Keep it simple and make it last Simplicity is key to durable building performance

15 December 2020 by Jason Quinn

Building durability matters and should feature alongside other high-performance building metrics (air quality, thermal comfort, energy efficiency). We ought to design and construct buildings that are robust, that are easily maintainable and will have a long life span. I think it’s appalling that New Zealand regulations require vital building components like windows to last just 15 years.

Yesterday I was reproved for mocking the notion of heritage buildings in New Zealand. In my defence, I grew up in a place where people routinely live and work in buildings that are many hundreds of years old. This is common across the northern hemisphere where buildings were constructed and expected to last for multiple generations and are still functioning as intended after 200, 300, even 400 years. A building’s working life matters for a range of reasons but not least because of its embodied carbon.

We can reduce the amount of carbon emitted in the construction of a building by replacing steel and concrete with timber and engineered timber. But building it to last twice or three times as long will also slash the total amount of carbon emitted over its life.

Here’s a great indicator of building durability you probably haven’t thought of: simplicity of design. We need building assemblies that are simple to build and readily repeatable, so that builders can develop their understanding and expertise in how and why these assemblies work, getting more proficient with every project.

This doesn’t mean cookie-cutter builds devoid of personality and architectural flair. I’m an engineer and I obsess over function but I like ugly just as little as the next person.

The builders who retired 30 years ago pretty much used the same materials and construction methods their whole working lives. They had a lot of practise, doing the same things over and over. They understood the purpose of the simple materials they were using and the details of the construction.

That’s not true any more. New building materials are coming to the market all the time. Many are highly sophisticated and they come with their own very specific methods of application. It’s demanding a lot more from builders. They have to keep up with the latest developments and delve into the science of why and how they work and learn the critical line between success and failure. Application methods may vary between different brands of the same product!

I’ve been privileged to work with some of the best builders in New Zealand—probably some of the best builders in the world—on a lot of outstanding projects. These are high-end jobs, where the client’s gone to great length (and expense) to specify a high-performance building. Even these guys many times don’t fully understand the products.

Backing rods are an example I find particularly entertaining. Standard practice is to put a foam backing rod in place before sealing a joint. Ask a builder why they do that and some will tell you it’s because you don’t want sealant to squeeze out the back of the joint. Don’t waste the sealant!

Now that’s true, but not the real reason you do it. The rod behind a concave joint makes the sealant form an hourglass shape. When the joint moves, the sealant stays adhered at the wide end of the hourglass (at each side of the joint) and flexes in the middle where it’s thin. That’s the whole point of having the backing rod: it makes the joint flexible and long lasting (if you consider 10 years a long time).

Yes, the people applying the products need to read the instructions. But a constant flow of new products with highly specific requirements introduces risk into projects. So does abandoning the redundancy that kept older buildings by and large structurally dry and sound for many decades. We’re in trouble if a building’s only line of defence against water ingress is a correctly applied sealant with a short life span.

What do we need instead? One, we need redundancy built into the design. That’s one of the most important things to learn from the leaky homes disaster. Don’t base your building’s integrity on a belief that your wall assembly is perfect. Perfect happens in CAD drawings; it’s much harder to achieve on a building site. We need designs that take into account not all builders have the same level of skill and commitment, that weather happens, that manufacturing defects can occur, that new building materials don’t have decades of use we’ve observed on site.

So design layers of defence: your wall assembly should be water tight, yes. But plan for moisture getting in. Make sure there’s a way for moisture to drain away where it won’t do any damage.

Secondly, the small details need to be highly durable. Caulk has a life span of just three to five years. This is not good enough. I advise you don’t use it except for cosmetic purposes. Even sealant will only last 10-15 years. Design with that in mind, plan for replacement.

Think about how to apply these ideas to overall design. Having multiple types of wall construction isn’t a sign of an architectural building to be proud of; it’s a sign of a building that is harder and more expensive to build than necessary. The same visual effect could be achieved for instance through a single wall construction that allows for different cladding systems to be fixed to structural cavity battens. Or use a false facade that actually forms part of the cavity ventilation system. It’s much easier to build, much more durable and much less likely to have problems.

What’s that KISS acronym again?

The whole construction field would also benefit from manufacturers/suppliers’ instructions that are clear, concise and intelligible but that’s another story.


Three ways sealants fail

  1. Adhesion failure (fails to adhere to substrate)
  2. Cohesion failure (fails in the sealant eg splits in the middle)
  3. Substrate failure (the substrate breaks off but the sealant is still stuck)

Technical references:

Online and freely available

Sikaflex® Elastomeric Sealants – Joint design and movement calculation guide

Sikaflex® Considerations for facade joint design

Available for purchase

BRANZ BU601 Sealants for cladding joints

BRANZ BU584 Sealed-joint design – claddings

Note that these replace BRANZ Bulletin 440 and 441.