Roofing Material: A Weighty Decision!

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I remember one time as a kid when a great-uncle told me about the historic New England turkey drives. As I listened open-mouthed, he told me that New England farmers of the 1800s used to join up in the fall to collectively bring all the turkeys they had raised down to Boston. It was before the days of the interstate and refrigerated trucks, and as he recounted, they would shepherd thousands of turkeys across the countryside over a number of days.

vermont-wild-turkeys

The turkeys would collectively roost on the ridges of country barns every night. “And that, my friend, is why so many Vermont barns have big sags in the ridge line – it was the weight of all those fat turkeys on their way to market!”

To this day I am not totally sure if he was pulling my leg completely about the “historic turkey drives,” but one thing is for sure: there are indeed many historic barns with saggy roofs. In our voyages around New England viewing barns, we very often find sway-backs. That is, buildings where the roof has sagged dramatically under the weight of slate.

The history of our slate industry

Green Mountain Timber Frames has its base of operation right on the edge of a slate valley and it is fascinating to think about the interplay of geography, geology and population movements as it relates to this industry. Slate first gained attention in our area around 1834. The first known barn to receive a slate roof in our area was in 1848. By the end of the 1800s, the slate industry in our valley was booming, bringing in groups of skilled quarry workers from Wales, Czechoslovakia, Poland and many other parts of the world where people had experience with slate.

Before this, our barns were roofed using hand-split cedar shakes.

handsplit-shingles-or-shakes_historic-roofing

Hand split shakes – image courtesy of http://www.nps.gov

Builders would cut cedar logs into 30 to 36 inch lengths and then split them into shingles using a tool called a froe. In some places where there were not readily available streams for early water powered sawmills, builders would forego roof boards completely. Instead, they would heavily brace the rafter system to make it ridged, and then would install strips of wood called purlins spaced up to 36 inches apart. They would then apply long hand-split cedar shakes directly to the purlins.

From cedar to slate

Cedar is a natural, light, and wonderful roofing. Like the planking on an old wooden boat, it would have large gaps when dry. As soon as the first rain drops soaked in however, the cedar would swell and shed the water beautifully!  However, cedar roofs last only a few decades. As the cedar started to fatigue on our earliest barns, farmers began to replace it with slate. What could be better than a local product that will last as roofing for 200 years? What farmer really has the time to replace cedar every few decades?

There was just one little problem…cedar shakes weigh about 400 pounds per 100 square feet. Slate, on the other hand, weighs as much as 1,000 pounds on that same roof surface! On a typical 30 x 40 barn, this means that the new roof weighed as much as 9 or 10 tons. For a barn designed for a lightweight roof, trouble came a-visiting, especially when the heavy Vermont snows also piled high on top of the slate.

before-and-after-barn-transformation

How does geography fit into this?

Here at GMTF, we have recently viewed a number of barns in northern Vermont. These structures were far from the slate quarries and were thus more likely to receive roof makeovers with multiple generations of cedar, followed by lightweight metal roofs (once that industry came into full swing.) We find that, unlike with slate, many more of these northern barns survived with roof-lines in tact.

How do we restore these barns?

There are several building techniques that we use to rehab a barn that was built for cedar rather than our modern heavier insulated roofs.

We find that the old barns with ridge beams faired much better over the past couple of centuries.

1790-barn-with-5-sided-ridge-beam

This 1790s frame received slate mid-life, yet look how straight the roofline remained because of the 5-sided ridge beam!

To further strengthen a ridge beam roof system, we sometimes add vertical posts going from heavy timbers to the ridge beam. This transfers the weight straight down and minimizes the outward thrusting forces that otherwise can push the eve walls out.

historic-timber-frame-barn_green-mountain-timber-frames

In this historic barn, there are posts going up to the ridge beam to further strengthen the structure.

 

As builders became aware of the weight issue, they often added a “queen system.” This is a structural system that picks up the weight of the rafters at mid-span and transfers this weight down to the heavy girt timbers and posts. On many of our oldest barns, we use vintage timbers to create this type of a system.

barn-featuring-queen-system-as-rafter-support_green-mountain-timber-frames

This  “queen system” helps to support the rafters.

queen-system-for-rafters-with-vertical-posts_green-mountain-timber-frames

Queen systems come in many shapes. This one uses vertical posts rather than angled.

Another method we use to beef up the rafter system is to add collar ties to the rafters. These horizontal timbers create a triangle within each pair of rafters, which is one of the strongest geometrical shapes in nature.

collar-ties-on-north-hero-vermont-barn

We installed these old-new collar ties to help stiffed the roof on this North Hero, VT frame.

How can we repair slate fatigue in place? 

On a recent project, we were asked to help prepare the structure of an 1860s house for a new roof. The builders had sized their rafters for a light roof and then installed slate. The sag was tremendous. We did not want to disturb the original plaster of the ceilings beneath the rafter system. Our solution, after the slate was removed, included first adding collar ties in the attic.

new-collar-ties-to-strengthen-rafter-system-in-old-vermont-barn

New collar ties stiffen these old, undersized rafters.

Next, we screwed 2 x 4 rippings over the top of the existing rafters and snapped straight chalk lines.

img_4006

We used a string with chalk on it to snap a straight line

Here is how it looked after we used this technique on all the rafters:

curved-shims-on-new-roof-system_green-mountain-timber-frames

Making all these curved shims made us consider going into the wooden boat building trade!

Next, we put new plywood over the whole roof and it was ready for roofing!

new-plywood-roofing-on-vermont-frame_green-mountain-timber-frames

Ready, set, and on goes a new roof!

Restored slate roof by Green Mountain Timber Frames

The finished project has a much flatter and straighter roof.

Conclusion

It is so interesting, as we travel around New England looking at barns, to imagine the various economic, geographical and sociological forces that all played a role in barn designs! We are grateful to have learned from the craftsman of the past who figured out how to properly build roof structures to withstand the various forces that material and weather throw at them.


Want to live in your very own historic barn?
Let us know!

802.774.8972 or Luke@GreenMountainTimberFrames.com

  • To learn more about the fascinating history of slate, check out the fantastic Slate Valley Museum in Granville, NY.
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