Replacing Steel Windows

A guide for home-owners

Peter Clement

melia white house
Melia White House, a Grade II listed Art Deco hotel in London, built in 1936 (Photo: Silver Cloud Photography)

Replacement of existing steel windows in a historic building might be considered for a number of reasons, the principal one being that the present windows, through age, neglect or both, have deteriorated to such an extent that repair or restoration, either in situ or off site, is not an option. Other common reasons include the need to reduce energy loss from single glazed metal frames, or a desire to remove inappropriate earlier replacements. Any proposal to replace windows in a listed building would require the owner to seek permission, as listed building consent is required for alterations to these buildings, and in practice any replacement will involve a degree of alteration. The need for planning permission may also apply if the building is unlisted but in an area designated for protection, such as a conservation area or area of outstanding natural beauty.

Depending on the listing and the historic or architectural significance of the property within its locality, securing permission can sometimes be a lengthy process. Consequently, it is sensible to discuss any plans for replacing windows with the planning authority at the earliest opportunity. Also consider seeking guidance from an experienced steel windows professional and a conservation architect or heritage consultant, who could approach the planners on your behalf to find out how they might respond to any proposed window replacement and what conditions might be imposed. These early meetings could save a great deal of time and money by avoiding window styles, types or patterns that never had a realistic chance of gaining approval.

When considering an application, subject of course to the property’s status, planners will generally prefer to see a replacement which matches the existing as closely as possible. Some problems should therefore be expected where a proposal includes double glazing, since the addition of sealed units will inevitably affect the windows’ appearance when compared with the existing. Once you have received guidance from the planning department’s conservation team on what is and is not likely to succeed, this might be the time to approach a number of steel window manufacturers. Begin the process by discussing what product range they can offer and how close these are to what you and the planners are looking for.

Steel Frames

We tend to think of steel windows as belonging to the mid 20th century, typified by those of the major steel window companies of the era such as such as Clement, Crittall, Hope's, and Williams and Williams.

However, before the introduction of standardised rolled steel window sections at the end of the First World War, steel windows were already popular in domestic architecture, particularly as a result of the fashion for leaded light windows in the late 19th century.

These early examples include a wider variety of profiles and components, and may be mistaken for blacksmith-made wrought iron windows. Cast iron windows are also sometimes found in late 19th century houses, but they are readily distinguished from steel windows by their shape, and they often had more elaborate designs.

Today the steel window replica replacement industry remains dominated by hot rolled steel windows whose profiles are close in appearance to those which have existed in a similar form since late Victorian times and the basic ‘Z’ pattern section used to create an opening light remains universal.

Although there are now far fewer steel profiles available than there were, it is still possible that your manufacturer could find a suitable one from stock that either matches or is close to the hot rolled steel sections required. Failing this, those profiles that are available can in the hands of an enterprising manufacturer be milled down or engineered to suit. This may consist of welding on specific steel shapes and patterns, such as an ovolo or half round moulding, to get close to the original pattern.

The small number of windows that are very highly decorated may prove to have been fabricated from cast iron, not steel, and it should be feasible, subject to expense, to have new castings made if the project warrants it. However, steel windows too can vary significantly, and sometimes a rare and unusual example of historic fenestration requires highly bespoke steel replicas.

Should the project be of a size to justify the cost, new steel profiles can be rolled to order to replicate the originals precisely, and it is interesting to note that these one-off solutions are possible even on a smaller scale given sufficient desire and resources. Once made, the frames are hot dip galvanised to BS EN ISO 1461:2009 EN ISO 1461, weather stripped and if required have a surface coating of polyester powder paint to BS EN 13438. If the project requires a brush applied finish, it is helpful to inform the galvanisers who can then apply an acid etch to the steel surface to improve paint adhesion.

comet works windows
W20 steel windows at The Comet Works, a former Victorian armaments factory in Birmingham (Photo: Silver Cloud Photography)

Single Glazing

The selection of glass is at least as important as the choice of frame. After all, as a percentage of the window’s overall dimension, glass takes up the greater area, and getting the right effect can determine the suitability of the replacement steel window. If a small pane size is required and there is no need for safety glass, glass with slight distortions can be bought from a number of specialist companies, including genuine mouth-blown cylinder glass, machine-drawn horticultural glass, and modern float glass that has been artificially distorted in imitation of earlier forms. Making the correct choice is something that needs careful consideration. It is a far more straightforward exercise for a listed property if the replacement window is to be single-glazed, particularly if it includes leaded lights.

A sealed glazing unit with lines of lead applied to its surface will always appear different from an original genuine leaded light. One pane of glass has a single uniform reflection, whereas leaded lights have many small panes of glass joined together by the leadwork, so the surface sparkles with numerous reflections. Leaded lights are still crafted by hand today using the traditional methods. Most of the leading consists of H-shaped lead strips known as ‘cames’ while the outside perimeter is made up of border leads similar to a channel. Both are available in several profiles as this too can have a strong impact on the appearance of the window. For example, the architect Edwin Lutyens would often use a bold and heavy 1 inch/25mm lead came alongside a black steel frame and dark oak.

In the past, most glazing was fitted into the window frame from the outside with putty. When windows are replaced, securing the new glazing with internal beading will detract from the look of the window, as it can remove all or some of the window frame’s shadow lines. These shadow lines are the result of a thin line of metal projecting beyond the plane of the glass and putty. The lines help to break up the appearance of the window frame so it becomes less heavy and therefore easier on the eye. In contrast, the internally glazed option results in a flat, featureless and less characterful steel frame and consequently it is not usually recommended.

The traditional use of putty in outside glazing can be replicated with modern low modular silicone in a colour which mimics the appearance of the original putty. This is helpful when a pre-decorated primary frame coated with polyester powder paint is used as putty requires redecoration every four to five years. However, the surface of silicone and other sealants/mastics can discolour in heavily polluted areas and the dirt is difficult to remove.

Double Glazing

The number of different profiles available increased significantly as manufacturers began to focus on improving the insulation of their windows and doors, introducing deeper glazing rebate platforms to accommodate insulated glazing units (IGUs). In the late 1960s and early 70s the UK company Pilkington, which was already a world leader in new glass technology, was pioneering the development of different methods of double glazing steel windows. Two of its earliest double glazing units were the Insulight Mark VI and Glastoglas, the former being hermetically sealed while the latter used a vacuum-like technique. With the arrival of these and other companies’ solutions for saving energy, steel windows would never look quite the same.

Today the replacement steel window manufacturer will aim to engineer a window that offers excellent performance and closely resembles the window it is replacing. The object of the exercise is generally to install a high performing steel window that will not detract from the building’s appearance. More substantial construction schemes are governed by project documents including guidance papers for all the specific elements of the building, and windows specifiers often refer to the NBS specification L10 which calls for double glazed windows to achieve energy preservation, security and reduction of sound and solar gain.

Listed building applications for double glazing in historic buildings are commonplace as owners strive to improve thermal efficiency in their properties. However, the planning authority quite rightly gives great thought to the appearance of the double glazing and any related changes, and the extent to which these will change the appearance of the listed building. If the proposals are considered to have an adverse effect on the appearance and character of the building and are rejected by the planners, the applicant has the right to appeal.

There are no hard and fast rules as to what is and is not permissible and every application that includes replacing existing single glass with double glazing can only be judged on its own merit. On the assumption that the planners will consider double glazing, the choice of insulated glass units (IGU) is wide and varied. However, options are reduced if the new steel windows are to be manufactured using the slim sections of the type associated with private residences.

Profiles such as F7D and W20 were initially designed for single glazing and although their glazing platforms can accommodate an IGU, the space is limited when compared to modern glass rebates. There are alternatives like Clement's EB suite of steel sections which have similar sight lines to the F7D but deeper rebates. Other robust sections may be more appropriate for commercial and industrial applications such as the W40 and the MW40 which are better suited to larger thicknesses of glass but might still suit the design requirements of a listed residential building. IGUs can provide improved thermal insulation as well as protection from the sun and the addition of acoustic glass will further reduce noise. They are all available with a selection of spacer bars in different colours and a gas filled cavity, using argon, krypton or more rarely zenon which are better insulants than air.

Secindary Glazing and Thermal Blinds

In situations where the planning authority will not permit double glazing due to its adverse impact on a historic building, it is usually possible to fit an internal glazing system over the existing windows. These secondary glazing systems or, indeed tight fitting heavy curtains and thermal blinds, offer a remarkably effective alternative to renewing primary windows. Depending on the glass fitted, secondary glazing is good for reducing heat loss, the fading of soft furnishings, draughts, noise and solar gain, and for increasing safety and security. However, with larger panels of glass the windows can be awkward to operate, and they may make cleaning and maintenance of the original windows difficult.

Generally, the systems are made from aluminium and are easy to install, being screw fixed into place in the internal reveals. They are available as horizontal and vertical sliders, side hinged and top hung, and the supplier can design the frames to align with the existing window frames so that they are less visible during daylight hours. Research carried out for Historic England by Glasgow Caledonian University in 2015 showed that low-e secondary glazing reduced heat loss from a single glazed metal window by 68 per cent, giving a whole-window U-value of 2.1 W/m2K. They were also able to achieve a 54 per cent reduction in heat loss using a roller blind with low-e foil, while heavy curtains alone offered a reduction of over 40 per cent.

Window Fittings

A broad selection of steel window furniture and fittings is available to suit steel windows of different periods and styles, from an Art Deco apartment to a mock-Tudor farmhouse. When required and cost allows, bespoke furniture and fittings can also be made to match. There remains a small group of artisan crafts people in the UK and Europe who are able to carry out this specialised work. In addition, the industry can offer from stock multi-point locking high security systems or single point key locking as well as individual security devices to suit individual requirements.

Decorative handle plates, both elaborate and plain, were a popular way of personalising steel windows, particularly in the Edwardian period. If original steel windows featured these special details, the new replacements could look quite bare without them. They used to be hand cut and filed out of sheet steel by the factory apprentice, but today they are usually laser-cut to match.

Installing the Windows

Fitting windows into listed buildings may require more expertise than into more conventional properties. Single glazing, double glazing or genuine leaded lights can be either bead, putty or silicone glazed, externally or internally, and steel windows can go into brick, stone, masonry or wood. There are a number of points to consider during the process.

To ensure that the window remains weather tight, the bedding compound is traditionally applied between the perimeter steel sections and the structural opening or wood surround, commonly referred to in the trade as ‘solid bedding’. When modern day fitters remove old windows, they often find evidence of solid bedding which was applied 60 or 70 years ago but is still doing a great job.

Several good quality ‘universal bedding compounds’ (UBC) are available from builders merchants. The fixing itself is generally achieved with stainless steel screws which can be fitted directly through the frame by drilling a suitable hole and inserting a rawlplug before putting the screw in place. If for some reason a strong fixing cannot be found immediately beneath the frame, then fixing with a lug or strap into the internal window reveal is perfectly adequate.

Be aware that if you are fitting steel windows or indeed any windows direct to stone, particularly porous stone, that the surface will need to be carefully sealed before applying the mastic. This is to stop what is called ‘mastic bleed’ which can leave the stone permanently discoloured. For windows fitted into existing wood surrounds it is good practice to assess any damage and consult a qualified person on how to deal with minor repairs.

In many cases a replacement is not required and the issue can be dealt with in situ using filler or by scarfing in a new section of wood. It can sometimes be necessary to remove the steel window to make a full inspection. The installer should pay particular attention to the perimeter seal between the steel and any form of masonry or wood surround to masonry. The client and installer need to consider the sealant colour, as sealant taken from stock may not be an appropriate hue.

Bespoke colours are available but even then, perhaps the right course of action is to carefully point around the perimeter with mortar. In certain circumstances it will be important to engage a stonemason who can follow the progress of the window fitters and point as required. Completed correctly, this finishing touch can make all the difference to the final installation.

Lastly, it is important to ensure that the installer chosen has the necessary expertise and competency. The window company should be asked about the experience of their installation team regarding steel windows and not just their knowledge of fitting windows generally.

Steel windows are a very different product from aluminium, plastic or wood and represent less than five per cent of the total number of new window installations undertaken annually. As a result, many window installers have never fitted a steel window even if they have been in the trade for 15 or 20 years plus. Therefore, it goes without saying that any window team working on a listed building must work perhaps to a slower pace than they are used to and pay particular care and attention to avoid damaging any of the surrounding masonry or brickwork. A high quality installation takes time.

Recommended Reading

Paul Baker, Improving the Thermal Performance of Traditional Windows: Metal-Framed Windows, Historic England 2015

Historic England, Practical Building Conservation: Glass & Glazing, Routledge, 2012


The Building Conservation Directory, 2021


Peter Clement is Chief Executive of
Clement Windows Group

Further information


Metal Windows

Wrought Iron and Steel Windows

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