Rooflights in zinc roofing – three key questions

Rooflights in zinc roofing – three key questions

Thinking about designing your zinc roof to include rooflights? This article from SIG Design & Technology talks about the key questions you should ask before you start.

The advice here is for zinc roofing and focused on elZinc detailing. It is worth remembering that whilst the details and advice may be suitable for other metals and for other manufacturers’ zinc products, you should contact SIG for advice about these as the information is not always the same and manufacturers differ in their requirements.

Rooflights in zinc roofing - the key questions

There are three key questions which always come up when SIG Design & Tehcnology talk to architects about rooflights in metal roofing:
• What is the roof pitch?
• What are the width limits?
• What are the depth limitations? (particularly with flush or sunken rooflights).

Let’s deal with them one at a time.

Rooflights are a form of penetration to your zinc roof, and as such need to be designed and installed with care, so as not to affect the overall integrity of the roof and the protection it provides to the building. – FTMRC UK Guide to Good Practice

1. Roof pitch and rooflights in zinc roofing
One of the first questions SIG Design & Technology will ask you is, “what pitch is the zinc roof?”

The challenge is water ingress in the seams, which becomes more likely as the pitch lowers. During heavy rain, water can build up at the roof to the level of the overlap on the standing seam and this can cause the water to be sucked into the construction via capillary action.

Make sure you can still drain water off. The higher the pitch, the more effective your roof is at draining off water. With rooflights, the issue of water ingress can be greater due to the rooflight acting as an obstacle to rainwater getting off the roof.

Make sure your rooflight is approved for low pitch. At lower pitches, unless the water is managed efficiently there is a risk of ponding water at the head of the rooflight, if only temporarily. As a result, some rooflight manufacturers specify minimum pitches, so make sure your choice of rooflight will comply with your choice of roof pitch.

If you need advice on which rooflights you could use, SIG are happy to assist.

2. What are the width limits for rooflights in zinc roofing?
Another question SIG Design & Technology will commonly ask is, “how wide will your rooflights be?”

Once again, this is due to the need to drain water off the roof effectively. If the rooflight is wide and you are breaking more than say, three seams, diverting water around the rooflight becomes a problem, simply due to the quantity of water involved.

No matter what the size and position of the rooflight, your objective should be to allow all the water to drain away around the sides – letting water run over the face of the rooflight is not good practice.

The FTMRC recommends that rooflights are mounted on a weathered timber upstand a minimum of 100mm high and 50mm wide. Some manufacturers will provide flashing kits to cover the upstand, or SIG can supply them for their installers.

Soldered or welted installation? Depending on the pitch of the roof, the installation will be soldered (usually below 14º) or welted (above 14º).

On a soldered installation, the seams will be folded flat and soldered 300mm above the rooflight together with a deflector fillet, to allow the water to flow horizontally around the edge of the rooflight.

On a welted installation, the standing seams will stop a distance above the rooflight and a continuous flashing (known as a back gutter) introduced, again with a deflector fillet.

In either case you’ll need some space above the rooflight to allow for these changes to the standing seam, which affect where you can position them. The distance required will depend on the roof pitch and size of the rooflight – you can contact SIG for advice on this.

If you want to install a series of rooflights above one another, bear in mind that these pose an additional challenge and greater risk of flooding the surrounding standing seams.

As always it’s a good idea to talk to SIG about what you are trying to achieve – they will have some ideas on how to get there.

3. Can I have flush or sunken rooflights in my zinc roof?
The third question concerns the depth of your roof, if you want to incorporate flush rooflights, also known as sunken rooflights.

If you don’t want an upstand on your rooflight and prefer the surface to be flush with the roof surface, it is still good practice not to allow water to flow over the top. Instead, you will need a gulley around the back and sides to divert the water in place of the upstand.

The space to incorporate this gulley has to be accommodated within the depth of the roof, and therefore it may involve adapting the insulation and maintaining the ventilation space, even lowering or cutting into the rafters.

Make sure you can achieve your target U-values! All too often if you’re trying to achieve a particular U-value in the roof, cutting in the rooflights may make that just that much more difficult. Whilst you can use alternative insulation materials around the area, this can also have a knock-on effect on the cost.

Sunken or flush rooflights are by their nature more expensive for this reason. There are implications for the structure, the insulation, and to incorporate the sunken gulley.

However, at SIG Design & Technology, they recognise the desire to achieve a flush finish, and work with architects to make this possible within best practice.

If you’ve got a zinc roofing project which you’d like to incorporate rooflights into, please don’t hesitate to contact SIG Design & Technology.

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