The Art of Perimeter Detailing - Ridge

One of the key aspects of any roof system is the junction points and perimeter details, that form a roof system and yet more time is spent on the specification of the roof sheeting material and colour than the technical design of the ridge, barge, sidewall & head wall to name a few of the perimeter details.

With the cheapest priced contractors winning tenders you may wonder why your perimeter flashings are installed on the basis of commerce rather than technicalities and key hidden components not installed, all of which lead to an alarming amount of buildings suffering with water leaks.

Why are the perimeter details so important?

Perimeter details or junction points are the weakest point in any roof system, but are essential to close off the insulation from the elements, withstand the increased wind loads around the perimeter of the building and keep water from entering the building. Perimeter details and junction points are the areas where the most care in design, specification, installation and maintenance is required.

Over the last 3 years I have been asked to review many industrial, commercial and retail buildings and comment on why the roof or cladding is leaking. You will be alarmed to note that, of the buildings I have been asked to review, less than 10% of the roofs didn't leak and of the vast majority of buildings that are leaking, 80% of the leaks are due to a lack of understanding of the perimeter / junction points and inaccurate construction. (Missing key elements that make up the perimeter detail.)

The devastating impact of water ingress is under-considered, we can think of the basic impact of water and how the damage or mess it causes needs to be cleaned up and the entrance to water ingress filled, patched or repaired BUT it is more serious than that...... the impact of the guarantee of the insulation installed is dramatic.

Insulation guarantees are generally for structural integrity and the guarantee is clear that if the insulation is maintained and remains in the same condition as it was installed i.e. no water leaks then the manufacturer will guarantee the stated thermal, acoustic and fire performance, however if water is allowed to leak onto the insulation then the guarantee is invalidated. 

So the ability to ensure that a roof system keeps out water is key for the long-term thermal, acoustic and fire performance of your specified roof system.

What is the difference between a commercial and a technical flashing detail?

Perimeter flashings are made from flat sheets which are purchased by the flashing manufacturer in standard width sheets. Theses size sheets in Africa are approximately 925mm or 1225. ( I say approximately as non-standard widths can be purchased, depending on quantity.)

It makes good commercial sense to get as many flashings out of the standard sheet as possible. Generally speaking if you were to check the leading manufacturers of Metal Roofing & Cladding Systems, you will notice that the ridge flashing has a total girth of 462mm or 231mm for each leg either side of the apex of the ridge flashing. 

This means that 2 ridge flashings can be manufactured from a 925mm width standard flat sheet with no waste material.

The manufacturers details indicated in the brochures and guides are merely a statement of principle and not a 'one size fits all.' Every building needs to be engineered differently to suit the site requirements and building performance and therefore the size of the ridge will change to suit those requirements.

Does a commercial ridge flashing technically achieve the long term performance of withstanding water ingress?

The Technicalities of the Ridge Perimeter Detail.

To determine the overall girth of the ridge flashing the following should be considered:-

• Roof Pitch - The lower the roof pitch the larger the girth of the flashing that is required.
• Length of the roof sheet and the impact of thermal expansion
• Wind loads, Site location and Environment

All of the above have a dramatic impact on the size of the ridge flashing and also the gauge of the metal used.

Dim. ‘A’ – The lower the roof pitch the longer this dimension should be to stop wind driven rain.

Dim. ‘B’ – This dimension is determined by the purlin distance. (*In the below table we have assumed 100mm) 

Table of Minimum Dimensions to determine the correct girth of Ridge Flashings.

Therefore you will note that ridge flashings in the main especially for low pitch roofs should be nearly double the size and the gauge of steel should be 0.8mm and not the 0.5 or 0.55mm that is currently installed.

The Elements that make up a Ridge Flashing Detail.

Metal Serrated Closure

The primary purpose of the metal serrated ridge closure is to withstand the impact of wind and to slow the passage of wind driven rain. Although it will stop some water entering the detail it is not sealed to the roof sheet.

 The impact from wind behind the metal serrated ridge closure is minimal and therefore the wind driven rain entering the detail slows quickly. The steeper the roof pitch, the less the water will travel into the detail before it stops or contacts the polyethylene polyclosure.

Closed Cell Polyethylene Polyclosure

The primary function of the polyclosure is to stop wind driven rain and debris entering further into the ridge. It should be sealed on the bottom edge to the roof sheet with a butyl mastic sealant, to provide a seal. As the roof sheet thermally moves the polyclosure will move with the roof sheet, still providing a water tight seal.

Roof Sheet 'Turn - Up'

 The ‘turning up’ of the edge of the roof sheet is to strengthen the edge and to provide the final detail to stop any water ingress into the building.

 The turning up the roof sheet should be undertaken by the appropriate manufacturer’s tool to ensure that the roof sheet does not split and that the turn up is uniform.

All the above elements are important to ensure water does not enter the building.

The Art of Forming a Ridge Detail On-Site?

If after spending the time in correctly specifying the correct girth and gauge of the ridge flashing and the contractor ignores the specification or omits to install the various hidden elements that make up a ridge flashing detail, then you will suffer roof leaks.

In my visits to the various buildings to review the water leaks, the major problem was the omission of the elements that make up the ridge, followed closely by the use of commercial flashings rather than technical flashings.

CAUTIONARY NOTE

For the long term performance of the ridge detail, it is essential that all components are installed correctly. Should any of the components be omitted or later removed, debris build-up causing water traps will speed up the corrosion of the roof sheets within the ridge detail. Once rust holes have been formed, the ridge detail will no longer be water-tight.

Long-Term Performance Guaranteed!

Currently the only long-term guarantee you will receive on the ridge detail is actually on the coating that is on the metal coil that makes up the ridge flashing, and not on the long-term performance of the ridge detail, with contractors providing a 1 - 5 year guarantee on their workmanship and therefore on the issue of water ingress.

Solution: Tried and tested ridge flashing details for a full array of conditions and roof pitches to ensure long-term performance. The performance of which needs to be clearly stipulated into the guarantee issued by the manufacturer of the roof system. The contractors need to match their workmanship warranty term to the same length of term as the roof sheet i.e. if the roof sheet guarantee is for 15 years then the contractor should guarantee his workmanship for 15 years stipulating clearly that they will guarantee the long-term performance of the ridge detail against water ingress.

For more technical information on perimeter details and junction points for 'pierced' fix and 'secret' fix roof systems, from an installer of such metal systems please click on this link..... how to form metal perimeter roof details!