Measure a Reveal for Roller Blinds using a Bosch GLM 50 Laser

A reveal, in fitting terms, is the depth of the recess between the front face of the wall and the glass line. It is also the surface a recess-mounted roller blind clamps into, so a small error in the measure can turn into a bracket that binds or a fabric edge that rubs.

The Bosch GLM 50 is useful because a steel tape can sag across a 1.8m opening and read long by 4 to 6mm at the centre. The laser reads a flat perpendicular distance with a stated accuracy of plus or minus 1.5mm. In the GLM 50 menu, set the measurement reference to the back edge of the tool, put the front of the unit against one jamb, and fire the beam at the opposite plaster face. The number on the display is the first width to record.

Width readings inside the recess

Take three widths: 50mm down from the head, at the vertical midpoint, and 50mm up from the sill. Plaster reveals are rarely parallel, especially in older openings. In a 1980s cavity-wall opening, a sill reading 5 to 9mm wider than the head is a familiar result; settlement and skim variation can both be part of that difference.

For a recess-fit roller, use the smallest of the three widths as the controlling figure. The fabric, tube, end caps, and brackets all have to pass through the tightest part of the drop, so the wider readings cannot rescue a pinch point higher up or lower down. Keep all three numbers in the notes, because the spread tells you how the blind will sit visually inside the opening.

If the spread is greater than 10mm, a clean recess fit becomes unlikely. A face-fix above the reveal is then the more honest option, because the larger fabric width can hide the uneven jambs instead of exposing them as a tapered side gap.

Depth checks against glass, handles, and motors

A standard 32mm roller tube with brackets needs roughly 55 to 70mm of clear reveal depth to sit behind the front face without the fabric roll touching a window handle or a tilt-and-turn hinge. Use the glass line or the closest projecting fitting as the inner reference. The frame edge gives a depth that is too generous when a handle, hinge, sensor, or seal sits proud of it.

Point the GLM 50 along the reveal depth at both top corners and both sill corners. A projecting espagnolette handle or a Somfy motorised blind sensor housing may take 20 to 30mm of clearance at one local point, while the other corners still look workable.

On a chain-operated blind, bracket depth is fixed by the manufacturer, often 40 to 45mm to the tube centre. On a Somfy motorised tube, the motor crown adds nothing to depth, although the manual override eye or the sensor bracket can protrude. Put any planned Somfy sensor in its intended position before taking the depth reading, otherwise the ordered blind can bind against its own control.

Where the depth falls below 55mm, a slimline 25mm tube is available. That smaller tube caps the maximum drop to around 2.1m before the fabric roll becomes too fat for the shallow pocket.

Diagonal readings for squareness

Fire the GLM 50 from the top-left internal corner to the bottom-right, then from the top-right to the bottom-left. Use the tool’s Pythagoras or point-to-point function, with the unit braced into each corner so the two diagonal readings are taken from the same internal fitting plane.

Equal diagonals indicate a square opening. A difference of 8mm across a 1.5m by 1.2m recess indicates a racked opening with a parallelogram twist, and a rigid rectangular blind dropped into it will show a triangular light wedge of 3 to 5mm down one side.

There is no measurement correction that makes an out-of-square recess square. The fitting choices are to accept the light gap, order to the smaller dimensions and centre the blind, or move to a face-fix with an oversized fabric width. In a bedroom blackout job, that wedge is usually fatal and pushes almost every racked reveal to face-fix with 40mm side overlap. In a kitchen where daytime privacy is the brief, a centred recess blind with a 4mm gap is usually unremarkable.

Deductions before ordering

Bracket systems consume clearance, and each supplier states its own deduction. A common spring-and-pin chain bracket removes 6 to 10mm in total across the pair. With a 10mm deduction, a 1200mm controlling reveal becomes a 1190mm fabric width order.

Motorised systems use different allowances. A Somfy Sonesse or comparable tube often specifies a total deduction of 15 to 20mm because the motor bracket is bulkier than a spring pin.

Write the deduction beside the laser reading as soon as the measurement is taken. A frequent error is to order the raw laser figure of 1200mm, receive a blind that jams on both jambs, and lose the cost of a made-to-measure unit that cannot be returned.

For drop, measure from the intended bracket line at the head down to the sill with the GLM 50 pointed straight down. Add 150 to 200mm so the fabric bottom bar overhangs the sill fully when lowered. A roller that stops 20mm short of the sill leaks a horizontal light bar every morning.

A typical chain-bracket example shows the arithmetic. The reveal reads 1204mm at the head, 1201mm at the middle, and 1206mm at the sill. The controlling width is 1201mm.

With a 10mm chain bracket deduction, the order width is 1191mm. The depth reads 62mm at three corners, yet only 41mm at the top-right where a tilt-and-turn hinge sits. A 32mm tube is marginal in that corner, while a 25mm slimline tube is the safe order.

The drop in the same case reads 1420mm to the sill. Ordering 1570mm of fabric gives the overhang needed for the bottom bar to pass the sill fully.

North-facing loft reveals

A north-facing loft conversion rarely gets direct sun, so the blind’s role shifts from glare control to condensation and ventilation management. North facing loft ventilation depends on air moving across the glass. A tightly sealed recess blind dropped flush to a Velux or dormer can trap a still-air layer against cold glazing, with overnight condensation pooling on the sill.

Measure the loft reveal with the same three-height width sequence and the same corner-depth checks. In the drop calculation, leave a deliberate 15 to 20mm bottom gap so warm room air can circulate behind the fabric.

Glazing type changes the depth figure. A retrofit sealed unit or laminated glass noise reduction pane sits proud of an older single-glazed rebate by 8 to 14mm, so a reveal measured before the glazing upgrade will be too deep after the new glass goes in. Measure after the glass is installed.

Where a Ventrolla sash weatherseal has been added to a period casement in a loft, the brush or wiper seal may project into the reveal by 4 to 6mm along the meeting rail. Use the seal as the contact face for the GLM 50 depth reading. Reading to the timber behind it lets the blind crush the weatherseal on every close.

Laser target limits

The GLM 50 reads to a hard surface, and dark, matte, or glossy finishes can weaken the return signal at longer range. Against black-painted trim beyond 8m, the tool can drop out or read erratically by 5 to 10mm. For a reveal deeper than 6m or for a dark far jamb, the supplied target card or a strip of white paper gives the beam a cleaner return and keeps the last reading tied to the plaster face.