Clement W20 Range of Steel Windows

Product Summary

A traditional, tried and tested robust window, designed in the 1960s, but still in demand today. W20 may be used in all types of building where strong, durable frames, efficient opening casements and hard wearing fittings are required.

Part L Regulations

Compliant to Part L, using the centre pane glass U value method.

Manufacturing Specification

Made to suit your individual requirements W20 windows are generally manufactured in accordance with BS 6510:2010 specifications for hot rolled steel windows. The mild steel sections used for Clement window and door frames are precision rolled in Switzerland using recycled steel to suit Clement’s unique profiles and tolerances. All frames and ancillary profiles are hot dip galvanised to BS EN ISO 1461 and available with a factory applied polyester powder coating to BS EN ISO 13438 from the RAL colour range, exceeding the minimum paint thickness over the zinc of 60 microns.

Manufacturing Description

Frames are manufactured from hot rolled mild steel profiles with corner joints mitred, welded and dressed square and flat. Small panes can be formed with T glazing bars whose ends are tenon riveted and/or welded to the frame and cross joints are interlocked and welded with rigid joints. Composite windows can be assembled by connecting windows horizontally and/or vertically with mullions and/or transoms of hot rolled slim steel profiles. Box sections are available as tubes or as box mullions which can be either hot rolled or manufactured from sheet steel. Pressed metal cills are available in a choice of profiles. Trickle vents can be fitted in accordance with Part F of the Building Regulations.

Locking system

The W20 range of windows is available with multi point locking to PAS24 or traditional single point locking and other security devices if required.


W20 windows can be fitted into timber subframes, or direct to brickwork, concrete or stone. Windows are installed using fixing lugs or direct using stainless steel screws.


W20 can be supplied with various glazing options, from both inside or outside including:

  • Clear glazing
  • Genuine T bars
  • Leaded lights, using real lead that is soldered by hand in a diamond or rectangular pattern in variable widths of lead
  • G+, no grille in cavity

Combinations of fixed lights, top hung, side hung, bottom hung, horizontal and vertical pivot windows are available as well as single and double doors in both ‘open in’ and ‘open out’ configurations.

W20 windows are available as single or insulated glass units up to 16 mm. When working on highly sensitive, Listed Buildings there is an option for outside single glazing with either putty or silicone fronting. All opening vents are weather-sealed using EDPM.

In accordance with Glass & Glazing Federation best practice, Clement steel windows are factory glazed, however, our concealed fixings mean that fixed light windows need to be glazed on site after the frames have been installed. Glazing beads are made of aluminium.

W20 windows can be supplied shaped, with shaped heads, and ‘curved on plan’.

Dimensions (Provided for guidance purposes only)

Nominal profile width: 32mm
Typical sight lines: fixed lights perimeter: < 40mm
Hinged casements: < 70mm

Windows and doors for composite panels are purpose made, generally within the limits shown in the table below; sizes outside these limits may be discussed with one of our sales consultants.

  Width Height Perimeter
WINDOWS Min (mm) Max (mm) Min (mm) Max (mm) Max (mm)
Fixed light 300 3000 300 3000 9200
Top hung 300 1800 300 1800 8400
Side hung 300 900 300 2400 6400
Bottom hung 300 1800 300 1500 6400
Horizontal pivot 400 1800 400 1800 6400
Vertical pivot 400 1500 400 2400 6500
Single 600 900 2000 2500 6600
Double 1200 1800 2000 2500 8400

Source: Steel Window Association


W20 windows meet the weather-tightness (air infiltration, water penetration and wind loading) performance criteria within BS 6375-1:2009 appropriate to the specified design wind pressure.

BS EN6375 part 1

W20 side hung vents have been weather tested to BS EN6375 part 1 achieving the following results:

  • Air permeability 600Pa, class 4
  • Water tightness 750 Pa, class E750
  • Wind resistance 2000Pa, class C5

BS 6375-1:1989

Air permeability was measured in terms of opening joint length (m³/h/m) against progressively increasing test pressures through 200Pa (class A), 300Pa (class B) up to 600Pa (class C). Class B, or a maximum value at 300Pa of about 16 m³/h/m, was the UK standard requirement. Water tightness was measured in resistance to leakage at progressively increasing test pressures, 300Pa being considered the most severe UK requirement. Wind load resistance entailed deflection and gusting tests at pressures ranging from 1200Pa to 2400Pa.

 Test Pressure Class BS 6375-1 1989 Edition
 Window Type Air Water Wind BS 6375-1 Exposure Category
Fixed light 600 300 2400 2400
Top hung 300 200 2000 2000
Side hung (open out) 300 200 2000 2000
Side hung (open in) 200 100 1600 1200
Bottom hung 200 100 1600 1200
Horizontal pivot 200 100 1600 1200
Vertical pivot 200 50 1600 1200

Source: Steel Window Association


W20 windows when double glazed with argon filled low-E warm edge insulating glass units, will have a typical “U” value of 2.9.


The average sound reduction index (SRI) of a double glazed unit is approximately 31 dB, but this varies with window type and size and glass thickness.


In accordance with approved Document Q we can achieve PAS24:2016 on top hung and side hung vents, as well as single and double doors when using our multi point locking systems.


Clement Steel Windows Glazing


We offer five different styles of glazing, from clear glass, to genuine lead in an arrangement to match the pattern of your original leaded lights, to the G+ and Fenestra joint systems which are unique to Clement. All of these options have superb features which faithfully replicate those of original steel windows.

Clement Steel Windows Colours


Our steel windows are hot dip galvanized and available in a choice of highly durable Akzo Nobel powder paint colours. Clement Windows provide an average paint coating in excess of 100 microns on flat surfaces – the industry standard calls for 60/70 microns. Order from a wide range of RAL colours to match your scheme or choose different colours for the inside and outside of your windows.

Clement Windows Accessories


Browse a select range of accessories which perfectly complement our stunning steel windows, doors and screens. Carefully crafted in a number of different styles and available in a choice of colours and finishes, our range of handles and other fittings are made from the finest quality materials and manufactured with the latest technology in mind.