Structural glazing: How to get it right
Expansive swathes of glass will give your house wow factor in spades. Julian Owen sums up the design possibilities and potential pitfalls to look out for
If you’re looking to add some minimalist finesse to your house design, structural glazing is a fruitful area for exploration. We think of glass as a fragile material, so when it’s employed to form selfsupporting structures, it seems to defy gravity, adding real aesthetic prowess. Plus, it allows high levels of light to wash over your home interiors.
Regular glass is relatively delicate and traditionally has been used in rather limited sizes, with the edges protected and held in place with frames of timber, metal or PVCU. Although made from the same basic constituents, structural glass is a different beast entirely. In many ways, its construction has more in common with plywood than a standard doubleglazed window. It is made from up to four or five layers of toughened glass that are bonded together as laminate sheets, which in turn are set into double- or triple-glazed units. The result is a thick, strong component which is self-supporting.
The larger the size required and the more load it’s carrying, then the stronger and thicker the glass panel will need to be; though the maximum dimensions are limited by the manufacturing process. Typical thicknesses can range from 6mm to 80mm depending on the use and location. The panes are laminated, with thin plastic-based films between each layer to add strength and maintain the unit’s integrity if it should crack. The largest single sheet that can be made in the UK is 6m x 3.2m, though bigger sizes are available abroad – for a price. But once you increase dimensions above 2.4m, this will put the pane into the oversize category, which comes hand-in-hand with a significant jump in the price.
What is structural glass?
To fit into this definition, a pane needs to be capable of supporting its own weight. It should also be able to resist the loads that are imposed on it without the support of a frame, taking some element of loading beyond its own weight. This type of glazing is different to so-called frameless windows, which are usually inset into a concealed frame buried in the surrounding construction.
To create the right aesthetic, it is essential that the glass looks like it’s
floating, free from support. This means that the fixings holding it all together are designed to be as discreet as possible. The attention to detail required is such that this is strictly the territory of a specialist.
One of the beauties of structural glass is that it can be used to connect two separate parts of a building; its transparency contrasting with the solidity of conventional construction materials. Two sections of a building that are joined with a glass link will still appear to be distinct from each other, as the glazing gives the illusion of having no mass. Plus, large areas of glass with minimal joints defy our expectations, creating a dramatic visual effect. Another benefit is the high level of daylight invited into any internal spaces where it is used, establishing a seamless connection with the world outside.
Design ideas
Large glazed walls can be formed from structural glass with minimal joints between the sheets, requiring nothing more than a thin mastic seal. This is perfect if there is a great view to be framed with as little interruption in sightlines as possible.
Glass boxes, meanwhile, are the modern spin on the traditional conservatory and – so long as they are immaculately designed and detailed – make a spectacular addition to a house. The problem of heat loss is still there, but bespoke glazed structures can be designed to have a better level of insulation than standard conservatories.
This type of extension is regularly used to solve the design problem inherent in adding a modern structure to an existing building with a strong character, which might be ruined by a conventional extension (particularly if the original is listed). Adding a glass link creates a complete visual break between new and old, which are then seen as different compositions, leaving the original house pretty much unaffected. The result is often stunning and usually wins the enthusiastic support of planning departments and Historic England.
In a similar way, the unobtrusive qualities of self-supporting glass can be used when converting threshing barns. This approach allows the large openings in each side to look pretty much as they did when the building was used for farming activities.
Structural glass also comes into its own where daylight is limited in the lower levels of a house, such as a basement. No one should try to walk across ordinary glass because it’s so weak that there is a high risk of falling through, but structural glass will easily cope with the load, allowing gloomy areas to be lit from above. Panels of structural glass set into the floor or into the ground outside avoid the risks of damp and are more secure than lightwells. Privacy can be preserved by sand blasting the surface, which allows light to filter through without giving a grandstand view of the occupants in the room below.
Glass can also be used to make entire staircases, potentially turning a dark, central landing into a bright design feature running from the top to the bottom of the house.
Potential challenges
One possible obstacle you might encounter is how to fit a frameless door leaf into a glass wall, because the bolts needed for hinges put a lot of localised strain on the surrounding sheets. If the window is to meet the ground at the same level inside and out, careful design is needed to ensure that, as the water drains down the outside, there is no damp penetration through to the interior.
Glazed roofs and walls featuring substantial spans need extra support, and if you wish to avoid spoiling the minimalist effect with any kind of frame, fins of glass projecting out at 90° will do the job. Additional support can be provided by thin stainless steel cables in tension.
As the glass sheets get thicker, they tend to develop a greenish tinge caused by the presence of iron-based impurities, which isn’t visible to the human eye in standard windows. Reducing the iron content increases the transparency of the material – along with the cost. The alternative is to have a coloured tint added during the manufacturing process.
Overheating is one risk associated with installing large sheets of glass that catch the sun’s rays, especially if the window is south facing. Adding a solar control coating to the spec can help to alleviate this effect.
Another fairly pricey feature that can be very convenient – as well as impressive – is glass that has a small electric charge running through it. When turned off with the flick of a switch, it turns opaque. This high-tech option is especially appealing if you are pursuing a minimalist ideal that rules out curtains or blinds.
Choosing your supplier
There are a couple of elements to bear in mind when researching which glazing company to use. The first is to be aware that you will probably have to pay a sizeable deposit when you place the order, and then possibly the
outstanding amount before the fitting team arrive. Added to this, typical lead times can be as long as 16 weeks, which means that you might have to select your supplier well before you know who is going to be your main contractor for the rest of the project.
In addition, there is a co-ordination problem once the main building work has started on site. Normal windows have plenty of tolerance and are often put in place shortly after the openings have been constructed and the roof is sealed. Fitting structural glazing, by contrast, is an exercise in precision. Unless your builders are confident in their ability to work to within a few millimetres of accuracy, the glazing supplier will have to measure the real opening before fabrication. This means there will be an interruption in completing the watertight shell.
Before you place your order, it’s essential to ask who will be fitting the glazing. You might think this is obvious – it’s the company you are speaking to about buying it, isn’t it? Unfortunately, this isn’t always the case. The marketing blurb of some apparently respectable firms creates the impression they’re suppliers, but they pass the actual contract on to a separate company that may prove to be less competent or financially stable than the glossy brochure implies. So always do your research.