AI
Thursday, September 26, 2002 6:47:38 PM
Probably nothing about log home construction stirs up more controversy and confusion than the issue of settlement. The image of a log home slowly slumping around their shoulders puts some log home owners is a state of near paranoia. The image isn't helped by the wealth of seemingly conflicting information confronting log home shoppers.
Actually, there isn't much mystery surrounding settlement in a log home. It's simply a physical process that has a greater or lesser effect on a log home depending on a number of factors. An understanding of its causes and effects will take the fear out of settlement.
Several types of settlement can affect construction of any home, not just those made of log. Foundations and foundation slabs settle if the weight upon the soil exceeds the soil's bearing capacity. Slight settlement is probably the rule rather than the exception when it comes to foundations. Evidence of this type of settlement may be cracks in a foundation slab, concrete walls or the mortar joints in block walls. These are the effects of uneven settlement as the soil beneath the foundation accepts the weight of the new home. Rarely more than a cosmetic concern, this is not the type of settlement that most concerns log home enthusiasts.
Another kind of settlement, resulting mostly from the shrinking of wood, also affects all types of construction, but especially log homes. This is because of the nature of wood and how it is used in log home construction.
Imagine a living log as a bundle of wet spaghetti noodles laid out straight. The noodles represent wood fibers. Water both surrounds the noodles and is contained within them. When the noodles start to dry, the water surrounding them evaporates first, followed by the water that is contained within them. As the water surrounding the noodles (in trees this is called free water) evaporates, nothing happens to affect the size of the noodles, but as the water contained in the noodles (called bound water in trees) evaporates, the noodles start to shrink.
Shrinkage takes place in three dimensions. The noodles can shrink in length, across their diameter and along a tangent, which is a line that runs at right angles to any straight line drawn through the center of the bundle (diameter). The least shrinkage takes place in length. In wood fibers, lengthwise shrinkage is pegged at about 0.01 percent. Thus, a log 40 feet long (480 inches) would shrink only .048 inches, or less than one-sixteenth of an inch.
The two types of shrinkage of most concern to log home owners and builders are shrinkage in diameter (called radial shrinkage) and shrinkage along the tangent (tangential shrinkage). Together, they reduce the cross section of the log. As the logs shrink, gravity goes to work and they settle. Thus, an 8-foot-high wall made up of 8-inch diameter logs will be less than 8 feet if the logs shrink to 7 3/4 inches in diameter. How much will the wall settle? That depends on a number of factors, including the starting moisture content of the log, the final moisture content, which is affected by the environment where the home is built, the joining and fastening systems, the sealing methods and the design of the house.
Before leaving the mechanics of settlement there's one more point to understand. Recalling our noodles, I pointed out that shrinkage doesn't start until water contained within the noodle (bound water) starts to evaporate. In wood fibers, this point is called "fiber saturation" and occurs at about 28 percent moisture content in most species. Shrinkage continues until the moisture level in the wood reaches equilibrium with the moisture level in the environment. This level varies around the country and even within the local environment where the house is located. The equilibrium moisture in the arid Southwest will be noticeably less than in Florida or New England. In the area where I build homes, I estimate equilibrium at 8 to 12 percent.
If our logs start out with moisture contents above fiber saturation, differences in moisture content mean little. If one log is at 45 percent moisture content and another is at 30 percent moisture content, neither has begun to shrink and both have the same shrinkage potential. So we don't need to worry about differences between individual logs. Only when they reach fiber saturation, which will be the same for all of them, will they start to shrink.
Logs dried below the level of fiber saturation, on the other hand, have started to shrink, so they are closer to their final dimension. A number of log home manufacturers either kiln-dry their logs or expose them to prolonged air drying. From the stand point of shrinkage, the drying method doesn't matter; only the moisture content of the wood at the time of construction is important. Thus, an air-dried log at 14 percent moisture will shrink less than a kiln -dried log at 19 percent. We can generalize that kiln-dried logs will be drier than most air-dried logs, but this isn't always the case.
The moisture levels I've been talking about are above equilibrium moisture content regardless of the drying method used. This means there is still some potential for shrinkage, although we still can't say exactly how much. Just because logs shrink doesn't mean they will settle at all or settle to the full amount of the shrinkage. Fasteners and seals between logs can prevent shrinkage from turning into settlement. If I build a log wall using spikes or screws, install heavy foam between the log joints, even when the logs shrink, settling may be reduced. Instead of the wall being reduced in height, I may see joints between the logs widen a little (although expansion of the caulk may conceal this).
If we anticipate settlement, how do we deal with it? Handcrafters and manufacturers use several techniques. Because handcrafters often work with green lumber or wood that is above saturation, they have proven expertise in handling settlement. Over the years I've seen many manufacturers adopt some of the handcrafters methods. The most basic is to allow some space over windows and exterior doors. With this space, log settlement will not result in the windows and doors bearing the weight of several courses of logs, the second-story floor system and the roof. The size of the space is determined by the amount of settling expected. In a home using large, air-dried logs, this could be three inches or more. In a kiln-dried home or one with very dry logs, the space may be less than an inch. The settling space is concealed behind the trim so it isn't noticeable in the finished home.
Windows and doors are installed in "bucks" (dimensional lumber frames set inside the log opening). If the system used calls for settlement allowances, the bucks are installed using methods that allow the logs to settle. Nails driven through the logs and into the bucks will prevent logs from settling unless they are driven into vertical slots that will allow the nail to move with the logs. The buck is sized according to the window or door, not the size of the log opening, which should be taller. Thus, the settling space should be above the buck. I once visited an owner-builder project where the bucks fit snugly against the logs all the way around the openings. The owners couldn't understand the space at the top of the window unit to the top of the buck. They thought it was a blueprint error until I pointed out that the settling space belonged outside the buck, not inside.
Allowing the outside walls of your house to settle raises the question, what about the inside? This depends not only on the amount of settlement expected but on the design of the house. In settling systems, the most common approach is to build in a way for inside framing to accommodate what happens to the log walls.
In many deigns, the girder supports the second floor in the middle of the house. Beams run from the log walls to the girder. As the log wall settles, the second floor will be forced out of level unless a means is included to lower the girder. Installing the posts that support the girder on adjustable jacks usually accomplishes this. As the log walls settle, the jacks are adjusted so that the center height of the first floor ceiling matches the height of the log wall. Obviously, if partition walls are framed tight from floor to ceiling, they will prevent post adjustments from having any effect. The answer is to build a settlement space into partition framing. This space is covered with trim that is secured only along its top edge. When the second floor's height is adjusted, the trim simply slides down over the wall.
In log homes where little settlement is anticipated, interior construction details like those just described can be omitted. Consider a 28-foot-wide house that is expected to settle an inch or less. The distance from the log wall to the center of the house is 14 feet. An inch of settlement will result in an inch difference between the outer wall and center. So take that inch difference and spread it over 14 feet. Over time, the floor will develop a slope of one-fourteenth of an inch per foot. Such a slight slope will be barely noticed. Still, some builders deal with this small amount by giving the second floor a slight downward slope toward the center. As the log walls settle, the floor levels itself.
Partition walls that attach to logs, kitchen cabinets, chimneys and stairs require attention to accommodate settling. Remember, if you build a house to allow for settlement, any vertical framing members nailed to the logs can prevent the log from settling. To avoid this, partition wall framing is secured to logs using nails driven through slots. As the logs settle, the nail slides down the slot. Because cabinets are secured across several courses of logs, settlement can cause pressure on fasteners. The same slotting principle allows logs to move behind the cabinets. Depending on the amount of settlement anticipated, cabinets could be secured to furring strips to isolate them from the logs.
Chimneys also cross the axis of settlement. Construction should allow the roof to settle without breaking the seal that keeps the chimney from leaking. The easiest way to accommodate settlement here is to design the house so any chimney passes cleanly through the roof, rather than through an intersection of wall and roof.
Stairs might require special attention if much settlement is anticipated. Handcrafters may keep stairs isolated rather than securing them to walls, so the stairs can be taken loose and repositioned when adjustments are made. If less settlement is expected, stairs may be set at a slight pitch that decreases as the house settles. Because the amount of settlement is spread along the stairs, the pitch on each stair is unnoticeable.
Because so many factors can affect settlement, it's almost impossible to predict the amount of settlement that may take place. In some cases settlement may take place but go unnoticed by the home owners. How home owners react to settlement varies almost as much as the homes themselves react. This is why the best way to evaluate manufacturers' claims regarding settlement is to visit several of their homes and talk to the residents. Ask to see homes that have been standing for at least three years, preferably in the area where you will be building.
---------------------------------------------------------------------------------------------------------------------------------
Excel
A fellow who says it can't be done is likely to be interrupted by someone doing it.
Actually, there isn't much mystery surrounding settlement in a log home. It's simply a physical process that has a greater or lesser effect on a log home depending on a number of factors. An understanding of its causes and effects will take the fear out of settlement.
Several types of settlement can affect construction of any home, not just those made of log. Foundations and foundation slabs settle if the weight upon the soil exceeds the soil's bearing capacity. Slight settlement is probably the rule rather than the exception when it comes to foundations. Evidence of this type of settlement may be cracks in a foundation slab, concrete walls or the mortar joints in block walls. These are the effects of uneven settlement as the soil beneath the foundation accepts the weight of the new home. Rarely more than a cosmetic concern, this is not the type of settlement that most concerns log home enthusiasts.
Another kind of settlement, resulting mostly from the shrinking of wood, also affects all types of construction, but especially log homes. This is because of the nature of wood and how it is used in log home construction.
Imagine a living log as a bundle of wet spaghetti noodles laid out straight. The noodles represent wood fibers. Water both surrounds the noodles and is contained within them. When the noodles start to dry, the water surrounding them evaporates first, followed by the water that is contained within them. As the water surrounding the noodles (in trees this is called free water) evaporates, nothing happens to affect the size of the noodles, but as the water contained in the noodles (called bound water in trees) evaporates, the noodles start to shrink.
Shrinkage takes place in three dimensions. The noodles can shrink in length, across their diameter and along a tangent, which is a line that runs at right angles to any straight line drawn through the center of the bundle (diameter). The least shrinkage takes place in length. In wood fibers, lengthwise shrinkage is pegged at about 0.01 percent. Thus, a log 40 feet long (480 inches) would shrink only .048 inches, or less than one-sixteenth of an inch.
The two types of shrinkage of most concern to log home owners and builders are shrinkage in diameter (called radial shrinkage) and shrinkage along the tangent (tangential shrinkage). Together, they reduce the cross section of the log. As the logs shrink, gravity goes to work and they settle. Thus, an 8-foot-high wall made up of 8-inch diameter logs will be less than 8 feet if the logs shrink to 7 3/4 inches in diameter. How much will the wall settle? That depends on a number of factors, including the starting moisture content of the log, the final moisture content, which is affected by the environment where the home is built, the joining and fastening systems, the sealing methods and the design of the house.
Before leaving the mechanics of settlement there's one more point to understand. Recalling our noodles, I pointed out that shrinkage doesn't start until water contained within the noodle (bound water) starts to evaporate. In wood fibers, this point is called "fiber saturation" and occurs at about 28 percent moisture content in most species. Shrinkage continues until the moisture level in the wood reaches equilibrium with the moisture level in the environment. This level varies around the country and even within the local environment where the house is located. The equilibrium moisture in the arid Southwest will be noticeably less than in Florida or New England. In the area where I build homes, I estimate equilibrium at 8 to 12 percent.
If our logs start out with moisture contents above fiber saturation, differences in moisture content mean little. If one log is at 45 percent moisture content and another is at 30 percent moisture content, neither has begun to shrink and both have the same shrinkage potential. So we don't need to worry about differences between individual logs. Only when they reach fiber saturation, which will be the same for all of them, will they start to shrink.
Logs dried below the level of fiber saturation, on the other hand, have started to shrink, so they are closer to their final dimension. A number of log home manufacturers either kiln-dry their logs or expose them to prolonged air drying. From the stand point of shrinkage, the drying method doesn't matter; only the moisture content of the wood at the time of construction is important. Thus, an air-dried log at 14 percent moisture will shrink less than a kiln -dried log at 19 percent. We can generalize that kiln-dried logs will be drier than most air-dried logs, but this isn't always the case.
The moisture levels I've been talking about are above equilibrium moisture content regardless of the drying method used. This means there is still some potential for shrinkage, although we still can't say exactly how much. Just because logs shrink doesn't mean they will settle at all or settle to the full amount of the shrinkage. Fasteners and seals between logs can prevent shrinkage from turning into settlement. If I build a log wall using spikes or screws, install heavy foam between the log joints, even when the logs shrink, settling may be reduced. Instead of the wall being reduced in height, I may see joints between the logs widen a little (although expansion of the caulk may conceal this).
If we anticipate settlement, how do we deal with it? Handcrafters and manufacturers use several techniques. Because handcrafters often work with green lumber or wood that is above saturation, they have proven expertise in handling settlement. Over the years I've seen many manufacturers adopt some of the handcrafters methods. The most basic is to allow some space over windows and exterior doors. With this space, log settlement will not result in the windows and doors bearing the weight of several courses of logs, the second-story floor system and the roof. The size of the space is determined by the amount of settling expected. In a home using large, air-dried logs, this could be three inches or more. In a kiln-dried home or one with very dry logs, the space may be less than an inch. The settling space is concealed behind the trim so it isn't noticeable in the finished home.
Windows and doors are installed in "bucks" (dimensional lumber frames set inside the log opening). If the system used calls for settlement allowances, the bucks are installed using methods that allow the logs to settle. Nails driven through the logs and into the bucks will prevent logs from settling unless they are driven into vertical slots that will allow the nail to move with the logs. The buck is sized according to the window or door, not the size of the log opening, which should be taller. Thus, the settling space should be above the buck. I once visited an owner-builder project where the bucks fit snugly against the logs all the way around the openings. The owners couldn't understand the space at the top of the window unit to the top of the buck. They thought it was a blueprint error until I pointed out that the settling space belonged outside the buck, not inside.
Allowing the outside walls of your house to settle raises the question, what about the inside? This depends not only on the amount of settlement expected but on the design of the house. In settling systems, the most common approach is to build in a way for inside framing to accommodate what happens to the log walls.
In many deigns, the girder supports the second floor in the middle of the house. Beams run from the log walls to the girder. As the log wall settles, the second floor will be forced out of level unless a means is included to lower the girder. Installing the posts that support the girder on adjustable jacks usually accomplishes this. As the log walls settle, the jacks are adjusted so that the center height of the first floor ceiling matches the height of the log wall. Obviously, if partition walls are framed tight from floor to ceiling, they will prevent post adjustments from having any effect. The answer is to build a settlement space into partition framing. This space is covered with trim that is secured only along its top edge. When the second floor's height is adjusted, the trim simply slides down over the wall.
In log homes where little settlement is anticipated, interior construction details like those just described can be omitted. Consider a 28-foot-wide house that is expected to settle an inch or less. The distance from the log wall to the center of the house is 14 feet. An inch of settlement will result in an inch difference between the outer wall and center. So take that inch difference and spread it over 14 feet. Over time, the floor will develop a slope of one-fourteenth of an inch per foot. Such a slight slope will be barely noticed. Still, some builders deal with this small amount by giving the second floor a slight downward slope toward the center. As the log walls settle, the floor levels itself.
Partition walls that attach to logs, kitchen cabinets, chimneys and stairs require attention to accommodate settling. Remember, if you build a house to allow for settlement, any vertical framing members nailed to the logs can prevent the log from settling. To avoid this, partition wall framing is secured to logs using nails driven through slots. As the logs settle, the nail slides down the slot. Because cabinets are secured across several courses of logs, settlement can cause pressure on fasteners. The same slotting principle allows logs to move behind the cabinets. Depending on the amount of settlement anticipated, cabinets could be secured to furring strips to isolate them from the logs.
Chimneys also cross the axis of settlement. Construction should allow the roof to settle without breaking the seal that keeps the chimney from leaking. The easiest way to accommodate settlement here is to design the house so any chimney passes cleanly through the roof, rather than through an intersection of wall and roof.
Stairs might require special attention if much settlement is anticipated. Handcrafters may keep stairs isolated rather than securing them to walls, so the stairs can be taken loose and repositioned when adjustments are made. If less settlement is expected, stairs may be set at a slight pitch that decreases as the house settles. Because the amount of settlement is spread along the stairs, the pitch on each stair is unnoticeable.
Because so many factors can affect settlement, it's almost impossible to predict the amount of settlement that may take place. In some cases settlement may take place but go unnoticed by the home owners. How home owners react to settlement varies almost as much as the homes themselves react. This is why the best way to evaluate manufacturers' claims regarding settlement is to visit several of their homes and talk to the residents. Ask to see homes that have been standing for at least three years, preferably in the area where you will be building.
---------------------------------------------------------------------------------------------------------------------------------
Excel
A fellow who says it can't be done is likely to be interrupted by someone doing it.
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