How to specify aluminium windows
What an aluminium window clause has to cover, from the thermal break and frame finish to escape, security and smoke venting, and how to write it in CAWS and in Uniclass.
Photo: Governor Macquarie, CC BY 2.0, via Wikimedia Commons.
How do you specify aluminium windows?
To specify aluminium windows, name a thermally broken system and fabricator to BS EN 14351-1 and BS 4873, state the whole-window U-value, the BS 6375 weathertightness and wind classes for the site, and the finish by standard and quality scheme (powder coating to BS EN 12206-1 or anodising to BS EN ISO 7599). Then settle per window type the security rating, escape and restrictor requirements, safety glazing, and whether any window is a smoke vent or fire-resisting.
Aluminium windows are bought as a system: one company designs and tests the extrusions, another fabricates and glazes the windows, and often a third installs them. Specifications that name only the system and a RAL colour leave the fabricator to decide the glass, hardware, finish quality and performance classes, and two tenders on the same system can differ more than two different systems.
This guide covers Uniclass Pr_30_59_98_02 Aluminium window units. Aluminium-clad timber windows are composite window units, Pr_30_59_98_15, and continuous runs of aluminium glazing from slab to slab are usually window walling or curtain walling rather than windows, so check which you are actually describing.
What does a aluminium window clause need to cover?
Some properties are specific to aluminium windows. Others belong in every product clause. A clause that leaves either out gives the contractor room to substitute.
Properties of aluminium windows
- Coat build-upFilm thickness and pretreatment under BS EN 12206-1.
- Frame profileSeries depth and face width set sightlines.
- Opening configurationCasement, tilt and turn, parallel opening or sliding.
- Operation methodManual, or actuator-driven for vents and high windows.
- Power supplySupply and controls for any powered opening lights.
- Thermal performanceWhole-window Uw; depends on the thermal break.
- Glazing specificationIGU make-up, coatings, g-value and light transmittance.
- Glazing beadsInternal beads; gasket or wet seal glazing.
- Structural performanceNever guessWind class to BS EN 12210 for the design pressure.
- Weather resistanceAir and water classes to BS 6375-1.
- Category of useOperation and strength to BS 6375-2.
- Acoustic performanceRw + Ctr from the acoustic report.
- Free ventilation areaTrickle vents or opening area for Part F.
- Security ratingNever guessPAS 24 where accessible, or higher for risk.
- Means of escapeNever guessClear opening for escape windows in dwellings.
- Window opening restrictionNever guessRestrictors and guarding to Parts K and O.
- Safety glazing classificationNever guessBS EN 12600 class in critical locations.
- Reaction to fire classificationNever guessOnly where the fire strategy sets a class.
- Fire resistanceNever guessIntegrity and insulation where a window sits in a fire wall.
- Smoke controlNever guessWindows acting as automatic smoke vents.
- IronmongeryHandles, locks, hinges, stays and friction hinges.
- FixingsBrackets or through-frame fixings, isolated from other metals.
- AccessoriesCills, flashings, add-on profiles, actuators.
- Third-party certificationKitemark or equivalent for product and finish.
What every product clause needs
- Manufacturer and product referenceSystem house and series, and the fabricator.
- Product standardBS EN 14351-1, with BS 4873 for aluminium.
- Installation methodSystem installation guide and BS 8213-4.
- Technical submittalsDoP, Uw calculation, test reports, finish certificates.
- Operation and maintenance informationCleaning regime that keeps the finish warranty valid.
- MaterialThermally broken aluminium alloy extrusions.
- FinishPowder coat or anodised; colour and gloss inside and out.
Uniclass Pr_30_59_98_02 Aluminium windows. Not every property applies to every project; where one does not, say so rather than leaving it blank.
Fields marked Never guess depend on the project and should never be filled from a default or a previous job. If the information is not available yet, the clause should say so with a TBC and a reason.
Which decisions does the clause depend on?
Most properties follow from the product you choose. These depend on the project, so they need a decision from the designer before the clause is written.
- 01
Thermal break and whole-window U-value
Part LAluminium conducts heat far better than PVC-U or timber, so the frame is the weak point and the polyamide thermal break decides most of the result. Approved Document L sets a limiting window U-value of 1.6 W/m²K for new dwellings and 1.4 W/m²K or WER band B for new or replacement windows in existing dwellings; Volume 2 has its own limits for other buildings, and the energy model normally needs better. The trap is a series that meets the figure only in its deepest, widest-break version, or only at the reference size. Narrow sightline ranges usually perform worse than standard ones.
How to decide
- Take the target Uw from the energy model, not just the limiting value.
- Ask for Uw for the actual window types or the reference size, calculated to BS EN ISO 10077.
- If slim sightlines matter, check the slim series reaches the U-value before showing it on the elevations.
- Check condensation risk on the internal frame face in high-humidity rooms.
Write it asThermal performance: Thermally broken system, whole-window U-value (Uw) not more than 1.4 W/m²K, calculated to BS EN ISO 10077. Submit calculation for each type. - 02
Wind load and weathertightness
Never guessAluminium windows are common on taller and exposed buildings, so the wind load is often higher than on a typical house. BS 6375-1 links the design wind pressure and exposure to classes of air permeability, watertightness and wind resistance under the BS EN 12207, 12208 and 12210 classifications. The class for wind resistance controls frame deflection, and on large lights it decides whether the series needs a deeper section or a reinforcing mullion. The pressure has to come from a calculation, not from a previous job.
How to decide
- Get the design wind pressure from the structural engineer or the facade designer, to BS EN 1991-1-4 and the UK National Annex.
- Choose classes from BS 6375-1 for that pressure; state them in the clause or schedule.
- Check the largest light of each type, and corner windows, where pressures are higher.
- For buildings with a facade consultant, ask whether project testing is expected.
Write it asWeathertightness: BS 6375-1, air permeability class 4, watertightness class E1200, wind resistance class C4. Design wind pressure: as window schedule. - 03
Powder coating or anodising
FinishThe finish is a product in its own right and the part most often disputed after handover. Polyester powder coating is specified to BS EN 12206-1, and anodising to BS EN ISO 7599 with a stated film thickness class. The quality schemes Qualicoat and Qualanod certify the applicator's process, including pretreatment, which is what decides durability near the coast or in polluted air. A RAL number on its own says nothing about gloss, film thickness, pretreatment or the applicator's guarantee.
How to decide
- Choose powder coat or anodising; anodised colours are limited but the finish is part of the metal.
- State colour, gloss level and whether inside and outside colours differ.
- Coastal or swimming pool environments: ask for the applicator's marine-grade pretreatment and guarantee terms.
- Require approved finish samples and a guarantee from the applicator, not only the fabricator.
Write it asFinish: Polyester powder coating to BS EN 12206-1 by Qualicoat-approved applicator, external colour and gloss as finishes schedule. Submit applicator's guarantee. - 04
Escape, restriction and security
Never guessIn dwellings, Approved Document B needs an emergency escape window, where one is required, to give an unobstructed opening of at least 0.33 m² and at least 450 mm high and wide, with the bottom of the opening not more than 1100 mm above the floor. Approved Document K asks for guarding where sills are below 800 mm, and Approved Document O raises that to 1100 mm in new residential buildings where windows open to control overheating. Window opening restriction must not defeat escape. For security rating, Part Q asks for easily accessible windows in new dwellings in England to be secure, with PAS 24 as the usual route.
How to decide
- Flag escape windows on the schedule and check the clear opening, not the sash size.
- Use releasable restrictors with a release catch on escape windows; permanent restrictors elsewhere if needed for safety.
- Mark easily accessible windows in new dwellings and require PAS 24 for those types.
- Commercial and higher-risk buildings: agree any higher security rating with the client or Secured by Design officer.
Write it asSecurity: PAS 24 to windows marked easily accessible. Escape windows: clear opening not less than 450 x 450 mm and 0.33 m², restrictors releasable. - 05
Safety glazing, fire and smoke
Never guessGlazing within 800 mm of the floor is a critical location under Approved Document K, so specify a BS EN 12600 safety glass class there. The other questions are whether any window needs fire resistance, smoke control or a reaction to fire classification. Windows near a boundary or in a compartment wall may need fire-resisting glazing tested as part of the frame and classified to BS EN 13501-2. Opening windows used as automatic smoke vents in stairs and corridors are part of the smoke control system and are tested as natural smoke and heat exhaust ventilators to BS EN 12101-2. Regulation 7(3) exempts window frames and glass from regulation 7(2), but not spandrel panels set in the frame.
How to decide
- Mark critical-location panes and any glazing acting as guarding (laminated).
- Ask the fire engineer which windows, if any, are fire-resisting or smoke vents, and give those their own type marks.
- Smoke vents: specify actuator, power supply, controls interface and free area as the smoke strategy requires.
- Check any opaque infill panel against regulation 7(2) on relevant buildings.
Write it asSafety glazing: Critical locations to BS EN 12600 Class 1(B)1 or better. Smoke vent windows (W05): to BS EN 12101-2, free area as smoke control strategy.
The values under "Write it as" are examples. The right value depends on the location, the use and the fire strategy.
Which standards should the clause cite?
Cite the product standard the declaration of performance refers to, the code of practice for installation, and the test method behind each value you set. A value without its test method cannot be checked against a submittal.
- BS EN 14351-1
- Windows and doors. Product standard, performance characteristics. Windows and external pedestrian doorsets
The harmonised product standard; the declaration of performance lists the characteristics it covers, such as U-value, air permeability, watertightness and wind resistance.
In the clause "Standard: BS EN 14351-1, performance as declaration of performance".
- BS 4873:2016
- Aluminium alloy windows and doorsets. Specification
The UK specification for factory-made aluminium windows, covering materials, construction, glazing and performance.
In the clause "Manufacture: to BS 4873". Do not cite the withdrawn 2009 edition.
- BS 6375 series
- Performance of windows and doors
Part 1 weathertightness classes and how to select them for exposure, Part 2 operation and strength, Part 3 additional performance such as security and durability.
In the clause "Weathertightness: BS 6375-1, classes as schedule. Operation and strength: BS 6375-2."
- BS EN 12206-1
- Paints and varnishes. Coating of aluminium and aluminium alloys for architectural purposes. Coatings prepared from coating powder
Requirements for powder coatings on architectural aluminium, including pretreatment and film properties.
In the clause "Finish: polyester powder coating to BS EN 12206-1".
- BS EN ISO 7599
- Anodizing of aluminium and its alloys. Method for specifying decorative and protective anodic oxidation coatings on aluminium
How to specify anodised finishes, including film thickness class and appearance.
In the clause "Finish: anodised to BS EN ISO 7599, film thickness class as finishes schedule".
- PAS 24
- Enhanced security performance requirements for doorsets and windows in the UK
The attack test for complete windows, named in Approved Document Q for new dwellings.
In the clause "Security: PAS 24, third-party certified for the window as fabricated".
- BS EN 12600
- Glass in building. Pendulum test. Impact test method and classification for flat glass
The impact classes used to show a pane is safety glass.
In the clause "Glass in critical locations: BS EN 12600 Class 1(B)1 or better".
- BS EN 12101-2
- Smoke and heat control systems. Natural smoke and heat exhaust ventilators
Product requirements and tests for natural smoke vents, including windows used as vents.
In the clause "Smoke vent windows: to BS EN 12101-2".
- BS EN ISO 10077 series
- Thermal performance of windows, doors and shutters. Calculation of thermal transmittance
The method for calculating whole-window U-value from frame, glass and edge values.
In the clause "Uw not more than 1.4 W/m²K, calculated to BS EN ISO 10077".
Confirm the current edition of each standard at every issue. Avoice's State of UK Specifications 2026 found a withdrawn standard cited in 40% of specifications at contract issue.
For the regulations behind these choices, see Part L and the Future Homes Standard, Approved Document B explained, or search the Avoice Regulations library.
How do you write it in CAWS?
Aluminium windows go in CAWS section L10, with one product clause per window type. The type clause names the system and fabricator, the standards, the weathertightness classes, the thermal performance, the glazing, the finish, the hardware and any special function such as a smoke vent, and refers to the execution clauses for installation.
The general clauses at the front of L10 (evidence of performance, control samples, site dimensions) and the execution clauses at the back (protection, corrosion protection, building in, fixing of aluminium frames, sealant joints, ironmongery) apply to every type. Fire-resisting windows and smoke vents need their own type clauses because their test evidence is specific to them.
- Manufacturer
- Submit proposals
- Product reference
- Submit proposals
- Standard
- BS EN 14351-1 and BS 4873
- Frame
- Thermally broken aluminium alloy extrusions, polyamide thermal break
- Configuration
- Top hung casements and fixed lights, as window schedule
- Exposure category/ design wind load
- BS 6375-1, classes and design wind pressure as window schedule
- Operation and strength characteristics
- BS 6375-2
- Thermal performance
- Uw not more than 1.4 W/m²K
- Glazing details
- Double glazed units to BS EN 1279, solar control low-e, g-value as energy model
- Safety glazing
- Critical locations to BS EN 12600 Class 1(B)1
- Beading
- Internal, gasket glazed
- Security
- PAS 24 to windows marked easily accessible
- Background ventilation
- Trickle vents as ventilation strategy
- Finish as delivered
- Polyester powder coating to BS EN 12206-1, Qualicoat-approved applicator
- Colour
- TBC External colour sample panel not yet approved by client. Resolve before ordering.
- Ironmongery/ accessories
- Lockable handles, friction stays, releasable restrictors, as clause 820
- Fixing
- As clause 782
Illustrative, not from a real project. One property is left as a TBC with a reason so it can be tracked and closed before issue.
Clauses the type clause relies on
- L10/110
- Evidence of performance Test reports and certificates for the system as fabricated.
- L10/140
- Control samples Corner section and finish samples approved before fabrication.
- L10/710
- Protection of components Protective tape on finished faces and glass until handover.
- L10/740
- Corrosion protection Isolation of aluminium from dissimilar metals, mortar and treated timber.
- L10/782
- Fixing of aluminium frames Fastener type and positions from corners and hanging points.
- L10/810
- Sealant joints Perimeter sealant type, joint size and preparation.
Clause numbers vary between NBS versions and practice masters. The numbers here follow typical NBS L10 specifications; check your L10 actually contains the clauses the type clause refers to.
How do you write it in Uniclass?
In Uniclass, the installation goes in a window system clause, Ss_25_30_95_95 Window systems: the window type in its opening, how it is fixed and sealed, how it meets the cavity closer, membranes and cill, and the performance of the installed window.
The aluminium window itself is a product clause under Pr_30_59_98_02. That keeps the system choice, finish and performance in one place, so a curtain-walled ground floor and punched windows above can share the same window product while having different installation details.
- Description
- Aluminium windows in rainscreen-clad framed wall
- Window
- Pr_30_59_98_02 Aluminium window units (W01)
- Position in opening
- In line with insulation zone, as head, jamb and cill details
- Fixing
- Stainless steel brackets to structure, isolated from aluminium
- Perimeter seal
- EPDM membrane air seal internally, breather membrane lapped externally
- Cill
- Pressed aluminium cill with stop ends, finish to match
- Installed performance
- Weathertightness classes as window product clause
- Manufacturer
- Submit proposals
- Standard
- BS EN 14351-1 and BS 4873
- Material
- Thermally broken aluminium alloy
- Opening configuration
- Top hung casements and fixed lights
- Thermal performance
- Uw not more than 1.4 W/m²K
- Weather resistance
- BS 6375-1, classes as window schedule
- Security rating
- PAS 24 where marked easily accessible
- Coat build-up
- Polyester powder coating to BS EN 12206-1
- Finish
- TBC External colour sample awaiting client approval. Resolve before ordering.
- Third-party certification
- Product and finish applicator certified
Illustrative. Performance of the finished system sits in the system clause; properties of the product sit in the product clause.
In NBS Chorus the aluminium window product clause also carries the NBS reference 45-25-99/310. Use whichever your practice's masters use, but do not mix CAWS and Uniclass references in one specification.
What goes in the spec, and what goes in the schedule?
The specification defines each type once. The schedule and drawings say where each type goes.
In the specification
What the product is and how it performs and is installed. Written once per type.
In the schedule or drawings
Where each type goes and how much of it. Changes with every plan revision.
Give smoke vent windows and fire-resisting windows their own type marks so their test evidence attaches to the right product. The schedule then says where each type goes.
For coordination between these documents, see Why your window and door schedules don’t match your specification.
What are the common mistakes?
Most problems are a missing property, a property filled with the wrong kind of value, or a decision left silently open.
- A RAL number as the whole finish spec. Colour alone leaves pretreatment, film thickness, gloss and guarantee open. Cite BS EN 12206-1 and an approved applicator.
- Slim sightlines drawn, standard series priced. Narrow-frame ranges often cost more and perform worse thermally. Decide which you want before the elevations go to planning.
- Steel fixings straight onto aluminium. Bimetallic contact corrodes in the wet. Require isolation and stainless fixings in the corrosion protection clause.
- Smoke vents written as ordinary windows. A window in the smoke strategy needs BS EN 12101-2 evidence, actuators, power and a controls interface. Give it its own type.
- Centre-pane U-value quoted. Aluminium frames pull the whole-window figure up a long way. Ask for Uw.
- Old BS 4873 cited. The 2009 edition was withdrawn in 2016. Masters that still cite it get flagged at QA and look out of date to tenderers.
How Avoice helps
Avoice's spec writer drafts aluminium window clauses from your schedules and elevations and keeps the special types separate.
- Drafts from your drawings and schedules
It reads window types, opening lights and colour notes from the window schedule and elevations, chooses the L10 or Uniclass heading, and fills in the system, glazing and performance the information supports.
- Leaves a TBC where your information does not settle it
If the finish sample is not approved or the design wind pressure has not been issued, the line says TBC with the reason and joins the project TBC register for closing before tender.
- Writes in your format and your style
CAWS or Uniclass, built from your practice's own L10 masters rather than generic text.
- Checks the clause against the rest of the set
QA/QC flags a smoke vent window on the fire strategy drawings that the specification treats as a standard casement, a withdrawn BS 4873:2009 reference, or a colour in the clause that differs from the finishes schedule.
- Keeps product data in one place
Declarations of performance, U-value calculations and finish guarantees sit in the Product Room linked to the clause, and a linked product edited once updates every clause that uses it.
Related guides: How to specify uPVC windows, How to specify curtain walling, How to specify sealants.
Software can draft and check. The designer still chooses the product and is responsible for the specification.
Draft your aluminium window clause in Avoice
Upload your drawings and schedules. Avoice drafts the clause in CAWS or Uniclass, cites the standards and flags anything your information does not settle.
This guide is for general education and is not professional advice. Check current editions of standards and Approved Documents, the manufacturer's data, and the requirements of your project before relying on any clause.