Recognising the value of geographic exposure for roofing

When it comes to roofing, there are four major exposure zones in the UK, ranging from sheltered to very severe.

In the roofing sector, we also utilise the same four category concepts for help when defining roof installation specifics. Each one takes into account the approximate amount of driving rain (L/m2 each rain spell).

The most crucial aspect for a roofer is understanding that different sections of the UK experience varied amounts of rain, wind, and exposure to weather. Any capable roofer should be aware of the classifications and the locations they cover.

Importantly, applying wisdom when choosing product parameters is necessary. For instance, you may find yourself constructing a roof in a low or moderate part of the country, but the structure is very tall and situated atop a hill. It is so, clearly, far more severely exposed to wind-driven rain than a standard house, sitting at a much lower level to the ground and therefore meaning there’s a better level of protection.

These circumstances often call for rational and tailored specification and a “belts and braces” approach. Ultimately, it’s better to over-specify than to be wishful, and hope for the best to happen.

Depending on the specific location of the roofing project being undertaken, a variety of measures must be applied to a number of significant areas of roof construction. Below, the importance of these factors are explained.

Step Flashings

A step flashing is a term that many roofers are familiar with, it refers to a stepped succession of triangular-shaped wings that are installed into the brick or masonry against an abutment, such a chimney or party wall. This form of flashing can be put as a “continuous” step flashing in regions with low and moderate exposure. This flashing may be put in lengths up to 1.5 metres and features several brick shapes carved into it. Continuous step flashings, when built properly, might appear more aesthetically pleasing, but they are vulnerable where the rake of the flashing meets the point below where the lead goes into the chase. A 25mm depth must be provided in the brick joints. As it is impossible to place any pointing material behind the lead flashing once it has been installed, it will inevitably be possible for water to seep into each piece during wind-driven rain. So, this flashing feature is not appropriate for places that experience heavy or very heavy rain, where it is crucial to employ individual step flashings.

Individual Step Flashings

Individual step flashings are single, triangular pieces that are individually installed on each brick or stone course. Each step for this specific flashing detail must provide at least 50mm of overlap to the final triangle installed. The size of the triangle being installed must be taken into account while adjusting the amount of cover. Even yet, these flashings will leak due to wind-driven rain, which can move in an oblique direction. For instance, if just 50mm are utilised on huge stones, the corresponding steps could be up to 500mm tall. These flashings should be utilised in the appropriate regions since they are often far more dependable than continuous step flashings.

Lead Hips and Ridges

It’s crucial to indicate the proper code and the kind of clipping materials to be utilised when placing hips and ridges. The position, orientation, and exposure of the structure will all influence how much clipping is necessary, but it’s still vital to adhere to the previously mentioned guidelines. The length of each clip should be between 300 and 500 mm in the centre, depending on how much lead sheet is being dressed down onto the tiles or slates, but it should not be less than 150 mm. This needs to be increased to a minimum of 200mm in areas that are subject to strong winds and rain. The clip length may need to be increased if the overall pitch is less than 30 degrees.

Felts and Underlays

Underlays are one of the many parts of the roof that could be impacted by exposed regions in the UK. In places with severe or extremely severe exposure, vertical lap lengths should be increased, and manufacturers have varying recommendations for each type of underlay for pitched roofing. For instance, a 100mm vertical lap on one type of underlay may be for low exposure locations, but when exposure and pitch are taken into account, this lap distance will increase to 175 or 200mm on some manufacturer specifications. Again, understanding these distinctions is essential to ensuring that a customer’s roof has the potential to last.

Headlaps and Sidelaps for Slating

Increased headlaps are needed for slate roof installations in a number of areas within the UK. In order to accommodate more laps on the respective courses, it will be necessary to reduce the batten gauges and margins, which will increase overall headlap and decrease the angle of creep on sidelaps. If the process has been planned to use pre-holed slates, it is crucial to check the pertinent paperwork to make sure the holing gauge on the slate type bought is compatible with the exposure and pitch of the finished roof. Slates that do not comply to the pitch and exposure are at risk of failing if the holes are not redrilled and the margins are not reduced. The contributing factors that work together to provide enough shelter and water protection on a pitched roof utilising natural slate are shown on the diagram. As demonstrated, the exposure of the slate margin or tail area determines the area of capillary spread (or angle of creep as it is occasionally termed). Consideration should be paid to reducing angle of creep and how it may affect the roof’s driven rain protection in areas of severe or very severe exposure.

To increase protection, two components work together:

  1. Raising headlap minimises the area of the margin that is exposed.
  2. Using broader slates, which provide more protection from wind-driven rain and allow water to flow farther inside the grey triangle (the area of capillary spread).