The short answer
A composite door is installed by removing the old door and frame, fitting the new frame square and level into the opening, fixing it securely, hanging and adjusting the slab, then sealing and finishing. The installer first takes out the old door set and prepares the opening. The new frame is offered into the opening, packed and adjusted until it is plumb, level and square, then fixed through the frame into the surrounding structure with screws or fixings at intervals. The door slab is hung and adjusted so the gaps around it are even and the multi-point lock engages cleanly. Finally the frame is sealed on the outside with weatherproof sealant, any internal gaps are insulated and trims fitted, and the lock, hinges and seals are checked. A correct fit is what lets the door's security and weather performance actually work.
A composite door is only as good as its installation. This page walks through how a professional fits one, so you know what should happen and why each step matters for security and weatherproofing.
Installation steps
- 1Remove old door and frame
- 2Fit frame plumb, level, square
- 3Fix frame into structure
- 4Hang and adjust the slab
- 5Seal, insulate and finish
Removing the old door and preparing the opening
Installation starts with removing the existing door set. The old slab is taken off its hinges, then the frame is cut or unfixed and worked free of the opening. This is done carefully to avoid damaging the surrounding brickwork, plaster or render, especially as the new frame needs a clean, sound opening to sit in. Any old sealant, fixings and debris are cleared away.
With the opening clear, the installer checks and prepares it. They confirm the opening is the right size for the new frame, with an appropriate fitting tolerance all round, and that the structure is sound — a door frame must be fixed into solid material, so any crumbling masonry or a missing lintel is a problem to resolve first. The threshold area is checked too, since the sill must sit level and the threshold height suits the floor levels inside and out (and any accessibility requirement). Good preparation here is what makes the rest of the job straightforward; a poorly prepared opening leads to a frame that will not sit square or fix securely.
Fitting and fixing the frame
The frame is the heart of the installation, because the door's security and operation depend on it being fitted accurately. The installer offers the frame into the opening and uses packers (shims) behind the frame to adjust its position until it is plumb (vertically straight), level (horizontally true) and square — checked with a spirit level and by measuring the diagonals. Getting this right is essential: a frame that is out of square will give uneven gaps around the slab and may stop the multi-point lock engaging properly.
Once aligned, the frame is fixed into the surrounding structure with appropriate screws or frame fixings, placed at intervals around the frame and into solid material — typically near the hinges and lock keeps where the loads are highest, so the door cannot be levered or sprung out. The packers stay in place behind the fixing points to keep the frame true and prevent it being pulled out of shape as the screws tighten. This secure, square fixing is a major part of what gives a composite door its forced-entry resistance: even a PAS 24 door loses performance if the frame is loosely or badly fixed.
Hanging the slab, sealing and finishing
With the frame fixed, the door slab is hung (on many systems the slab and frame come pre-hung, but the slab is still adjusted on site). The installer adjusts the hinges and keeps so the gaps around the slab are even on all sides and the door closes without catching. They then check the multi-point lock: lifting the handle should throw the hooks or bolts smoothly into the keeps, and turning the key should deadlock cleanly. The keeps are adjusted so the locking points engage fully — a lock that only half-engages is both insecure and prone to wear.
Finally the door is sealed and finished. The external joint between the frame and the structure is filled and finished with a weatherproof sealant to keep water and draughts out, internal gaps are insulated (often with low-expansion foam) and covered with trims or architraves, and the threshold is sealed so water drains away outward. The installer checks the weather seals compress correctly, confirms the door opens, closes and locks smoothly, and tests the cylinder and handle. A well-finished installation leaves a door that is square, weathertight, secure and easy to operate — and that, far more than the slab alone, is what determines how well a composite door performs over its life.
Why a professional fit matters
It is tempting to think of a door as a product where the quality is all in the slab, but with composite doors the installation is at least as important as the door. A PAS 24 door set is tested as a whole assembly fixed into a frame, and its proven security resistance depends on that frame being fitted square and fixed securely into solid structure. Fit the same door into a loose or out-of-square frame, with the keeps misaligned so the hooks barely engage, and much of the tested security is lost. The same is true of the weather performance: even sealing and a properly compressed gasket all round are what keep the door watertight and draught-free, and those come from careful fitting, not from the slab.
This is why using a competent installer is worth it, and why building-regulations certification ties into the fitting. Replacing an external door is controlled work that must meet the energy requirement and be certified — most installers do this through a competent-person scheme such as FENSA or Certass, self-certifying the installation and issuing a certificate. A good installer will also survey the opening beforehand, flag any structural issues such as a failing lintel or unsound masonry, and order the correct size and threshold for the situation. The result is a door that not only looks right but delivers the security, weatherproofing and energy performance it was specified for — and comes with the paperwork that proves the work was done to standard.
Frequently asked questions
How is a composite door frame fixed in place?
The frame is packed with shims until it is plumb, level and square, then screwed or fixed into the surrounding structure at intervals, with fixings concentrated near the hinges and lock keeps where loads are highest. Fixing into solid material is essential, as it gives the door its resistance to being forced or levered out.
Why does a composite door need to be fitted square?
Because the gaps around the slab must be even for it to close properly, and the multi-point lock only engages cleanly into its keeps when the frame is true. An out-of-square frame causes catching, uneven seals, a lock that does not engage fully, and reduced security.
Is the door sealed during installation?
Yes. The external joint between frame and structure is finished with weatherproof sealant, internal gaps are insulated and trimmed, and the threshold is sealed so water drains outward. Correct sealing keeps the door weathertight and draught-free, and is a standard part of a proper installation.
Sources & further reading
Figures on this page are typical UK ranges drawn from published sources and depend on your specific door and opening. They are guidance, not a quotation.