Magflap on composite and uPVC doors: how it fits

Most new front doors in the UK are uPVC or composite. They are made from plastics, foam and engineered timber rather than solid wood, so it is fair to ask whether a letterbox draught excluder will screw on properly. Here is how these doors are built, where the screws hold, and how to fit a Magflap without drilling.

How a uPVC door is built

A uPVC (PVC-U) door has two parts. The frame around the edge of the door is a hollow plastic extrusion with several internal chambers, and steel reinforcement slides into those chambers to keep it rigid. Most UK systems are about 70mm deep.

The middle of the door is an infill panel, usually 20, 24 or 28mm thick. A typical panel has thin PVC-U skins (around 1.5mm) over an insulating core of foam or polystyrene, and better panels add a layer of MDF or plywood for strength.

The letterbox is usually cut through the infill panel, though some doors have it in a rail of the frame. The outside letterplate and the inside trim are normally joined by bolts through the door, often with a sleeve lining the hole. Letterbox makers warn that overtightening those screws can dent or crack the plastic face, and the same care applies to anything you screw to the door.

How a composite door is built

A composite door is a solid slab rather than a frame with a panel. There are two main kinds:

Either way, the letterbox hole is usually cut at the factory and fitted with a letterplate and sleeve that adjusts to the door's thickness. The door hangs in a reinforced uPVC outer frame.

Why letterboxes in modern doors still let in draughts

The doors themselves are well insulated, but the letterbox is a slot straight through the door. A letterplate flap rarely closes tight, and on many doors the sleeve and inner trim leave a clear path for air. The Energy Saving Trust lists the letterbox as one of the places to draught-proof on a front door, with a letterbox flap or brush.

A Magflap covers the hole with a steel flap that magnets pull shut against a seal as soon as the post is through, so the air path is closed whatever the outside letterplate is doing.

Will a Magflap fit my composite or uPVC door?

The door's material is not the issue. What matters is the area around the letterbox. Check four things:

• Door thickness at the letterbox: 20mm or more. The screws are 20mm long. Most uPVC panels are 24 or 28mm and composite doors about 44 to 48mm, so nearly all are fine. There is no maximum.
• The hole fits inside the plate. MK3 Horizontal: the hole must fit inside the 340 × 100mm plate (post opening 290 × 70mm). MK3 Vertical, for upright letterboxes: 90 × 260mm plate (post opening 65 × 220mm). Measure the hole through the door, not the outside letterplate.
• Flat door all round the hole. The plate needs a flat surface on all four sides. On composite and uPVC doors, watch for raised mouldings, decorative panel edges, glass or a handle close to the letterbox.
• Bolts behind the letterbox. Many letterplates are held on by bolts or a sleeve that stands proud on the inside. The standard Magflap clears 9mm (it is 9mm deep including the gasket). If yours stick out further, choose the additional gasket, which clears 18mm.

Our letterbox sizes guide shows exactly what to measure, and the free printable fit template lets you hold the plate size against your door before you buy.

Can it go on the outside of a uPVC or composite door?

Yes, on many doors, but the outside needs more checking than the inside. Inside, the door is usually flat round the letterbox. Outside, there is the letterplate: on most uPVC and composite doors it is a telescopic letterplate, an outer plate with its own flap (often around 310 × 76mm) bolted through the door to the inside trim, with a sleeve lining the hole.

The Magflap plate has to lie flat on the door with all six screws in solid panel. A full-size outer letterplate is nearly as long as the Magflap's 314mm screw spacing and stands proud of the door, so in most cases it has to come off first. A small, low outer plate can stay underneath only if it sits inside the solid line on the fit template and stands no more than 9mm proud (18mm with the additional gasket).

An outside fit is compatible if:

• The outside letterplate comes off without loosening the inside trim, or fits under the plate as above. On a through-bolted letterbox the same bolts often hold both sides, so check what you are undoing.
• The outside face is flat under the whole plate: no raised mouldings, decorative panel edges, glass or handle in the way.
• All six screws land on solid panel, not on glass or the metal letterbox sleeve.
• The door is 20mm thick or more at the letterbox.
• The door's own letterplate doesn't have to stay. Flat front doors are usually fire doors, and new-build and Secured by Design doors are certified with a tested (TS008) letterplate in place. On those, leave the letterplate alone.

Fit it inside instead if:

• any of the checks above fail;
• you rent, or you'd rather not take the letterplate off a door that is still under its maker's guarantee;
• you want the screws out of reach. Outside, anyone with a screwdriver can get at them.

Keeping the weather out. Screw holes in the outside skin can let rain into the panel core. Put a dab of clear exterior silicone in each hole before you drive the screw, and don't cover any drainage holes in the door or frame. In strong sun, especially on dark doors, the plastic expands more than the steel plate, which is another reason to tighten the screws only until they are snug.

Where the screws hold

The Magflap comes with six 3.5 × 20mm self-tapping screws. Self-tapping means they cut their own thread as they go in, so you don't need to drill pilot holes in the door.

• uPVC panels: the screws go through the plastic skin into the core behind it, and hold best where the panel has a board layer inside.
• Solid-core composite doors: the screws go through the skin into engineered timber, which holds very well.
• Foam-core composite doors: foam on its own grips a screw weakly, so most of the hold comes from the GRP skin. The Magflap is light, so this is enough, but tighten each screw only until it is snug.

If a screw suddenly gets hard to turn, it has probably met the metal letterbox sleeve or steel reinforcement in the frame. Don't force it. Back it out and check the position of the plate. If a screw keeps turning without tightening, the thread has stripped; stop and email us at info@magman.uk with a photo.

How to fit a Magflap to a composite or uPVC door

1. Choose the side. Inside suits almost any door and keeps the screws out of reach. For the outside, run the checks above first and take off the outside letterplate if it won't fit under the plate.
2. Clear the area. Take off any old brush or internal flap and wipe the door clean round the letterbox.
3. Position the plate. Hold the Magflap centred over the hole with the flap at the bottom, check it sits flat, and mark the screw holes with a pencil.
4. Start each screw by hand. Use a screwdriver, not a drill or impact driver, so you can feel the screw bite. Keep it straight, at right angles to the door.
5. Tighten until snug, then stop. Overtightening can crush the foam or crack the skin and strip the thread. Tighten the screws evenly so the gasket sits flat.
6. Check the flap. Push a letter through. The flap should open freely and the magnets should pull it shut on its own.

It usually takes about 10 minutes.

Security, guarantees and fire doors

Letterbox security. Modern doors built to PAS 24 or Secured by Design usually have a letterplate tested to TS008, which is designed to resist "fishing" through the letterbox. Fitted inside, a Magflap leaves that letterplate in place. Fitted outside, it usually means taking the letterplate off, so on a Secured by Design or fire-rated door, fit it inside. For more protection, the locking keyset lets you lock the flap shut, for example when you're away, and the Maguard security cowl fits behind the letterbox and restricts the opening to 45 degrees.

Your door's guarantee. Fitting a Magflap means no drilling and no cutting, but the screws do make small holes of their own. If your door is new and still under its maker's guarantee, it is worth checking their terms on fixing accessories.

Flat front doors. The front door to a flat in a shared block is usually a fire door (FD30S), and its letterbox is a fire-rated part of it. Don't add anything to a fire door or its letterbox without checking with your landlord or building manager first.

Questions people ask

Do I need to drill my composite or uPVC door?

No. The Magflap MK3 is fixed with six self-tapping screws that go straight into the door with a screwdriver. You only need a drill and wall plugs if you are fixing into brick or masonry.

Will the screws hold in a foam-filled composite door?

Yes, if you tighten them gently. Most of the hold comes from the GRP skin, and the Magflap is light. Tighten each screw until it is snug and stop; overtightening strips the thread.

Can I fit a Magflap on the outside of a uPVC or composite door?

Yes, if the outside passes the checks: the outside letterplate comes off (or fits under the plate), the door is flat under the whole plate, and all six screws land on solid panel. If not, fit it inside.

My door is thick. Is there a maximum?

No. The Magflap fits doors 20mm thick or more, with no maximum, because it screws to the face of the door rather than through it.

The bolts on my letterplate stick out on the inside. What do I need?

The standard Magflap clears 9mm. If the bolts or sleeve stick out more than that, choose the additional gasket when you order, which clears 18mm.

Will a Mini Magflap fit my door's keyhole?

The Mini Magflap fits mortice lock keyholes only. Check yours with the 34mm circle on the fit template.

Read next