Build a Under-Stair Drawer With Blum TANDEMBOX Runners in 8 Steps for 30% More Storage

Why the runner choice decides the build

Blum TANDEMBOX antaro runners are sold with 30 kg and 65 kg load ratings. For an under-stair drawer, the usual choice is the 30 kg version in a 550 mm nominal length, because the cabinet normally has to fit below a sloping stair string and still pull fully clear of it.

Full extension matters here. A drawer that comes out from the side of a staircase has to expose its whole box, otherwise the rear section remains buried under the third or fourth tread. The TANDEMBOX mechanism gives 100 percent extension, with the BLUMOTION soft-close damper built into the rear bracket, so a 900 mm cabinet can pull clear of the stair string when the sizing works.

Size the runner from the cabinet’s internal depth. If the triangular void measures 780 mm deep at floor level and the cabinet box is 550 mm deep, the correct order is for 550 mm runners. The remaining 230 mm can stay as open shelving or unused space behind the moving box.

A mismatch between runner and cabinet depth causes problems at the front and rear of the travel. A 600 mm cabinet on 550 mm runners leaves the front proud by the depth difference, and the BLUMOTION damper fails to engage because the drawer reaches the closed position before the trigger point is reached. Confirm the cabinet internal depth first, then choose the runner length.

Steps 1 and 2: measure the void and cut the carcass

Measure the under-stair void at three depths: at the floor, at 200 mm up, and at 400 mm up. The readings will usually differ because the stair string slopes across the side of the space. The drawer cabinet has to pass under the lowest point of that slope for its full movement, so take the smallest usable height and subtract 20 mm for clearance.

That measurement becomes the outside height of the cabinet. In many staircases, the front face of the carcass ends up around 500 to 620 mm tall. Mark the width from the opening you want to fill, allowing for an even reveal around the finished front.

Cut the carcass from 18 mm birch plywood or 18 mm MDF. Birch plywood holds runner screws better under repeated 30 kg loading, which matters because each TANDEMBOX cabinet profile is fixed with four 4 x 16 mm screws into the cabinet side. Cut two side panels, a base, and a back. A top panel is optional because the stair treads already form the ceiling of the cavity, although adding one stiffens the box against racking when the drawer is loaded and extended.

Steps 3 to 5: fit the profiles and assemble the drawer

Mount each TANDEMBOX cabinet profile with its front edge set back 1.5 mm from the cabinet front. Blum’s ZML.0010 drilling jig locates the screw holes at System 32 spacing and takes most of the judgement out of the layout.

Fasten the left and right profiles at exactly the same height. A 2 mm difference between sides can make the box bind at half extension and make the soft-close action stutter. Check the marks before drilling, then check again after the screws are seated.

The drawer box is built from TANDEMBOX side profiles and a steel back. The side profiles clip onto the runners at the front using the locking dowels in the box base. Push the box home until both front catches click.

Lay a 600 mm spirit level across the open top of the drawer box and check that it sits level. If one side is high, look first at the cabinet profiles, since an uneven fixing height shows up as drag or twisting during travel.

Racking faults tend to appear during these middle steps. The usual cause is a carcass that was out of square before the profiles were fitted. A small diagonal error in the cabinet can show as an uneven drawer-front gap once the box is clipped on, especially when the drawer is extended and carrying weight.

Set the BLUMOTION damping to suit the expected load. The rear locking device has a small adjustment wheel; turn it toward the plus mark for heavier contents. A drawer carrying 25 kg of tinned food needs more damping force than an empty drawer, and the factory setting assumes a mid-weight load.

Adjust the front in three planes

The TANDEMBOX front bracket allows height, side, and tilt adjustment after the front is clipped on. Use the height cam at the back of the front fixing bracket, the side adjustment on the front face, and the tilt setting to bring the reveal to an even 2 to 3 mm all round.

Step 6: fit the front panel and finish the oak

Matching the drawer front to an existing solid oak parquet floor or oak flooring turns the unit from hidden storage into visible joinery. A 20 mm solid oak front can be cut to the cabinet front dimensions plus the reveal allowance. It mounts to the TANDEMBOX front fixing bracket through standard EXPANDO fittings or by screws driven into the rear face.

The extra weight of oak has to be counted as part of the drawer load. At 20 mm thick, a solid oak front adds roughly 4 to 6 kg, and the 30 kg runner rating covers the drawer box, the front, and the contents together. That is why the front material belongs in the load calculation before the drawer is filled.

Danish oil suits oak in a hallway because it cures by oxidation into the timber surface and leaves a low-sheen finish that does not form a film over the grain like polyurethane varnish. Apply it in thin coats with a lint-free cloth. Three coats, each left for 6 to 8 hours and lightly de-nibbed with 320 grit between coats, gives a finish that resists typical scuffs from a high-traffic drawer. End grain absorbs the first coat heavily, so the cut edges need an extra pass.

Steps 7 and 8: install, level, and load-test

Slide the assembled cabinet into the under-stair void and shim it level on the slab before fixing. Packers under the base carry the drawer weight into the floor, so the carcass is supported along its base as the loaded drawer moves forward.

Fix the back of the carcass to the staircase carriage or the adjacent wall with at least two 5 x 60 mm screws driven into a stud or into plugged masonry. These fixings stop the cabinet creeping forward when the 30 kg drawer is pulled out, which is a common failure point on under-stair units that have only been pushed into place.

Load-test the drawer before relying on it for storage. Place 25 to 30 kg of weight evenly in the box, pull it to full extension, and watch the front edge of the cabinet. If the cabinet lifts at the front, it needs stronger fixing at the rear or better support through the floor.

A drawer that feels smooth when empty and binds under load usually has a runner height mismatch that appears only when the box flexes under weight. Once the cabinet is fixed and the runners are aligned, the BLUMOTION should pull the loaded drawer through the last 50 mm of closing travel after a gentle push.

The volume the drawer actually recovers

A staircase with 14 risers over a 2.6 metre run encloses a wedge that tapers from full ceiling height at the bottom to nothing at the top. The bottom third contains the useful height for a side-pull drawer, which is where the 30 percent recovery figure comes from. The upper two thirds is lower, awkward to reach, and normally accessible only by crawling.

Access above the drawer would need a separate mechanism, such as a top-hinged hatch through the tread riser or a pull-out tower fixed to the carriage. The upper void above the moving box remains low, awkward, and outside the reach of a side-pull drawer.