Short answer: order the bifold after the threshold and the floors are understood, not before. A low threshold needs a way for water to leave. A high threshold needs you to accept the step. The frame cannot invent a level that your slab and your patio do not have.
The short answer
Bifold doors stack to the side and leave a wide opening. That is the luxury. The ordinary part is a long aluminium or timber threshold that must stay straight, must meet the internal floor, and must not become a trough. On extensions around Stoke-on-Trent and Cheshire we see more callbacks about the step and the rain than about the colour of the handles.
The survey has to know three levels: the internal finished floor, the structural opening, and the external paving. If those three are still “to be decided by the builder”, the door quote is a guess. Manufacture should wait until the levels are real.
What the threshold is doing
The threshold carries the bottom track or the bottom seal, depending on the system. It has to be packed level even when the brick course is not. It has to be fixed down so a run of four or five leaves does not rock. And it has to finish against your floor covering — tiles, oak, or screed that has not been laid yet.
There is always a compromise between wheelchair-friendly, trip-free, and weather-tight. Exposed elevations with no overhang need more honesty. A door on the weather side of a Staffordshire house, with paving sloping the wrong way, will test any gasket. A canopy, a recess, or a change of levels is sometimes the real product, and the bifold is only the glazed part.
Drainage you should be able to point at
Ask where water goes. Some thresholds drain through the front face, some into a channel in front of the door, some rely on the patio being lower and falling away. Block paving laid after the doors, proud of the drainage holes, is a classic leak. So is a deck that traps the weep path.
Internal drainage — water that has crossed the track being collected and led back outside — only works if it is specified and if the channel is maintained. It is not something to invent with silicone on the first storm. If the quote says “low threshold” and does not mention drainage, it is incomplete.
Floor build-up and who controls it
New extensions often have a floor zone that is still moving: insulation, screed, then a finish. The door fitter needs the finished floor height, or a written allowance. If we set the doors to structural slab and you later add 20 mm of tile and adhesive, the leaves may foul, or the seal may miss. If we set them high and the floor never comes up, you have a step you did not order.
On a renovation, the old floor may slope. A long bifold will show that slope as a tapering gap. Sometimes the right work is a localised floor build or a packer the eye can accept. Sometimes the opening should be sliding doors with a different threshold, because the bifold’s stack and track are less forgiving. That is a survey decision, not a showroom one.
Structure, lintel and the stack
Wide openings need a lintel or beam that is already designed for the span and the load above. Door companies do not become structural engineers because they sell a five-leaf set. If the opening is being knocked through, the builder’s drawing should exist before the doors are made. The stack side needs a place to park the leaves without covering a socket, a radiator, or the only route to the kitchen.
Furniture and curtains matter. A bifold that opens inside needs clear floor. A bifold that opens outside needs a patio that is not the bin store. Traffic doors — the one leaf you use every day — should be on the side you actually walk through, with a handle you can live with, not only a full concertina when guests arrive.
What the quote must pin down
- System name, number of leaves, and which leaf is the traffic door
- Threshold type and how water leaves it
- Internal finished floor level assumed, and what happens if it changes
- External paving level and who adjusts the fall
- Colour, glass, blinds in the glass if any, and handles
- Who supplies the lintel design and who signs off the opening size
- Making good inside and out, including plaster and render returns
When to book a survey
Book the survey when the opening exists, or when the structural drawing does. A photo of a hole in the wall is not a size. If the floor is unfinished, we can still advise, but the order waits for levels. Bring the kitchen plan if the stack will hit units.
We fit across Staffordshire and Cheshire and we would rather delay manufacture than set a low threshold into a patio that slopes back to the house. If a bifold is the wrong door for the exposure, we will say so and talk about sliding or lift-and-slide instead.
Questions we hear on survey
Can the threshold be completely flush?
It can be close, on the right system, with drainage and a floor that was built for it. “Completely flush” with no fall and no drain is how water comes in. Ask to see the section, not the lifestyle photo.
Who is responsible if the builder’s floor is wrong?
Whoever changed the level after the survey without telling the door manufacturer. Write the assumed floor build-up into the quote so the argument has a number attached.
Do bifolds leak more than sliders?
They have more meeting stiles, so there are more seals to set. A well-installed set on a protected elevation is fine. An exposed opening with a blocked drain will leak whatever the brochure promised.
Should I order the doors before the extension is built?
You can choose the system and the colour. Do not freeze the manufacturing sizes until the opening and the floors are measurable. Lead times are managed by ordering the right thing once, not the wrong thing early.
