Composite Doors: PAS 24:2022 Security, Part L U-Values, Secured by Design and Part M Thresholds

Quick Answer: Composite doors consist of a GRP (glass reinforced polymer) skin over a timber/PU foam core. PAS 24:2022 is the enhanced security test — required for all new dwellings (Approved Document Q) and recommended for all external doors. U-value for replacement doors must be ≤1.4 W/m²K (Part L1B). Secured by Design certification provides additional police-tested security. Part M requires a threshold ≤15mm for new dwellings.

Summary

Composite doors have become the dominant external door type for UK housing because they offer the appearance of timber with better thermal performance, lower maintenance, and superior security ratings compared to older uPVC doors.

From a trade perspective, composite door installation sits at the intersection of two regulatory frameworks: energy efficiency (Part L) and security (Approved Document Q — introduced October 2015). For new-build properties, both must be satisfied. For replacement in existing dwellings, energy performance must be met but Q is advisory.

The installation workmanship — particularly the threshold detail and weathersealing — determines whether the door performs to its specification or develops problems within a few months. Composite doors are typically supplied with the frame; a good frame means nothing if it is installed out of plumb or without adequate cavity tray above.

Key Facts

• PAS 24:2022 — Enhanced security for windows and doorsets. Replaces PAS 24:2016. Tests include manual attack simulation, lock performance, weather resistance. Required for all new dwellings under Approved Document Q.
• Approved Document Q — Requires all accessible doors in new dwellings to meet PAS 24 minimum. This includes back doors, patio doors and garage-to-house doors.
• Secured by Design (SBD) — Police-backed certification scheme. Doors meeting SBD criteria must pass PAS 24 plus additional requirements (3-star cylinder, TS 007 multipoint lock, 10mm hinge bolts). Strongly recommended even where not mandatory.
• Part L1A U-value (new dwellings) — Door leaf ≤1.4 W/m²K; doorset ≤1.8 W/m²K.
• Part L1B U-value (replacement) — Door leaf ≤1.4 W/m²K. Same threshold as new build.
• Part M threshold — New dwellings: maximum 15mm threshold height from external to internal. Zero-threshold preferred (for wheelchair access). Threshold bars must be low-profile.
• Anti-snap cylinders — TS 007 3-star cylinder (anti-snap, anti-drill, anti-pick). Lock snapping is the most common break-in method in the UK. Never fit a cylinder that is not anti-snap rated.
• FENSA/CERTASS — Composite door installation in existing dwellings is notifiable under Building Regulations. Either use FENSA/CERTASS registered installer (self-cert) or submit Building Notice. Applies to external doors (not internal).
• Hinge type — Steel hinge bolts on the lock side are included in most PAS 24 compliant frames. Ensure they engage correctly.
• Cavity tray — Essential above door frame head in cavity wall construction. Without it, water tracks down to frame and causes premature rot/failure.
• Silicone pointing — All joints between door frame and masonry must be pointed with neutral-cure silicone (not acetoxy — attacks metals).

Quick Reference Table

Detailed Guidance

Understanding Composite Door Construction

A composite door typically consists of:

1. GRP (glass reinforced polymer) outer skin — high-density glass fibre skin moulded to timber grain texture. Weather-resistant, colour-stable, does not warp or swell.
2. Timber frame (solid hardwood stiles and rails) — provides structural rigidity and lock/hinge fixing points. Critical: the hardwood must be dry and sound; softwood core = poor security rating.
3. PU foam core — closed-cell polyurethane foam fills the interior for thermal insulation (contribution to U-value).
4. Outer weatherseal — typically two-part: brush pile and compression foam; some use magnetic seals.
5. Multipoint locking — 3, 5, or 7-point hook/roller/deadbolt system. The lock cylinder pocket must accept TS 007 3-star cylinders (check gauge — some frames have non-standard cylinder lengths).

Security Components

Anti-snap cylinder:

• The UK's most common break-in technique is "lock snapping" — insert a spanner-type device, snap the front half of the euro cylinder off, then turn the remaining section to open
• TS 007 3-star cylinders have a sacrificial break point and reinforced core that prevents snapping: the back half remains functional even after the front snaps off
• Check the cylinder gauge: the cylinder must not protrude more than 3mm beyond the escutcheon — protruding cylinders are easier to grip and snap
• Products: Ultion, Avocet ABS, Kitemark-certified options

Multipoint lock:

• Minimum 3-point (1 deadbolt + 2 rollers)
• For Secured by Design: 5-point minimum; all points must engage fully when key-locked (not just on the lever/handle)
• Test: after fitting, apply pressure to door at lock side while lifting handle — any movement indicates incomplete engagement

Hinge security:

• Hinge bolts (security studs) on the hinge side prevent doors being forced from hinges
• Standard on PAS 24 frames: check they are present and engaging in the striker plate correctly

Part L U-Value Compliance

A replacement door in an existing dwelling must achieve ≤1.4 W/m²K for the door leaf (not including glazed panels). The doorset (frame + door + any glazed panels) will typically be ≤1.8 W/m²K when sidelights are included.

Manufacturers issue EN 14351-1 performance declarations. The installer must retain a copy or make it available on request. When installing under FENSA, the declared U-value is recorded on the certificate.

Factors affecting U-value:

• Glass specification in any glazed panels (double vs triple; gas fill; low-E)
• Foam core density and thickness
• Frame material (steel reinforcement in frame increases thermal bridging)

Installation: Frame and Threshold

Frame in cavity wall (most common new-build situation):

1. Build reveals to correct width (typically 100mm wall plus frame width)
2. Check reveal plumb, level and square
3. Cavity tray DPC: insert stainless steel cavity tray before frame head. Allow 150mm minimum tray behind face; slope up to outer leaf; weep holes at 450mm centres in outer leaf. Without this, water tracks to frame head and causes rot/failure.
4. Insert frame: shim level at sill; check plumb both faces; square by measuring diagonals.
5. Secure frame: minimum 3 fixings per jamb and 2 per head. Fixings must resist horizontal pull-out.
6. Pack frame and seal: expanding foam or mineral wool at perimeter; do not over-foam (distorts frame).
7. External pointing: backing with closed-cell PE backer rod; neutral-cure silicone.

Threshold detail:

• Part M requires ≤15mm for new dwellings
• Low-profile threshold bars (typically 6–8mm) with weather brush on inside of door
• External threshold must slope away from door (drainage fall 1:40 minimum)
• For Level Access (0mm): requires upstand drain channel outside; door must have multi-seal system

Notifying Building Control

Door replacement is notifiable under Building Regulations (Part L). Two routes:

1. FENSA/CERTASS registered installer — self-certify; issue FENSA certificate to customer; FENSA notifies LA Building Control. Certificate registered on Check-FENSA database (important for conveyancing).
2. Building Notice — submit to LABC before installation; LABC may inspect; issue completion certificate on satisfaction.

FENSA certificates can be regenerated if lost: contact FENSA at fensa.com with property address.

Frequently Asked Questions

What is the difference between PAS 24 and Secured by Design?

PAS 24:2022 is the published British Standard test for enhanced security — it is a performance specification that doors must pass (manual attack, lock testing, weather). Secured by Design (SBD) is a police certification scheme that requires PAS 24 as a baseline but adds further requirements, including specific cylinder standards and a site audit by the local Police Architectural Liaison Officer. SBD is a product certification; PAS 24 is a test.

Should I use a wooden or steel letterbox plate?

Steel letterbox plates are harder to force and are required for SBD certification. They should have an anti-intruder baffle inside to prevent manipulation of the cylinder or locks via the letterbox. Recommend steel over aluminium for security-rated doors.

My composite door is warping — is this normal?

Composite doors can exhibit seasonal twist (warping) due to differential solar heating — a dark door on a south-facing elevation in full sun can reach surface temperatures of 60–70°C. Most reputable manufacturers use a steel reinforcement frame to resist this. If warping is severe (door catches or doesn't seal), raise with manufacturer. Minor twist is normal and typically self-corrects with temperature change. Adjust hinges if necessary.

Regulations & Standards

• Approved Document Q (2015) — Security: external doors and windows in new dwellings

• PAS 24:2022 — Enhanced security performance requirements for doorsets in dwellings

• Approved Document L1B — Part L replacement door U-value (≤1.4 W/m²K)

• Approved Document M — Access: threshold height (≤15mm, 0mm preferred)

• BS EN 14351-1:2006+A2:2016 — Windows and doors: product standard and performance characteristics

• TS 007:2014 — Diamond Standard for physical security of cylinders and hardware

• Secured by Design — door specification

• FENSA — replacement door registration

• Master Locksmiths Association — lock snapping guidance

• security locks — Cylinder and lock selection guide

• fire doors — FD30 fire door requirements

• part p notifications — Notifiable work overview

• glazing regs — Glazing safety requirements