Resilient Designs Cost Less Than You Think

In last month’s article, Designing for Resiliency, I present some of my thoughts on how changing the design approach for wood trusses could aid in making future homes more resilient to the types of climate extremes that seem to be ahead of us. Whether you think that climate change is a man-made problem (I believe it largely is) or is part of a natural cycle (highly unlikely), it is very likely that no matter where you live you are looking at more extreme snow, rain, and wind conditions in the future than we have seen in the past.

In the past, I have confidently relayed to anyone who cared to listen that I had never seen – in 35+ years now – the failure of a properly installed truss roof. In fact, the only house roof failure I was ever involved in related to a 10-year-old house with inadequate truss bearing and no bracing that experienced approximately 200% of design load due to significant snow drifting on a lower roof followed by heavy rain in mid-February. Every other truss failure I have experienced was related to a lack of adequate bracing and/or poor installation practices.

Today though, I cannot be as confident looking forward 35 years. It seems entirely possible to me that my area could see 5 to 6 ft of snow in a short period of time, after all we got 3 ft in less than 24 hours around Christmas 2022. If that had been followed by several days of heavy rain, with all of the snow acting as a sponge and holding it on the roof for some time, then we could easily see roofs experience significantly higher than code-specified loads would predict.

It isn’t reasonable to build our homes as bunkers – but how should we proceed? It’s true that the building codes use historical data to predict future loading, but in a rapidly changing environment, this will be less valuable. In the absence of data, looking forward into the unknown is always going to be a bit of a guessing game. So to satisfy my curiosity and expand this discussion, I decided to run some test designs.

I decided to design a typical roof for a home located near me in Barrie, ON. The layout shown [See PDF or View in Full Issue] gives you some idea of the house I used, with a footprint of approximately 2,400 sq ft, a cathedral ceiling over the great room, and attic trusses over the garage. There are some piggybacks due to height constraints, and I’ve included lay-in gables at the hips and valleys as needed. I am sure that some of you may want to comment on the arrangement of the truss framing, but I hope you will accept that the intent is to provide relative costs. I designed the trusses essentially as I would for production, although I am sure that there are savings available in all cases.

First, we have the base conditions. The design snow load for a single-family house here is ~37 psf, with another 13 psf for dead loads. As I mentioned last month, we are not required to design for either wind or unbalanced snow conditions for small (<6,000 sq ft) residential buildings. I then added a design for wind equal to approximately twice what the current code would suggest if it were a requirement to anticipate stronger wind events – this is case 2. I then created case 3 by adding only unbalanced loading to account for some snow drifting and uneven distribution. And finally, case 4 considers both the wind load from case 2 and unbalanced load from case 4.

So, what did I learn? I expected that there would be a modest cost increase as the design requirements increased. I expected that case 4, with both wind and unbalanced loads, would be about 10–15% higher than the base case 1. Instead, what I found was that even for the worst case the increase was only 3%, and the increase in lumber volume was less than 1%. For reference, I included about $300 in hold downs for any of the wind cases. I should also note that the costs are shown in Canadian dollars, but of course the percentages still apply. It is also reasonable to suggest that an additional $300 or so should be included for the cost to install the hold downs, but I leave that to you.

To put this into further perspective, this house would sell in my area for somewhere in the $1 to $1.4 million range. The increase in the cost of the trusses at worst is less than 0.1% of the selling price. Unfortunately, I know that nearly all builders, architects, and home owners are unlikely to spend this small additional amount for something they can’t see. And how does this affect my own thinking? I know that I would rather spend an extra $600 – $1,000 on trusses for my own house and sleep easy than spend that same money on an upgraded vanity or light fixtures that could easily be upgraded at a later date.

Are you considering any design changes in preparation for future possibilities? Please share your thoughts with me, I’d love to hear from you.