Nailing a weather-tight roof from the start

 

 

When it comes to keeping the weather out, a building’s roof is the first, and most important, line of defence. Here’s a few tips to help make a roof perform as it should.

Get pitch perfect 

The slope of your roof is more than just about looks. It’s essential to meet the minimum roof pitch for your chosen materials, which can vary in their drainage performance. Even a roof that looks flat needs to have a certain gradient to stop it collecting water, under the Building Code's E2/AS1 rules. “Flat” roofs with a gradient of less than 3º require a membrane roofing system (installed by a licensed professional), while metal can also be used for medium-pitched roofs of 3-10º.

Place roof penetrations wisely 

From the start, designing a roof to avoid complex junctions is one of the most important steps you can take to minimise the risks of leaks, though the placement of pipes and other roof penetrations also comes into play. The Department of Building and Housing recommends putting them near the top or bottom of the roof slope, as mid-roof flashings are the hardest to get right, requiring a tricky combination of “over” and “under” flashing.

Install roof underlay

While you shouldn’t need an underlay for a pitched masonry tile roof, a good roof underlay otherwise improves weather resistance by reducing the suction effect that can draw water in through gaps. However, it’s important to make sure you’re using the right one. A wall wrap is not suitable as a roof underlay because water absorption is more important for roofs since they drain more slowly than a wall and need to cope with moisture for longer.

Sweat the small stuff

It’s often the little things that cause big problems. Common design and install issues with roof fittings include: 

• Not allowing for thermal movement (leading to buckled metal or torn fixings) 
• Using the wrong fasteners for the environment 
• Insufficient drainage capacity (especially at valleys and gutters) 
• Adding heavy gear like solar panels without considering load or flashing needs 

Don’t forget to use the right flashings around every pipe or penetration through the roof. This will depend on the roofing material, the size of the penetration and the slope of the roof – BRANZ has a useful guide to flashings here.

The right cladding for the right conditions

Even if a roof is watertight to begin with, it can fail much sooner than expected if the conditions are wrong for the materials. Masonry tiles made out of terracotta or concrete aren’t suitable for roofs that are walked over regularly, and always require sturdier framing than a lighter material. Meanwhile, the average lifespan of wooden shingles or paint for metal roofing can be halved in severe or damp climates. If your client has a certain material in mind that may not be suitable, you’ll need to be sure they understand the risks and maintenance required. Installation conditions can also make a huge difference. For example, asphalt shingles can crack if installed in temperatures lower than 10ºC.

For more details on getting your roof right, see the BRANZ website or the Department of Building and Housing weathertightness design principles.