6 Awkward Bay Angles Templated with a Bosch GLM 150 on a Victorian Terrace

142 degrees on the left mullion, 128 on the right. Those figures came off a splayed bay in a Bristol terrace, and they decide whether roller tubes meet cleanly in the corner or leave a light gap visible every evening. The GLM 150 reads the angle directly in inclination mode, stripping out protractor guesswork and the old habit of scribing a bevel onto scrap MDF for the workshop. Many fitters still template a Victorian bay as if all three faces sit at a tidy 135, then find the returns binding.

A canted bay’s splay came from whichever bricklayer laid the sill in the 1880s, without much help from a drawing. Two bays on the same street rarely match. The received measuring method treats the bay as a symmetrical trapezoid and prints a cutting sheet from that assumption.

Reading the internal angle at the head reveal

Put the GLM 150 into angle mode and lay it flat against the plaster reveal at head height. Sills on a 140-year-old bay have dropped, twisted, and collected enough paint to sit a degree or two away from the reveal above. The head reveal carries the blind bracket, so the fabric width has to respect that surface.

On the terrace bay, the sill gave 137 degrees at the left junction while the reveal directly above gave 142. A blind cut to the sill figure would have overlapped its neighbour by roughly 6mm at the top corner, enough for the fabric edges to kiss and jam on a 45mm roller tube. Measuring both faces from the reveal, then taking a 3mm deduction each side, kept the tubes parallel to the glass and clear of each other.

The GLM 150 stores up to 20 readings, so the six measurements across a three-sided bay, three internal splays and three face widths, can sit in memory before anyone touches a saw. That matters when the stepladder is already in place. Re-measuring a bay after the scaffold board has been dismantled is how a morning job turns into a full day.

Perfect fit frames and tapered sash beads

Perfect fit reveal measuring assumes a rectangular opening with sash beads deep enough to grip a 20mm to 26mm frame. A Victorian sash box rarely offers that. The staff bead on an original timber sash is often only 15mm proud, and on the canted returns of a bay it follows the splay angle, so a square perfect fit frame pushed into a 142-degree corner leaves a wedge of daylight at the outer edge.

The saving measurement is bead depth, taken at four points per sash with the GLM 150 nose tucked into the rebate. On the middle face of the terrace bay, the depth ran from 22mm at the cill to 17mm at the head, a 5mm taper caused by a century of paint layered onto the lower bead. A perfect fit frame needs a minimum consistent 18mm grip, so the head end failed and the frame moved to a face fix with side channels.

Honeycomb cell shades change the calculation. A cellular shade in a perfect fit frame weighs almost nothing, and the frame can be trimmed to follow the splay if it is ordered as a trapezoid. The double-cell 25mm fabric also buys back some of the draught performance lost when the blind moves outside the timber rebate, because the trapped air in the cells sits directly against the cold single glazing most of these bays still carry.

Warm-edge spacer bars at the cold corner

If the bay has been reglazed with sealed units, the spacer bar around each pane decides where condensation forms. Aluminium spacer bars conduct heat straight out at the glass edge, so the perimeter of every pane runs colder than the centre, and on a north-facing bay that edge is where the water beads first.

Warm-edge spacer bars, the grey composite or stainless strips such as Swisspacer or the Edgetech Super Spacer, drop the edge conductivity enough for the glass perimeter to stay a few degrees warmer. On a bay, the corner mullions are already the coldest timber in the room because they present two external faces to the weather. Pairing warm-edge units with a blind that traps air, again the honeycomb cell, keeps the corner glass above the dew point on most winter mornings. A roller blind does nothing here because its fabric hangs 30mm off the glass with open sides.

The mullion post that takes the fabric width

Between each face of a canted bay sits a timber mullion post. On the terrace it measured 44mm wide. That post is dead space no fabric can cover, and it is where two adjacent blinds have to break.

Somfy Glydea drives on a curved bay track

The Somfy Glydea 35e is a track motor for curtains and wave headings on a bent track, and it is frequently confused with roller tube motors. On a three-sided bay, the curtain has two internal angles to negotiate. A Glydea drive pulls the leading carrier through both bends, so the track itself has to be bent to the measured 142 and 128 degrees, because a factory 135-degree bend leaves the fabric fouling the return glass on the wider splay.

Those stored readings pay off in the workshop. The track bender sets the jig to the measured figures, then the Glydea carrier can run the full sweep without the motor stalling on a tight corner. A stalled Glydea trips its obstacle detection and reverses, which reads to the homeowner as a fault when the cause is a geometry error baked in at the bending stage.

The motor draws its power at the master end, so on a bay the Glydea drive unit goes behind the shorter return where the mullion post already hides the housing. The 44mm post that stole fabric width then earns its keep by concealing the motor head.

Roller retrofit into an existing bay pole

A bay window roller retrofit usually starts with a bent curtain pole already in the room and an owner asking for rollers on each face. The tempting layout puts three rollers hard against the three panes and ignores the corners. In plan view, the two corner rollers then overlap because each tube projects into the same shared corner space at its own angle.

The fix comes from the stored bay readings. Deduct the tube projection from each face width at the corner end so the tubes stop short of the mullion centreline. On the terrace bay, the left face lost 18mm and the right face lost 22mm at the shared corner. That uneven deduction only makes sense once the two different splay angles are written down. Fit them symmetrically and one blind rubs the other every time it rolls.

Timber sash draught proofing fits alongside this retrofit because pulling the old pole brackets off exposes the staff bead, which is the moment to route in a brush-pile carrier. The bay sashes on this terrace leaked air worst at the meeting rail, where the two sashes cross, and an Aquamac 21 gasket into the parting bead cut the draught you could feel with a wetted finger. Doing it while the blind hardware is off the wall saves a second visit and a second set of holes.

A cutting sheet stays silent on one awkward mismatch: the head-height readings may diverge from the sill once long settlement has worked through the bay. The same reveal that accepts a bracket cleanly above can drift away from the timber below, leaving the template true in one plane and suspect in another.