By Engineer Arnon Kfir, May 18, 2021

Are you planning or building a new, modern home and want the roof edge details to be designed and installed safely?

If so, it is important to give careful thought to the design and installation of cornices, coping elements, special roof corners, and other building-edge details.

Over the years, I have inspected many projects and investigated numerous cases of damage and failure. Some of these failures caused damage to property and to third parties. I therefore decided to share my conclusions in the hope of preventing situations that could potentially result in serious injury.

Many of these dangerous failures can be prevented through greater awareness, proper workmanship, and responsible maintenance.

What Causes These Failures?

When we talk about an edge detail on a building, there is almost always some form of gap between the edge element and the main building structure.

This gap may be relatively large, such as the space created around a pipe or rounded bullnose detail, or it may be very small, such as the gap beneath flashing installed on a standard panel roof.

In either case, failure can occur for one or more of the following reasons:

• There is no proper seal between the elements, allowing wind to enter the space between the cornice and the building or roof.
• The flashing is made from thin sheet metal or has not been bent and formed correctly.
• Flashing or sealing materials are missing.
• Incorrect sealants, screws, or fasteners are used.
• There is insufficient overlap between adjoining elements.
• The steel or aluminium components are unsuitable, for example because they are too weak, too thin, or too flexible.

What Is the Connection Between Pressure and Force?

When wind enters through a gap left during installation, or through an opening created by deterioration or partial detachment, it creates pressure inside the space between the roof edge detail or cornice and the building.

As wind speed increases, the force also increases.

The pressure acts from inside the cavity outward, creating an uplift force that attempts to push the edge detail or cornice away from the building.

The main purpose of this article is to make readers aware of the importance of preventing conditions in which this type of internal pressure can develop.

Are There Warning Signs?

A cornice does not always detach during the first winter or the first strong wind. It may remain in place until a severe storm occurs.

In many cases, however, the problem can be identified well before a major failure takes place.

Such defects may be found during routine inspections and preventive maintenance, which building owners are required to carry out under Israeli Standard 1525 for building maintenance.

These types of failures can be detected during scheduled inspections in accordance with the standard, including an annual inspection by the person responsible for maintenance and an inspection by an engineer every five years.

Any defects discovered during these inspections should be brought to the attention of the responsible engineer, who should provide instructions for the required repair work.

Possible warning signs include:

• A visible gap that has developed between elements
• A piece that has already fallen away
• Loose screws
• Screws that have popped out of position
• A small flashing section that has moved or become detached
• A missing side cover
• A sagging element, possibly caused by the weight of water that has entered it
• Any other visible movement, opening, or deterioration

What Are the Most Common Causes of These Failures, and How Can They Be Prevented?

1. Wind Entering Through Open Gaps

As mentioned above, one of the main causes of detachment is wind entering through an opening and creating pressure from inside the cavity.

This pressure can lift the outer covering away from the structure.

The first priority, therefore, is to ensure that all gaps remain properly closed.

This can be achieved through the correct use of:

• Flashing
• Suitable sealing materials
• Appropriate rivets and screws
• Proper installation methods

2. Incorrect Screws and Sealing Products

In my experience, many contractors and installers working in this field have insufficient knowledge about the correct fasteners and sealing products for thin sheet metal.

This lack of knowledge often leads to poor choices of screws, fasteners, and sealants.

Personally, I generally recommend the use of rivets rather than screws wherever appropriate.

Rivets may cost slightly more, but they often provide a stronger and more reliable long-term connection.

A useful rule to remember is:

Connections between thin sheet-metal elements should be made using non-self-drilling screws where screws are required.

3. Insufficient Overlap Between Edge Components

Roof edge details are often limited in length by the size of the machinery used to manufacture them.

As a result, several sections may need to be joined together.

Special attention should therefore be given to the way these parts are connected.

Where possible, use:

• Large overlap areas
• Male-and-female joining methods
• Concealed connection systems
• High-quality mastics, adhesives, and sealing materials between sections

The objective is to create a strong, continuous, and properly sealed connection.

4. Sheet-Metal Edges That Are Too Narrow

Another common failure occurs when the sheet-metal edges are not wide enough.

Attempts to reduce material usage can leave installers with very little surface area for proper fastening.

As a result, screws or rivets may be installed too close to the edge of the sheet metal.

This significantly increases the risk of failure.

In addition, insufficient distance between the fastener and the edge of the material may not comply with the relevant Israeli standards.

Adequate material width must therefore be provided during both design and manufacturing.

5. Inadequate Supporting Structure

The supporting structure beneath roof edge details and flashing is also extremely important.

These components are usually fixed either to concrete or to roof cladding systems such as insulated panels or corrugated sheet metal.

Installers must ensure that flashing is fixed to a proper supporting structure made from profiles with a thickness of at least 2.0 mm, where required by the design.

Angles fixed to concrete walls should generally be installed at relatively close spacing, such as approximately 600 mm, subject to the engineer’s instructions.

The thickness of the supporting material should not be less than 2.0 mm, where specified.

Summary

To ensure that cornices and roof edge details remain securely in place and are able to withstand wind forces, several basic principles should be followed.

Plan the installation properly from the beginning, using suitable materials, high-quality fasteners, and effective sealing systems.

Do not try to save material at the expense of safety during the design or manufacturing stages.

Use appropriate construction methods such as:

• Male-and-female connections
• Large overlaps
• Folded and reinforced edges
• Strong and sufficiently thick materials

Every gap, hole, or opening through which wind could enter the cavity must be properly sealed.

Regular inspections and appropriate maintenance must also be carried out.

Do not postpone repairs, and never ignore early signs of movement, deterioration, or failure.

This article does not address the quality of the raw materials themselves. It assumes that all materials used have been approved by the client’s engineer and supervising professional and that they comply with the requirements of the relevant Israeli standards.

About the Author

Arnon Kfir is an engineer and specialist in roofing systems and building envelopes. He conducts engineering arbitrations and prepares professional expert opinions for court proceedings. He is also registered on the list of court-appointed experts.