Norsound NOR400 Acoustic and Fire-Rated Letterplate is a bolted, telescopic letterplate developed for suitable fire-resisting and acoustic timber doorsets between 40mm and 80mm thick.
The NOR400 combines a robust metal letterplate assembly with a concealed intumescent liner. It is designed to allow postal access through a door while limiting the reduction in fire and acoustic performance caused by cutting a letterplate aperture into the door leaf.
The stocked range is available in three finishes:
Cutting a postal aperture through a door creates a direct opening through the door leaf. On an acoustic or fire-resisting doorset, an ordinary letterplate can significantly weaken the performance of the complete assembly.
The NOR400 is designed for applications where the door must provide:
The letterplate must be selected and installed as part of a suitable tested or assessed doorset. It does not independently convert an ordinary door into a fire-rated or acoustic door.
Published NOR400 technical information states that the letterplate can retain up to 95% of the fully caulked door’s acoustic performance when used with the appropriate Norsound sealing arrangement.
This means the result depends on the complete door assembly, including:
The NOR400 should not be assigned a standalone dB or Rw rating. Acoustic performance belongs to the complete laboratory-tested doorset or building element.
The NOR400 is supplied with an intumescent liner for installation around the outer telescopic sleeve.
When exposed to fire conditions, the liner expands around the letterplate aperture to help restrict the passage of flames and hot gases.
Published product data identifies fire resistance in accordance with BS 476-22 when the letterplate is installed within a suitable fire-resisting timber doorset.
Fire performance depends on:
No standalone FD30 or FD60 classification should be applied to the letterplate independently from the doorset evidence.
The internal and external letterplate sections extend through the prepared door aperture and are bolted together.
This telescopic arrangement is suitable for supported timber doors between approximately 40mm and 80mm thick.
The through-bolted construction helps:
The fixing bolts must be tightened evenly without distorting the letterplate or restricting flap movement.
The NOR400 incorporates an anti-snap flap designed to open through approximately 180 degrees.
This allows the flap to move fully out of the way as letters and postal items pass through, reducing the risk of the flap being forced against a restricted stop.
Both flaps should return freely to the closed position after use.
The letterplate has an internal opening of approximately 278 × 44mm and is described as providing an A4-compatible aperture in accordance with the referenced postal-opening standard.
The nominal product dimensions are:
The required cut-out in the door is not the same as either of these dimensions.
The door aperture must be measured from the outer telescopic sleeve after the complete supplied intumescent liner has been wrapped around it.
The NOR400 is manufactured using a high-quality zinc construction with plated or finished visible surfaces.
Published product information identifies:
The finishes should still be cleaned and maintained using methods appropriate for decorative metal door hardware.
Avoid abrasive cleaners or tools that could scratch the plated surface.
The stocked finishes are:
The selected finish applies to the complete visible letterplate assembly.
Do not assume that the NOR400 independently provides ambient-temperature cold-smoke control.
Where a smoke-control doorset is required, confirm that the complete door, letterplate, aperture treatment and sealing arrangement have suitable current smoke-test or assessment evidence.
Before ordering or installing the NOR400, confirm:
| Product Name | Norsound NOR400 Acoustic and Fire-Rated Letterplate |
| Product Reference | NOR400 |
| Product Type | Telescopic acoustic and fire-resistant door letterplate |
| Application | Suitable fire-resisting and acoustic timber doorsets |
| Door Thickness | 40–80mm |
| Overall Size | Approximately 310 × 76mm |
| Internal Opening | Approximately 278 × 44mm |
| Material | High-quality zinc construction |
| Fixing | Bolted telescopic assembly |
| Intumescent Protection | Self-adhesive intumescent liner wrapped around the outer telescopic sleeve |
| Supplied Liner Length | Approximately 1250mm; the complete supplied length must be used |
| Fire Performance | Fire resistance in accordance with BS 476-22 as part of a suitable tested or assessed doorset |
| Acoustic Performance | Up to 95% of the fully caulked door’s performance when used with an appropriate Norsound sealing arrangement |
| Standalone Acoustic Rating? | No — the achieved dB or Rw rating belongs to the complete tested doorset |
| Flap Operation | Anti-snap flap opening through approximately 180 degrees |
| Postal Opening | A4-compatible aperture under the referenced postal-opening standard |
| Durability | 30,000 operating cycles |
| Corrosion Resistance | Salt-spray tested to BS EN 1670 Class 5 |
| Stocked Finishes | Gold, Polished Chrome and Satin Silver |
| Standalone Fire Rating? | No — fire performance applies to the complete tested or assessed doorset |
| Standalone Cold-Smoke Rating? | No separate ambient-temperature smoke classification should be assumed |
| Unit of Sale | One complete letterplate assembly |
| Finish | Product Code |
|---|---|
| Gold | NOR400/GLD |
| Polished Chrome | NOR400/PC |
| Satin Silver | NOR400/SS |
| Size Options | Finish Options | Total Variations |
|---|---|---|
| Fixed NOR400 size | Gold, Polished Chrome and Satin Silver | 3 |
Suitable intumescent sealant may also be required by the approved installation arrangement. Confirm whether this is supplied separately.
Published NOR400 product information continues to reference fire resistance in accordance with BS 476-22.
The publicly accessible Warringtonfire assessment located during this review covered NOR400 installations in suitable 30- and 60-minute timber fire doors but stated a validity date ending on 11 November 2025.
Before relying on the product for a current certified project, obtain:
The NOR400 must be installed in accordance with the current fitting instructions and the evidence for the complete fire-resisting or acoustic doorset.
Machining an unsupported letterplate aperture into a certified door can invalidate its fire and acoustic evidence.
Before cutting the door, confirm:
Check the NOR400 for damage before installation.
Do not install the product where:
Remove the release backing from the supplied intumescent liner.
Wrap the liner around the outside of the outer telescopic sleeve, ensuring that:
Do not cut or shorten the liner.
Use the complete supplied length.
Measure the overall width and height of the outer telescopic sleeve after the intumescent liner has been installed.
Use these completed measurements to establish the required door aperture.
Do not determine the cut-out from:
Mark the opening in the position permitted by the doorset evidence.
The manufacturer’s fitting instructions state that the aperture should be no more than 1100mm from the bottom of the door.
Before cutting, check that the marked opening:
Drill appropriate starting holes and cut the opening using suitable equipment.
Create a clean aperture that closely accommodates the telescopic sleeve and liner without crushing or scraping away the intumescent material.
Remove all loose door-core material and machining debris.
Do not enlarge the aperture beyond the dimensions required for the protected sleeve.
Where required by the current fitting detail, apply the specified intumescent sealant continuously around the letterplate or aperture.
The sealant should close the required perimeter gaps without entering the postal opening or obstructing the flaps.
Use the sealant specified by the manufacturer or doorset evidence rather than an ordinary decorative caulk.
Insert the internal and external sections through the prepared opening.
Check that:
Fit the supplied bolts through the designated fixing positions and tighten them evenly.
Do not overtighten the bolts, as this could:
Remove excess uncured sealant using the method recommended for the selected product.
Maintain the continuous required seal behind the faceplates and around the protected aperture.
Open and release the internal and external flaps several times.
Confirm that:
Verify that:
Inspect the NOR400 as part of the door’s fire and acoustic maintenance programme.
Check periodically for:
The NOR400 does not independently provide a fire-resistance period or acoustic rating.
Performance depends on the complete doorset, the surrounding seals, the permitted aperture, the complete intumescent liner, the required sealant, the bolted fixing and the quality of installation.
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