Building Regulations: Facts vs Fiction

It is not often that I put pen to paper in industry publications, however I feel compelled to write to dispel some of the myths surrounding the proposed changes to the exempt status of conservatories under the Building Regulations (England and Wales).

It is true that a BRAC committee has been set up to consider the elements of the Building Regulations that could be extended to cover domestic conservatories. The proposals being discussed include thermal performance and ventilation, among others. There are no proposals to bring conservatories of less than 30 sq.m. under Part A (structure) of the Building Regulations and there is absolutely no proposal to force manufacturers to produce conservatories using products from systems companies who have formed some kind of alliance or who manufacture both window and roof systems.

The major problem which would be faced by the Local Authority Building Control Officers, should conservatories be subject to any part of the Building Regulations, will be the processing and checking of applications for each and every conservatory installation. The amount of conservatories installed each year would render this a mammoth and impractical task. As such, it is assumed that the conservatory industry will become "self-regulating" under a scheme similar to that provided by FENSA for window installation. The suggestion that the Building Control Officers would have the resources (or desire) to check structural calculations for each installation is pure fantasy.

Perhaps it is worth considering the elements relating to structural stability of a conservatory and comparing them with elements of structures which are already subject to Building Regulations approval.

The structural integrity of a conservatory is dependant on the following factors:

1. The design and quality of the foundations and basework

2. The design and configuration of windows and structural couplings

3. The design and configuration of the roof

4. The quality of installation

The important key-words in the above list are design, configuration and quality.

To suggest that a conservatory would only be structurally stable when the same manufacturer (or those which have formed an alliance) of window and roof systems are used is ridiculous. That’s like saying that you can only build a house using the same manufacturer for bricks, roof trusses, roof tiles, lintels and floor joists. Of course you can mix and match products from different suppliers providing that the overall structure is properly designed.

To imply that all conservatories will require full structural calculations is also stretching the bounds of probability. Let’s take a look at house construction again: lintels, floor joists, roof trusses etc. are chosen from tables produced by the manufacturers, there is no need to provide structural calculations for each element. Steel beams are subject to structural approval but that is because each case can be vastly different, some may be supporting single elements such as a floor whereas others may be supporting a combination of floors, walls and roof. The calculations for such beams are undertaken not only to prove structural adequacy but also to ensure that the design is economical.

There are also general guidelines that are followed for the design of traditional strip foundations, maximum lengths of brickwork before expansion joints are required, minimum sizes and spacing of brick piers etc. It is not normally necessary to provide full structural calculations for these elements in standard house design.

Let us now consider domestic conservatories:

Assuming that the foundations and basework are adequately designed, the next consideration would be the windows. PVCu windows (almost without exception) are designed to fit within openings in the building structure. Their function is to let light in, to keep the weather out, to provide a degree of insulation, to withstand wind loads, and to give the option of opening vents. They are not designed to carry vertical loads from the roof.

Vertical loads are catered for by the addition of structural couplings between each window. These structural couplings, which can be either straight or angular, were originally designed for bay windows and are intended to transfer the imposed roof load to the basework. They can also be used to provide additional resistance to wind loads.

All window manufacturers supply a range of these couplings and generally give tables or simple formulae for their maximum load bearing capacity. The spacing of the couplings is dependant on various factors: the load which is to be transferred, the height of the couplings and the maximimum span of the structural element which they are supporting (the eaves beam).

Let’s face it, domestic conservatory windows are generally between 1500mm and 2100mm tall, have mullions at between 600mm and 900mm centres and, due to transport and handling restrictions, are generally not wider than 2400mm. This obviously restricts the amount of design that would be necessary and the only consideration within the above parameters would be the spacing of the structural couplings (referred to above) and the effect of that on the maximum width of the windows.

Moving on to the roof, the majority of domestic conservatory roofs have pitches of between 20 and 30 degrees (or 2.5 degrees upwards on lean to styles) with glazing bar centres of between 600mm and 900mm. Most roof systems companies provide guidelines and tables for the selection and use of structural elements such as glazing bars, eaves beams, ridges, valleys and tie bars. They also provide general guidelines on the maximum sizes of conservatories. These guidelines have been produced following theoretical calculations and structural testing.

Of course, conservatory roof systems companies have no idea what sort of structure their roofs will be sitting on so their guidelines are restricted to the structural integrity of the roof system only on the assumption that it is supported by an adequately designed structure. The correct choice of the individual structural elements and correct spacing of tie bars mean that their roofs will be capable of withstanding both dead and imposed loads and of transferring them to the structure below.

Admittedly, the combination of roof and window frames as a complete structure does become something of a "grey area" when considering unevenly distributed loads or lateral wind loads. In these loading conditions the ability of the structure to resist "racking" becomes a consideration. Again, this is where the general guidelines come in to play. Lateral wind loads are higher on conservatories in exposed locations or with long or tall elevations, unevenly distributed loads can be the result of unusual designs or snow drifting in valley and box gutter situations. The K2 Structural Guide (and I’m sure most of the structural guides from reputable systems companies) will take account of the majority of these extreme conditions.

The transfer of lateral wind loads through a conservatory structure is difficult to prove by calculation. The loads are actually transferred through the glass units, acting within the frame in what is commonly referred to as "diaphragm construction". As any installer knows, a conservatory is easily pushed and pulled out of and in to shape until the structure is glazed, at which point it becomes much more rigid and stable. There are a number of factors that cannot be legislated for in this type of construction: the amount of friction between the beads and the glazing, the packing of the glass to the frame, the amount of panels through which the load is transferred, the amount of mullions and couplers and the nature of fixings between each window.

It is generally accepted though that domestic conservatories, designed in accordance with window and conservatory roof systems companies guidelines, are capable of resisting lateral wind loads. Of the 280,000 conservatories installed each year how many fall over? I have only seen one publicised failure of a complete conservatory structure (used extensively by one of our competitors) which, on close examination of the photograph, appeared to be as a result of the window frames not being fixed to the brick dwarf wall or the existing building. This goes to prove that even a correctly designed and manufactured combination of roofs and frames is still dependant on the quality of installation. Indeed, having spent 15 years in the conservatory industry, the majority of problems that I have come across (structural or weatherproofing) on conservatory structures can be put down to poor quality of installation rather than design or choice of window and roof manufacturers.

As stated above, around 280,000 conservatories are installed in the UK each year and it is generally accepted, in all circles, that they are robust structures which are generally structurally sound and have adequate weatherproofing ability. In the USA, however, conservatories are new and unfamiliar technology and K2 have had to prove the structural stability of their window and roof systems in conservatory structures. As previously discussed, the proving of a glazed structure by calculation alone is a difficult exercise, so structural calculations were supplemented by physical testing undertaken by a reputable test-house. This testing has led to approval of K2 conservatory products in 48 states, many of which have more onerous wind and snow loads than those experienced in the UK. K2 has also sought and obtained BBA and BSI approval for their products in the UK, the majority of this approval relating to theoretical and physical structural testing.

Of course, conservatories over 30 sq.m. have always been subject to Building Regulations (England and Wales) approval and K2 have always provided structural advice or full structural calculations in these circumstances.

In conclusion, there is no need for conservatory fabricators or installers to worry about any potential changes to the Building Regulations to cover the structural integrity of conservatories. If and when (the main consideration being "if") these changes come into force then the guidelines for design already exist and have been proven over many years of conservatory manufacture and installation. There is also no need to worry about the removal of the freedom-of-choice to mix and match manufacturers of roofs and window frames as any roof should be able to be married to any frame, providing that the guidelines outlined above are followed.