A Simple Guide to Structural Integrity of Roofs

A conservatory roof is a complex piece of engineering, and this guide attempts to explain the various forces acting on it in simple terms. I am not a structural engineer - this a guide only and should not be used as the basis for any calculations. As always, if in doubt seek professional advice before deciding if a design is structurally sound.

Forces

There are several major forces acting on a roof:

• The downward vertical load of the rafters, glazing and worst-case snow load
• The upward vertical load from worst-case wind (as the wind speed increases it creates a vacuum effect which pulls the sheets upwards)

In order to determine if the roof is suitable for its site location we need following information:

• The glazing weight per square metre
• The maximum expected snow load per square metre (site specific)
• The strength of the supporting rafters
• The maximum deflection acceptable for the chosen glazing (which is lower for glass than polycarbonate)

The site specific information is often looked up from a location map - contact Maitlands for more details.

Failures

A conservatory will begin to fail for a number of reasons:

1. Glazing cannot deflect beyond the maximum possible, and cracks or disintegrates
2. Fixings (screws, expansion bolts etc) cannot sustain the loads upon them
3. Cappings can no longer resist the loads upon then and become detached
4. The ring beams deflects causing doors or openers to jam

Remedies

If the given design is found to be insufficiently strong there are a number of remedies, including:

• Use stronger rafters to reduce glazing deflection
• Use tie bars or tie bar removal kits to reduce ridge deflection
• Use a portal frame
• Use alternative glazing which is lighter or has a higher deflection co-efficient

Tie Bars

As the weight supported by the ridge increases (because either the span is large or the ridge length is long, or both), there will be a large force pushing the ridge down and the rafters outwards. When this load exceeds safe limits, the ring beam will start to be pushed outwards, which in the worst case could result in a roof collapse. A tie bar will prevent this outward movement by "tying" the rafters together. Note that the vertical element of a tie bar is simply to keep the bars level - all of the work is done by the horizontal members. Very large roofs may require two or more tie bars.

Use of a tie bar is illustrated below:

Portal Frames

A portal frame provides a structural support upon which the roof sits. Typically portals are made of a steel or aluminium tubing, which is either welded or mechanically jointed. The portal is designed to transfer the calculated loads to fixed concrete footings. As a result the entire structure will by much stronger and capable of supporting a larger roof. A simple portal frame is shown below:

The roof would sit on this portal as shown here:

Automating Remedies

Resolving some of these structural issues manually can be very time-consuming and often requires the services of a structural engineer.  Better software systems can recommend solutions automatically for many designs although complex designs will still need to be referred to an expert. In some cases there can be several ways of solving specific structural issues, and this is where the experience of a qualified engineer is invaluable.