Step Guide: Fitting Undermount Runners
Correct operation, particularly for systems with integrated soft-close or push-to-open mechanisms, depends entirely on the exact parallel alignment of the cabinet runners.
Step 1: Marking the Position of the Cabinet Runners
Measure the desired height of the drawer inside the cabinet. Draw a horizontal line on the inside face of the carcass. This line typically marks the position of the screw fixing holes on the runner. Pay attention to System 32 if your cabinet already features continuous line boring. The runners are punched to align precisely with the 32 mm hole pitch.
The distance from the front edge of the cabinet to the first screw is determined by the drawer front design. For an overlay front, the runner is usually mounted flush or with a minimal setback of 2 mm from the front edge of the carcass. For an inset front, the runner must be set back by the thickness of the front panel (e.g. 19 mm) plus a small clearance gap.
Step 2: Screwing the Cabinet Runners into Place
Align the runner with the marked line. Use a bradawl or a 3 mm wood drill bit to mark the screw centres lightly.
Fix the runner using the chipboard screws. Use at least three screws per runner: one in the front third, one in the centre, and one towards the rear. Ensure the screw heads are fully countersunk flush into the metal profile. Protruding screw heads will obstruct the runner carriage later. Repeat the process on the opposite side of the cabinet. Use a tape measure to check that both runners sit at the exact same height and depth.
If you are installing full extension drawer runners, extend the runner completely once to ensure the mechanism does not catch or rub.
Step 3: Fitting the Locking Clips to the Drawer
Turn the wooden drawer upside down so the bottom faces upwards. The locking clips form the connection between the drawer and the runner. Place the left-hand clip into the front left corner of the recess created by the base setback and the side wall. The clip must sit flush against the inner side of the side wall and flush against the back of the drawer front.
Screw the clip diagonally into the front panel and/or laterally into the drawer side. Strictly monitor the screw length. With a front thickness of 16 mm, screws must not exceed 14 mm in length to avoid piercing through the front face. Install the right-hand clip in mirror image.
Step 4: Drilling for the Rear Retaining Hook
A crucial detail of undermount installation is the rear pin or hook on the back of the runner. This engages with the drawer back panel and prevents the rear of the drawer from tilting upwards when pulled out.
Measure the position of the pin on the runner. Transfer this measurement to the rear panel of the drawer. Drill a hole on the left and right at the marked points with a diameter of 6 mm and a depth of approximately 10 mm.
Step 5: Inserting and Adjusting the Drawer
Pull the cabinet runners out slightly. Place the drawer horizontally onto the runners. Push the drawer backwards into the cabinet with steady, even pressure.
When the drawer reaches the correct position, you will hear a clear click. The locking clips engage with the runners, and the rear hooks seat into the prepared drill holes.
Check the gap pattern around the drawer front when closed. Modern undermount clips feature small adjustment wheels or levers. These allow tool-free fine adjustment of the height (typically +3 mm) and sometimes the tilt of the front. Turn the thumbwheels until the reveals run parallel to adjacent doors or drawers.
Avoiding Common Installation Mistakes
Even experienced craftspeople occasionally make mistakes during undermount runner installation. Pay particular attention to the following points:
- Confusing Mechanisms: A common misunderstanding is treating soft-close and push-to-open as the same feature. Soft-close is a damper that gently brakes the drawer over the final centimetres and pulls it shut quietly. Push-to-open (or tip-on) is the exact opposite: a mechanical spring ejects the drawer when pressed, removing the need for handles. A runner cannot perform both via the same mechanical path (with the exception of specialised hybrid hardware). If you are planning handleless fronts, you must explicitly install push-to-open drawer runners.
- Incorrect Width Deduction: Calculating the clear internal cabinet width incorrectly causes the drawer to bind. If the drawer is just 2 mm too wide, the runner will rub heavily and push the slides outward against the carcass sides.
- Missing Rear Holes: Forgetting to drill the 6 mm holes for the rear hooks causes the drawer to sit awkwardly on top of the pins. The runner cannot engage properly, causing the drawer to drop downwards when fully extended.
FAQ: Frequently Asked Questions About Undermount Installation
What is the difference between full extension and partial extension runners? A full extension runner pulls out 100% from the cabinet carcass, allowing access to the entire depth of the drawer right to the back wall. A partial extension runner extends only to roughly 75% due to its design, leaving the rear part of the drawer inside the cabinet. Partial extension is often found on standard furniture with self-closing drawer runners, whereas full extension provides greater convenience.
Can I simply replace old roller runners with undermount runners? No, a direct like-for-like swap is almost never possible. Roller drawer runners are screwed to the sides or bottom edges and require side clearance (usually 12.5 mm per side). Undermount runners require a specific recessed base and exact internal width calculations. Existing wooden drawers would require major structural alterations to accommodate the change.
What runner length do I need for my drawer? The nominal length of the runner is determined by the external depth of your wooden drawer box. Measure the drawer from the back of the front panel to the outer face of the rear panel. If this measures 450 mm, select drawer runners 450 mm. The internal cabinet depth must be at least 3 mm to 5 mm greater than the nominal length of the runner to provide sufficient clearance.
Why is my soft-close drawer not closing completely? When soft-close drawer runners stop short over the final centimetres and fail to pull shut completely, it is usually due to an installation error. Either the cabinet carcass is out of square (runners taper inward towards the back), or the drawer is slightly too wide relative to the clear opening, exerting lateral pressure on the dampers. An improperly seated rear hook can also obstruct the mechanism.