Month: May 2025

Roof Deck Solutions

Posted on by Gary Snider

In this article, we look at a critical but often underappreciated part of timber frames – the roof deck.

In modern timber framing, whether with sawn or mass timber, the roof deck is the first layer that is attached above the ceiling beams or rafters.  It’s a big part of what makes timber frames strong and attractive.  We call them decks because they’re more than a ceiling finish; they carry roof loads and resist forces that could distort or damage a building.

Tongue and Groove

The most popular roof deck material that Cornerstone supplies to client projects is “2×6 tongue and groove boards”, in pine or fir.  These boards have a profile that allows them to fit tightly together and span up to 5’ between rafters.

When T&G boards are nailed onto rafters the high friction connection between boards makes the entire roof act as a single unit which can resist lateral forces, such as those created by high winds.  They also provide these great benefits:

  1. Continuous nail base – which means a nail or screw will always find a solid base to attach to. This makes installing the rest of the roof so much easier.
  2. Work stage – during construction, T&G decking supports workers, making their job faster and safer.
  3. Interior finish included – not only strong, but also good-looking, T&G boards provide an interior ceiling finish that goes well with timbers.

Tongue and groove boards come in other thicknesses and profiles as well. 1×6 is a favourite for accent walls where the look of T&G is desired but the strength of 2” material would be excessive.  And double T&G is made in 3” and 4” formats to be used for spans of 6-8’. It’s harder to find and is often a special-order item that may take 3 months or more to produce.

Mass Timber

Modern and minimalist designs, both residential and commercial, have long spans that require roof decking to match. Laminated wood decking answers this need, coming in 8” and 12” widths and lengths up to 60’.  Spans of 10-12’ are typical.

What’s Next

Micro CLT is a fairly new product that is made up of 3 ply’s of laminated wood that run crossways to each other.  The material comes in panels, in a variety of dimensions up to 6’ wide by 15’ long. Thicknesses range from ½” to 2-1/4” so it can be used as a wall, ceiling or structural roof deck.

The cross-lamination makes this material extremely stable and strong, while keeping all the beauty of natural wood.  Micro CLT often comes with a lap or T&G edge profile, allowing them to fit tightly together.  The panel format will enable them to be installed fast!

Fire Resilience of Mass Timber

Posted on by Gary Snider

Wood Burns

This fact may make the word “resilience” in today’s blog title seem a little optimistic. But time and again, real-world testing in Europe, Canada, and Australia show that in fires, large-section mass timbers maintain their design strength, thanks to how wood reacts to fire.

Fire Testing

To start, let’s look at a recent fire test conducted at the Ottawa Fire and Explosives Testing facility on June 10, 2022

The Canadian Wood Council was assisted by fire researchers at the National Research Council to conduct full-scale fire testing on a two-storey, mass timber office, with open floor plan.  The final test was designed to simulate a “worst-case scenario” and was run without the intervention of sprinklers or firefighters.

The office space was arranged with wood cribbing to about 120% of a modern office fuel load. Window and door openings were left open to allow unobstructed air inflow. An “aggressive ignition package” was used to start the fire, ensuring that flames reached the ceiling as rapidly as possible.

At 10 minutes following ignition the fire is at its maximum intensity and thereafter begins to cool.

By 25 minutes, only the remains of the “office furnishings” continue to burn at a low intensity.

The fire lasted four hours, ten minutes and self-extinguished once the fuel load was exhausted. Despite the intensity of the fire, the building remained structurally sound and, following the fire, was safe to enter.

Char Saves the Day

Why did the office building in the CWC fire test not collapse? The answer is found in the way big timbers burn. As fire envelopes a big timber, it develops a surface char layer, and two effects occur – the char is much less combustible and its also a poor conductor of heat. These characteristics of char dramatically slow the transfer of heat to the interior of timbers, which in turn reduces the rate of burn and limits the amount of material available for combustion. By sizing timbers to account for the char layer, a mass timber structure can endure a “worst case” fire and still maintain structural integrity.

The dramatic photos above leave no doubt that fire is a severe threat to structures and human life. They also show that mass timber acts in a way similar to non-combustible construction. It is the contents that burn while the structure keeps its capacity for loads. In multi-storey buildings this capability allows for fire survival in cases where evacuation becomes untenable and ‘shelter in place’ is the remaining option.

Sprinklers Work

Building codes require multi-unit residential buildings of four stories or more to have automatic sprinklers. They also have hardwired smoke detectors and fire alarm control panels that notify the local fire department. Thus, the probability of a “worst-case” scenario is very low. In Europe, an engineering consulting firm Arup conducted fire tests on a large mass timber structure and found that when a low-pressure water mist suppression system was in use, it effectively extinguished the fire with only “limited discolouration” of the ceiling panels above the fire ignition point and minimal water damage.

Conclusion

While fires will always be a risk, modern mass timber design and construction dramatically improve life safety. While mass timber is extremely difficult to ignite, when it is exposed to fire, its char-forming trait will protect its load bearing capacity. When an operating sprinkler system is in use, fires will be contained within the space where they were initiated.

Mass Timber- Finishing Well

Posted on by Gary Snider

It’s hard to imagine that a half millimeter could dramatically improve our appreciation of a material. But with a stain or finish, carefully chosen and applied, the natural beauty of wood can go from “that’s nice” to “this is gorgeous!”.

Twenty-one years ago, our CEO opened the first five-gallon pail of stain in Cornerstone’s finishing shop. Her unyielding pursuit of technical perfection is a path our professional finishing team has followed ever since.  As Cornerstone entered the world of mass timber our tools and methods adapted. The goal, achieve a super-durable finish that looks great, allows easy clean-up both during and after construction, and makes the owners and occupants proud.

Photo from: Northern Log

One of our first realizations was that mass timber can be very heavy and awkward to handle. Glulam billets that we transform into finished columns and beams can weigh several tonnes and be up to 60’/20m long. Every step of handling requires the right equipment, skilled operators and well considered safety precautions.  Moving a 60’ billet through a 24’ wide shop door with a standard forklift simply doesn’t work.  Thankfully, a company out of Belfast, Northern Ireland had already solved this problem.

A Combilift Sideloader carries loads to the side of the operator and can drive in all directions, making it an invaluable team member when pieces get long.

Standard industry practice for finishing mass timber is quite basic and consists almost entirely of sprayed wood sealers.  These are best described as undercoats, and their stated purpose is “to protect wood from weathering during storage, handling, shipping, and installation”.  Sadly, most proponents and owners of mass timber projects never realize the short-term nature of these finishes. Left as the only coating, they leave timbers looking dull, scuff easily and break down within months of application, leaving timbers vulnerable to moisture, UV damage, and dirt.

Cornerstone’s finishing approach is driven by our craft experience. We see mass timber structures like large furniture. They deserve quality finishes that preserve the beauty of an honourable material and that respect the hundreds of hours of craft work in each frame.  In our view, a finish must do three things: celebrate the beauty of wood, make cleaning and maintenance easy, and look good for years.

Our finishing process for mass timber follows these steps:

  • Billets arriving from the laminator need a good sanding to make them silky smooth
Put the palm sanders away! A set of wide billets gets an initial sanding with…a floor sander.
  • A penetrating base coat sets the client’s chosen stain colour into the wood.
  • A first topcoat of clear finish is applied to protect the base coat.
  • For exterior timbers, a second topcoat is added, providing extensive protection from weather while also enabling easy maintenance.
  • The result:

We may be more than a little biased, but the relatively small cost of quality finishing for mass timber is a true no-brainer. So much aesthetic goodness is developed and preserved, while so much hassle and future cost is avoided. 

It’s a beautiful half millimetre that should be part of every mass timber plan!