Sliding doors in London, slim frames, big glass

Sliding doors in London: the four things that decide the result

Nearly every enquiry we get for sliding doors in London starts with a photograph. Two huge panes of glass, almost no frame visible, a floor that runs flush out to a terrace. That result is completely achievable, but it is decided long before anyone places an order. It is decided by the structural opening, the number of tracks, the weight of the glass and how that glass is physically going to reach the back of your house. Get those four right and the door looks like the photograph for twenty years. Get them wrong and you live with a door that catches, drags and whistles.

We cut, toughen and laminate glass in our own workshop in Wembley and fit with our own teams, so we see both ends of the job. The pattern never changes. The problems homeowners end up living with are almost never about which brand is on the frame. They are about dimensions, levels and access.

Sliding doors or bifolds? Argue it room by room, not by fashion

Bifolds open fully. Sliding doors do not. That is the entire trade-off, and it matters less than most people expect, because the real question is not how wide the hole can get on six days a year. It is what you look at for the other three hundred and fifty nine.

A bifold at rest is a stack of frames. Six panels means six sets of stiles concertinaed at one end, standing in your view and eating either the reveal or the terrace. A sliding door at rest is glass with a single vertical line through it. If the space is a kitchen-diner you use every day and the doors spend most of their life shut, the slider wins on looks by a wide margin.

Orientation changes the answer again. A north-facing rear extension gets soft, even light and very little solar heat, so you can run large fixed panes without the room becoming unusable in July. A south or west elevation gets hammered in the afternoon, and what you actually want there is ventilation you can leave open in light rain. A lift-and-slide panel pushed back a few hundred millimetres does that neatly under the soffit. A folded bifold stack does not.

• Everyday kitchen-diner with the doors mostly shut: sliding, for the uninterrupted view.
• Narrow terrace, balcony or walkway where a folded stack would block the route: sliding, because nothing projects into the space.
• Garden room you throw wide open in summer and barely heat in winter: bifolds, if a genuinely clear opening matters more than sightlines.
• Opening under about 2.4 metres wide: think hard. Split across two sliding panels you can only ever open a little under half of it, and a single door leaf beside a fixed light may serve you better.
• Wide opening, heavy daily use, children and pets: sliding, because there is no bottom track full of folding hardware waiting to jam on grit.

How many tracks you order decides how much opening you actually get

This is the part most quotes skate over. On a two-track door one panel slides behind the other, so your clear opening is slightly under half the overall width once the interlock is deducted. A three-track door lets two panels stack together and gets you to roughly two thirds. Only a pocket system, where the panels slide away into a cavity built into the wall, gives you something close to the full opening.

Tracks also cost you depth. Every additional track adds width to the bottom frame and to the head, and that depth has to be absorbed by the floor build-up and by the structural opening above. If your builder has already poured the slab and set the steel at a fixed soffit height, the number of tracks may have been decided for you without anyone noticing. This is why the door wants specifying while the beam is being designed, not after the brickwork is finished and the plasterer has been and gone.

Glass weight, with the arithmetic done properly

Glass weighs roughly 2.5 kg per square metre for every millimetre of thickness. A conventional double-glazed unit with a 6mm pane on each side is 12mm of glass, so about 30 kg per square metre. Take one panel 1.8 metres wide and 2.7 metres high. That is 4.86 square metres, so around 146 kg of glass, and once the aluminium sash is built around it you are handling something close to 170 kg.

Now specify a laminated outer pane because the panel is oversized or because you want acoustic benefit. Two 5mm sheets bonded with an interlayer instead of a single 6mm pane takes the same panel to roughly 200 kg. Four panels of that size is most of a tonne of glass turning up on a lorry in a London street. Nobody walks that through a side gate on suction pads and good intentions.

If a company has quoted a four-panel slider for a mid-terrace with no side access and has not once mentioned how the glass is getting to the back of the house, they have not finished pricing the job. The number they gave you will move.

What separates a good sliding door installation from a bad one

• The track is bedded dead level along its entire length on a continuous bearing, not perched on packers with voids between them.
• The threshold has a drainage channel in front of it and a real route for water to escape, instead of relying on gaskets alone to hold back a London downpour.
• The opening is surveyed after the steel is in and the reveals are finished, not scaled off a drawing.
• Rollers and gear are selected for the actual weight of your panels, not for the system’s published maximum.
• The frame is fixed solidly at the jambs while the head is left with a deflection gap, so movement in the beam above does not land on the door.

That last point causes more return visits than everything else combined. A steel beam over a wide opening will deflect once the building is loaded onto it. If the door head has been packed tight to the underside, that movement is transferred straight into the frame, the panel starts catching at the top corner, and the homeowner gets told the door has dropped. It has not dropped. It is being squeezed.

How to compare sliding door quotes without getting caught

Prices for what looks like the same door vary enormously in London, and the reason is nearly always sitting in what has been left off the page. Ask every company the same short list of questions and the cheap quote usually explains itself within a minute.

• Which system is it, by manufacturer and series? Not “premium aluminium” but the actual product, so you can read the test data yourself.
• What is the interlock dimension in millimetres where two panels overlap? That single number is the look you are buying.
• Is the U-value quoted for the whole door or for the centre of the glass? Centre-pane figures always flatter the product and are not what Approved Document L is interested in.
• Who supplies, plans and pays for the lifting, and is a crane, a spider lift or a glazing robot allowed for in the price?
• Who makes good the plaster, the floor finish and the external brickwork around the new frame when the door is in?

We will tell you when you do not need the expensive answer. Plenty of rear extensions are served perfectly well by a two-panel slider in a mid-range system with a sensible interlock, and the money is better spent on the glass specification and a properly detailed threshold than on shaving ten millimetres off a frame nobody will ever look at twice. If a slim sightline system genuinely suits your opening we will quote it and explain why. If it does not, we will say so.