Logs take a stand against nature’s worst.
Old World Origins.
Log cabins built on the frontier or even in later settlements didn’t use fasteners. Logs supported themselves at the corners, which were formed where walls intersected. Although these early American dwellings weren’t erected with any intention of permanence, corner-supported log walls provided durability. They were the premise of log building and had been such long before Scandinavians introduced stacked construction to America.
In his 1942 work Der Blockbau (translated into English as The Craft of Log Building), German architect-scholar Hermann Phleps observes that log walls «derive their structural strength primarily from fitting and interlocking at the corners.» He traces this technique to the late Bronze Age and notes that Old World corner work evolved various styles and techniques. But, he points out, corner keying and fitting «is inadequate to securely hold the walls in alignment.»
To fasten logs so they wouldn’t move in or out of the wall, he says that European log crafters used wooden dowels or pegs (3 centimeters in diameter and 16 centimeters long) «driven down through the log- work an average of 1.5 meters apart, with the dowels staggered every other course by half this spacing.» Log builders in the Bohemian Forest region used wooden pins in place of dowels. Even stronger in-wall reinforcements were planks or boards «mortised right into the wall to hold three courses of logs together. At times, this vertical reinforcement would even extend the full height of the wall.» Strangest of all, Phleps notes, in East Prussia, «pebbles were used instead of pegs, these digging into the logwork as it settled.»
The need for fasteners results from the way log homes are built. Logs are stacked on top of each other. Without something to hold them in place, logs might topple off.
Handcrafters, who typically build with logs averaging better than a foot in diameter, often assure the stability of their walls by scribe-fitting (matching contours of top and bottom logs) and compression (the weight of the logs in the wall bearing down on each other).
As the logs are raised into place, most handcrafters use long spikes, either spiral or smooth, to fasten them to each other. «Some builders spike the corners and the walls near the corners, then drive heavy rebar into pre-drilled holes next to door and window openings, leaving long stretches without spikes,» Rex A. Ewing reports in Crafting Log Homes Solar Style. «I, on the other hand, like my spikes 3 to 4 feet apart and staggered with each course.»
In other words, handcrafters, being rugged individualists, have their own ideas about how to spike their logs. In the end, however, «if there are enough spikes in the walls,» Ewing says, «it’s doubtful the notches are going to shift.»
Phleps and Ewing describe handcrafted logs. Nine out of 10 log homes built in the United States depend on uniformly milled logs, whose diameters rarely exceed 10 inches. Fastening systems cannot be left up to individual builders.
Milled logs allow for precision fitting of logs with interlocking tongues and grooves and overlapping copes. Logs are supported along their entire horizontal surfaces, whether round or flat. Whatever is strengthening their bond does its work under cover. Looking at the inside and outside surfaces, these connecting and fastening devices are imperceptible. The logs’ good looks are preserved while their sturdiness is strengthened.
Fasteners are part of the engineering principles that ensure the structural integrity of the walls and compliance with building codes. Today’s codes are pretty sophisticated, so log homes must be rigid enough to resist vertical, lateral and shear forces resulting from wind, snow and earthquakes. To assure this rigidity, engineers apply complex mathematical formulas to such factors as log size, wood species, horizontal profiles, corner styles, moisture content and the size of the home. These engineering calculations can range from a few pages for an individual design to a few hundred pages for an entire line of standard plans.
The size of the home comes into play because a wall can act as a giant sail when subjected to a strong horizontal wind. If logs that make up the wall aren’t fastened correctly, this wind energy can contribute to racking and shearing, which can twist walls out of alignment.
The goal is to have all the logs perform together, as they did in the case of the swimming-pool home and the river-rafting home, and as they do whether they’re merely supporting the weight of the structure or allowing it to withstand extreme stress. Log-home producers assure wall logs work as a unit by calculating a specific fastening schedule for their homes. This schedule prescribes the frequency and location of each fastener. The grade of log, species, moisture content and locations of windows and doors all affect the fastening schedule.
The schedule will also vary according to the region where the home is built and the log producer’s building system. Producers may offer more than one fastening option or combine several types of fasteners on the same wall. Most companies include the fasteners with their log-home package.
Here are the most common fasteners used in log walls today.
Spikes are basically huge nails. Varieties include smooth shank, spiral shank and ring shank. For most wood species, the builder must first drill a hole in the logs to accommodate the spikes, which are then driven into the log with a sledgehammer. One drawback to this type of fastener is that the logs (particularly if they’re tongue-and-groove) can be damaged if the sledgehammer misses a spike.
Lag screws are pointed bolts. Again, holes must be drilled in the logs to accommodate the lag screws and then tightened with an impact wrench. They are installed at varying intervals so that one lag screw is never on top of another. The holes need to be countersunk because the lag screw won’t countersink itself. The producer can drill the holes before shipping the logs, or the builder can drill Through-bolts are threaded rods that are installed vertically in drilled holes throughout the entire wall assembly. Some producers’ building systems combine through-bolts with tension springs to accommodate movement. Once the entire wall is assembled, the through-bolts are tightened, typically at the home’s foundation level, so continued access to these bolts is essential. The bolts will be tightened regularly according to the producer’s recommendations.
Drift pins are often required in earth- quake-prone zones. These pins, made from galvanized pipe or re-bar, are set vertically in holes drilled into the logs. In high seismic areas, the fastening schedule may call for the pins to be as close as 8 inches apart, although typically they’re placed 4 feet apart and on both sides of windows and doors.
Threaded log-home screws offer the benefit of not requiring pre-drilled holes for their installation. Instead, the screws essentially drill their own holes. The screws have become popular because the installer drills and countersinks them with one effort, thereby saving time and money. Although smaller in diameter than other fasteners, most threaded screws are heat-treated for equal or greater strength.
The obvious first question is: Why are there so many different kinds of fasteners to hold logs together? And second: Which kind is the best? As is often the case, different people thought up their own ways to hold logs together. Of the ways that worked, some log-home companies preferred one, others liked another.
When considering your options, rest assured that no one type of fastener works best, or else every company would use it. Regardless of which fastener a producer specifies, it will be calculated into the engineering of your home to ensure its structural soundness. That’s why you should always follow the producer’s fastening schedule. If you need additional engineering and technical specifications, you can obtain them from the log producer or fastener manufacturer.
You may develop a fondness for a certain kind of fastener, but you aren’t likely to go wrong with whatever your log company provides. Fasteners are the scientific way log homes stand up best to nature’s worst.