An undermount runner is a drawer slide fixed to the underside of the cabinet, with the drawer box carried on two rails beneath it. Nothing shows when the drawer is open, and the drawer uses the full internal width of the opening.
How it carries the box
Two rails are screwed to the cabinet base, one each side. The drawer box has a locating hook at the front and a latch at the back that clips onto the runner. Fitting is a matter of positioning the rails square and at the right depth — after that the drawer clicks in and adjusts.
Because the box hangs on the rails rather than sitting between them, the box has to be narrower than the opening by only a couple of millimetres, and it needs a recessed or rebated base to clear the mechanism. That is the 20–30 mm of height it costs.
| Situación | Veredicto | Por qué |
|---|---|---|
| Cajones de cocina | fuerte si | Full width, soft close, hidden |
| Tocadores de baño | Sí: | Clean look, easy to wipe out |
| Shallow drawers under 90 mm | No | Mechanism needs clearance |
| Heavy pan drawers | Yes, check rating | 40–45 kg available at 500–600 mm |
| Retrofit to an existing box | No | Box and carcass both need rework |
The height budget
A 90 mm drawer front might only allow a 60 mm box once the runner clearance is taken. This is why undermount drawers are designed as a system: the box height, the front height and the runner are chosen together, not independently.
On a bank of three drawers in a 720 mm base unit, that height budget is what decides whether the top drawer is useful or decorative. Work it out before the cutting list, not after.
Adjustment and the common faults
Undermount runners adjust in three directions at the front latch: height, side-to-side and tilt. Tilt is the one people miss — a drawer front that is tight at the top and open at the bottom is a tilt adjustment, not a hinge problem.
The common faults are a rail fixed out of square, which makes the drawer bind halfway, and a box built 1 mm too wide, which makes it scrape. Both are workshop errors that show up on site and cannot be adjusted out.
Preguntas frecuentes
Are undermount runners worth the extra cost?
For kitchen and vanity drawers, usually yes: full width, soft close and nothing visible. For utility or workshop drawers, side-mount does the job for less.
How much height does an undermount runner need?
20–30 mm beneath the drawer box, depending on the system. That has to be allowed for when the box height is set.
Can undermount runners be fitted to an existing cabinet?
Rarely worth it. The rails need a clean, square base and the drawer box has to be rebuilt to suit.
Why does my drawer bind halfway out?
A rail fixed out of square, or a box slightly too wide. Neither can be adjusted out at the front latch — the rail has to be re-fixed.
