Skip to content

Fixing a 150-year-old mistake

May 4, 2021

Fixing a 150-year-old mistake

 
---
 
In 1869, Joseph A. Thatcher made a mistake.
Ol’ Joe was one of the founders of Zumbrota, Minnesota. As he and the other founders looked to increase the fledgling town’s fortunes, they determined that a bridge across the Zumbro River would make it easier to transport goods. Thatcher, a Civil Engineer, had seen a design for covered bridges in New England and he instructed a local contractor to build one like it.
A standard peaked roof, like the sort you see on those old-fashioned covered bridges, is in the shape of an inverted ‘V.’ When you turn the ‘V’ upside down and press down on it, as happens when a heavy snowfall piles up on the roof, the two legs of the ‘V’ want to spread and flatten out. To prevent this, engineers install cross-ties between the two sides of the roof to resist that tendency to spread. For reasons known only to him, Joseph A. Thatcher failed to insist that cross-ties be installed on the Zumbrota bridge.
 
 
 
Now, to be fair, Joe’s slipup remained undetected for 150 years. But the snowfall on February 24, 2019, included one flake too many and the roof of the Zumbrota covered bridge flattened itself like a pancake.
The structure is the only remaining covered bridge in all of Minnesota. It has been listed on the National Register of Historic Places and is the centerpiece of Zumbrota’s annual Covered Bridge Festival held on the third weekend in June. So the collapse of its roof was a big deal.
The City has been working with the engineers at WHKS on other projects for many years so they were immediately called to assess the damage and begin repairs. Because the structure is listed on the National Register, any repairs or modifications have to adhere to very strict guidelines to maintain its originality. 
 
 
 
Since February in Minnesota doesn’t lend itself to construction, it was decided to stabilize the bridge, inventory and store the old materials and wait till better weather. In the meantime, the engineers began to think about how to restore the bridge to period accuracy while still correcting Thatcher’s original error. The bridge had been studied and documented extensively, including measured drawings and a digital 3D scan, which made it much easier to understand how it had been originally designed and constructed.
The historic preservation guidelines insisted that as much original material as possible should be used in the restoration. But it wasn’t enough to simply reuse the wood. Each timber had to be reinstalled in exactly the same location as it had originally been placed. As the collapsed roof was dismantled, each piece of wood was assessed for condition, inventoried and stored for reuse. About one-third of the roofing boards and 40 of the 92 rafters were damaged or rotted to the point where they had to be discarded.
The historic preservation guidelines further insist that, if the original parts can’t be reused, they must be replaced with identical new materials. In 1869, Minnesota was covered in white pine forests. Enthusiastic loggers made short work of that and today less than one percent of the original forest exists. Cutting down an old-growth pine tree to use for bridge repair was out of the question.
 
 
 
Enter Zach Klaus. Zach is a volunteer at the History Center of Olmsted County. He is a history buff and the operator of an 1800’s-era, steam-powered sawmill that he demonstrates every year at the Living History Fair. He also owns a stand of reforested white pine in the Mississippi flood plain that he uses as raw material at the Fair. In a perfectly serendipitous moment, Klaus heard about the need for white pine boards to repair the bridge and just happened to have a stockpile of boards from his sawmill demonstrations, dried and ready to be used.
The roof structure was rebuilt using a combination of original materials and new white pine. It was also strengthened by sandwiching new laminated wood alongside the original beams. The new materials were treated with a stain to match the aged originals as closely as possible.
 
 
 
The only hurdle that remained was how to integrate the all-important cross-ties without changing the appearance of the structure. When homes are constructed in high wind areas, builders tie the roof to the walls using a flat steel strapping, which adds tremendous strength to the structure. One of the engineers had the bright idea to lay the flat steel strapping along the top of the original beams where it could tie the two sides together while remaining completely out of sight. The strapping provided the vital cross-ties without changing the appearance of the structure at all.
The bridge has been repaired, strengthened and repainted. Joseph Thatcher’s legacy is intact. And the bridge is good for another 150 years. At least.