Under-Stair Drawers Built with Blum Tandembox Runners Across a 2.4-Metre Void

The space below a straight-flight staircase forms a wedge in section. Under the upper treads, headroom can approach 1900 mm; near the base tread, it can fall below 200 mm. A drawer bank fitted into that wedge has to graduate: the innermost unit is the deepest and tallest, then each bay becomes shorter and shallower as the run approaches the newel post.

Blum Tandembox runners suit this layout because their double-wall steel sides carry the drawer box load directly through the runner system. A separate timber box side is removed from the main load path, which matters when one deep drawer under the top steps is filled with 30 kilograms of tinned goods.

Runner length governs the layout before the carcass does. Blum supplies Tandembox in nominal lengths from 270 mm to 650 mm in 50 mm increments, with dynamic load ratings of 30 kg or 65 kg depending on the pairing. Across a 2.4-metre horizontal void, the drawer depths have to change because the treads rake upward and the fronts must clear the string. For a domestic flight rising at 42 degrees, a workable sequence is a 650 mm drawer at the deep end, then 550 mm, 450 mm, and a shallow 350 mm unit near the base.

Datum, partitions, and runner positions

Strike a laser line along the finished floor and carry it up the wall face before panel cutting begins. Under-stair floors are often 3 to 8 mm out of level over 2.4 metres, and a runner pair fixed into a twisted carcass can bind within the first 200 mm of travel. Pack the carcass base until it follows the laser datum across the whole run.

Set out the carcass verticals from the deep end. The first partition sits at 600 mm from the back wall, giving a clear internal width that accepts a 550 mm Tandembox with the required 12.5 mm clearance on each side. Each drawer box needs 25 mm total side clearance inside its opening for the runner mechanism.

That tolerance is small enough to show in the finished bank. If the opening is tight, the drawer front can rub the neighbouring partition as soon as the cams are adjusted. If the bay is made too generous, the front gaps start to read against the raking string above. The aim is a drawer aperture that gives the runner its clearance while leaving enough front width to cover the construction lines.

Use the Tandembox drilling jig to set the runner screw centres. Blum gives the fixing dimensions in the Tandembox assembly instructions: the cabinet profile screws land at 37 mm from the front edge and repeat at 32 mm centres. A hand-marked runner can drift by a millimetre at each fixing, and that error stacks up across a bank.

By the fourth drawer, the accumulated drift can push the top front out of line with the base front. The error is especially visible under a stair because the sloping string already gives the eye a second reference line. Keeping the runner line disciplined at the start makes the front adjustment less dependent on the limited movement available in the cams.

Work from the deep bay outward when fixing the profiles. The largest and heaviest fronts sit where the drawer travel is longest, so that end deserves the cleanest datum and the most accurate first fix. Any small loss of tolerance toward the shallow end is less visible because the fronts are smaller.

The deep unit and its load

The drawer under the top four treads is the one that usually takes heavy, rarely moved items. At 650 mm deep with a 400 mm internal height, it can swallow a domestic vacuum cleaner or a stacked column of paint tins. When the drawer is open, that load sits at full extension, where the leverage on the runner front bracket is at its worst.

Blum rates the standard Tandembox pairing at 30 kg dynamic. The Tandembox intivo or the heavier Legrabox pairing reaches 65 kg. For the deep unit, use the 65 kg pairing and full-extension runners so the back of a 650 mm drawer comes completely clear of the front partition.

Partial-extension runners leave the rearmost 150 mm buried under the tread structure. That strip is also where the void is deepest and the recovered volume is most valuable. On a tall under-stair drawer, losing the back of the box changes how the storage can actually be used.

The base-tread drawer most banks lose

Under the bottom two treads, headroom collapses below 200 mm, and a conventional rectangular drawer front has no clean place to sit because the raking string cuts diagonally across the aperture. A shallow toe-kick drawer on a 270 mm Tandembox recovers that abandoned space: the front is cut as a parallelogram to follow the string rake, and the runner is mounted horizontally on a packer that lifts it clear of the sloping soffit above. The drawer holds shoes or cleaning cloths in a 90 mm internal height, though it is often skipped because the string profile has to be scribed, transferred to the drawer front, and checked before the runner position is finalised.

A worked depth-and-clearance calculation

Take a flight rising 42 degrees with a 250 mm going per tread. Over the 2.4-metre horizontal void, the soffit rises 2400 x tan(42), roughly 2160 mm of vertical gain, though the usable height is capped by the ceiling under the landing.

Set the deep drawer aperture at 700 mm wide and plan around a 650 mm Tandembox. The drawer box internal depth becomes the runner length minus the 35 mm reserved by the mechanism at the back, giving 615 mm of clear interior. Front-to-partition clearance remains 25 mm total.

The next aperture inward is set 620 mm from the back wall so its 550 mm runner clears the rising soffit by at least 50 mm at full extension. Repeat the reduction inward: a 450 mm runner in a 520 mm-deep aperture, then a 350 mm runner in the shallow bay near the base. The four drawers together recover approximately 1.2 cubic metres of the 1.6 cubic metres contained by the void, with the balance lost to carcass thickness, runner reserves, and the toe-kick soffit.

The 350 mm base drawer shows the constraint clearly. At that station, the soffit sits only 380 mm above the floor datum, so the drawer front is capped at 300 mm high and the box internal height at 180 mm. Push the runner deeper and the drawer back fouls the underside of the third tread.

Site order and front lines

Fit the carcass first, dry-assemble it, and shim it to the laser datum. Fix the runners next, with the cabinet profiles screwed to the marked centres from the deep end outward. Drawer boxes are then clipped on, followed by the fronts, which are hung and adjusted on the Tandembox front-height and tilt cams. Those cams give 3 mm of adjustment in each axis.

Do the front adjustment with all drawers closed and a consistent reveal set with 3 mm spacers between fronts. The raking string above puts each top reveal at a different angle against its neighbour, so the eye reads the vertical gaps between fronts first. Set those vertical gaps dead equal and the raking cut above becomes less noticeable.

The awkward part is that a drawer bank can be geometrically correct while still looking wrong against the underside of a staircase. That is why the straightest-looking installation is often the one whose top edges refuse to match the rake above.