Telescopic Letterbox Buying Guide: Sizes and Extensions

Telescopic Letterbox Buying Guide: Sizes and Extensions

Why choose a telescopic letterbox

A telescopic letterbox is a practical solution when you need to route post through doors or walls of variable thickness. The telescopic design comprises an outer faceplate, a sliding or nested tube assembly and an inner plate: this allows the unit to extend to suit composite doors, thick timber doors, or masonry walls without bespoke fabrication. For both UK homeowners and trade professionals a telescopic letterbox offers flexibility, cleaner aesthetics and improved draught control compared with a fixed-length post tube.

Understanding sizes and extensions

Telescopic letterboxes are specified by three main dimensions: the external plate aperture, the internal plate aperture and the assembled extension length. Aperture sizes commonly meet guidance in standards such as BS EN 13724 for posting apertures; many manufacturers supply flaps around 230mm wide to accept A4/C4 envelopes, with flap heights typically 30–40mm. Extension lengths vary widely: short units suit standard door thicknesses, while extra-long telescopic tubes accommodate deep cavity walls or stonework.

• Typical aperture widths: around 230mm; verify if you need a wider slot for larger mail.
• Typical flap heights: 30–40mm for standard post; increase height if you receive bulky mail.
• Extension ranges: small (30–50mm), medium (50–110mm), large (110–250mm) depending on supplier; always check the specific product datasheet.

How to measure: practical trade advice

Accurate measurement is essential before ordering a telescopic letterbox. For doors measure from the outer face of the external plate to the inner face of the internal plate: include door seals, trims and any surface-mounted escutcheons. For wall installations measure the full depth from the outer brick or rendering face to the internal plaster or lining face. Allow an extra 5–10mm to clear trims and to permit foam or sealant for draughtproofing.

• Measure at the centre of the planned aperture; doors can be tapered or uneven.
• For composite doors check the manufacturer’s guidance: some door cores contain insulation strips that affect cutting.
• For cavity walls include cavity width, inner leaf thickness and plasterboard depth; choose an extension that reaches comfortably into the internal face.

Materials, finishes and security features

Telescopic letterboxes are available in stainless steel, brass, and powder-coated steel or aluminium. Stainless steel is durable and corrosion-resistant; brass suits traditional properties and can be lacquered to retain finish. Consider matching the finish to door furniture and hardware coloured in satin chrome, polished brass or black. Security and weatherproofing are equally important:

• Anti-fishing cages or internal restrictors prevent unauthorised access to mail.
• Draught brushes or rubber seals reduce heat loss and noise ingress.
• Lockable internal letterboxes provide extra security; ensure clearance for the locking mechanism inside the door cavity.
• Check for fire door compatibility: altering FD30 or FD60 doors may invalidate performance ratings; consult the door manufacturer or a qualified installer.

Installation: trade and DIY practical steps

Installation varies by door and wall type but the following practical steps apply to most telescopic letterbox installations. If in doubt about structural work or fire doors, engage a competent tradesperson.

• Mark the centreline and aperture position on both faces. Verify level and vertical alignment before cutting.
• Cut the opening carefully: use appropriate blades for timber, metal or masonry; on metal and composite doors use pilot holes and jigs rather than large routers.
• Dry-fit the telescopic tube and plates: extend to the measured length and check that both plates sit flush to the faces.
• Secure the outer plate first using corrosion-resistant screws or rivets appropriate to the substrate. For PVCu doors use the recommended screw pattern to avoid splitting; for steel doors consider self-tapping or rivet fixings.
• Adjust the telescopic sections and secure the inner plate. Apply low-expansion foam or silicone sealant around the tube to seal and reduce vibration; avoid overfilling which can prevent movement of the internal flap.
• Fit weatherstrips, draught brushes and internal anti-fishing cages as required. Check operation: flap should open freely and close fully with a tight seal.

Common installation issues and professional tips

Tradespeople encounter recurring problems when fitting telescopic letterboxes; addressing these at the planning stage saves time on site.

• Door core obstruction: composite doors often have reinforcing bars or insulation blocks; probe with a long drill bit or consult the door maker to avoid snagging internal hardware.
• Rattle and vibration: secure inner and outer plates firmly and use rubber washers or mastic to dampen movement.
• Incorrect length selection: measure twice; a telescopic tube that is too short leaves gaps, while one that is too long may bulge internal finishes.
• Weather ingress into cavity walls: use through-wall flashings or a storm collar if the external face is exposed to driving rain.
• Fire-rated doors: any modification can compromise the rating; use approved, fire-rated letterplates where specified and record the change in the building maintenance log.

Choosing the right product for trade vs DIY

For trade contractors looking for speed and reliability select telescopic letterboxes with clear product datasheets, adjustable trim rings and pre-drilled fixing holes. Buy from reputable suppliers who can provide technical support and replacement parts. For confident DIY installers there are kits and templates available; ensure you have the correct cutting tools and take extra care with door security and fire protection. If unsure about structural openings or fire door compliance request an on-site survey.

Recommended Products

Based on this article, you may find these products helpful:

• In-Line Window Mechanism
• Offset Window Mechanism
• E-Type Gasket - Black 5m Roll
• E-Type Gasket - Black (Per Metre)
• B003 Bubble Gasket

Conclusion and next steps

A telescopic letterbox offers adaptability for a wide range of door and wall thicknesses; getting the measurements, security features and installation method right will ensure a neat, secure finish. Next steps: measure the finished depth from external face to internal face, decide on aperture and security options, then consult product datasheets or contact a supplier for availability and specific extension ranges. For further reading see our pillar resources at /guides/letterboxes or contact the Window Hardware Store trade team for technical advice and quotations.