Settlement/Shrinkage of Log houses

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SETTLEMENT by Eamon Murray

Settlement and shrinkage of logs is probably the single most important detail concerning the construction of a log house. If a log company or builder tells you there is no settlement – move on – you are in deep trouble if you continue with them.

 

Timber shrinks significantly in width not in length. This is why there is a huge difference between log buildings and timber frame. In timber frame the length is mostly vertical so there is no significant settlement. In log buildings the length is horizontal so each log shrinks horizontally by a small amount. Care must be taken when mixing log construction with timber frame or block work as in chimneys. Anything that is fixed to log walls or is free standing inside a log house must be accommodated. This includes wardrobes, walls presses & counter tops. Wall tiling & mirrors. Electrical & plumbing- as in ducts, pipe work, sinks & toilets. The chimney stack should be free standing and rafters should never be fixed to the chimney.  The Log House Company has designed a special flashing that accommodates movement.

 

Log construction is a beautiful natural building material and when you work with and understand the character of wood it is very satisfying. The Log House Company has researched this area thoroughly from the beginning. The good news is that the settlement stops after about 2 years.

If the house is a dormer with a timber frame and timber clad gable and standard roof trusses then you can exclude the upper floor from the settlement equation. It is when you mix logs, timber frame and roof trusses that you can run into trouble.

OK let’s look at how much settlement. Take a log size of 180mm high by 120mm wide. We are not concerned with the width of the log for the moment though this shrinkage affects the thickness of the log wall and can open corner joints a fraction. We are concerned with how many horizontal logs there are and by how much they shrink. Also we are dealing with two types of pine logs. Standard with a moisture content of 20% aprox and laminated with a moisture content of 16% aprox.

We have measured that each standard log settles by 4mm from 180mm to 176mm aprox. The standard ceiling height is 2400mm so the ceiling will come down by 53mm and a 6 metre ridge with log gable will come down by 133mm aprox. Also because of the logs in the gable the rafters will splay and move out from the side wallplate by about 30mm depending on roof pitch. The laminated log settles by 3mm from 180mm to 177mm.

There is nothing wrong with standard logs but The Log House Company now use laminated logs where possible. Because the lamination makes the logs more stable the timber can be dried to a lower moisture level. This means less settlement, less twisting and the corner joints don’t open.

Please Note: The Log House Company takes no responsibility for use of the above information. They are general guidelines and suggestions. Each project should be individually assessed.

Regards

Eamon Murray

SETTLEMENT 2 by Eamon Murray

In our first article on settlement we calculated each standard log settles by 4mm from 180mm to 176mm aprox and the laminated log settles by 3mm from 180mm to 177mm. This means the standard log shrinks vertically at a rate of 22mm/ metre and laminated at 16mm/ metre. Settlement stops after about 2 years.

Let’s make some observations based on standard log shrinkage. The min standard for ceiling height is 2.4 metre so your ceiling will lower 52mm (2 inches). For example if you tile your bathroom from floor to ceiling the pressure of the logs will break all the top row of tiles. If you construct a stud (timber frame) wall from floor to ceiling the consequences will be worse. Because the stud wall can take the weight of the logs, the log wall will open up and you will have gaps to the sum of 52mm. This is very serious and will leave an exterior wall open to draughts and weather.

If we take it that counter tops, sinks and toilet cisterns are aprox 900mm from the floor we can expect 20mm (3/4 inch) log settlement. Even the pipe work to these units has to be accommodated so it is important to have proper consultation with electrical and plumbing contractors. Even a 600mm (2 foot) high mirror fixed top and bottom to a log wall will have to make way for 13mm of settlement. The mirror will crack.

Most Canadian and some Scandinavian manufacturers have timber frame gables and upper floors. So the upper floor will not be affected by settlement. But anything that spans the two floors will be affected by the settlement of the ground floor logs. So where the upper floor and the roof meet the chimney stack the floor and the rafters will drop 52mm.

Where you have log gables and log upper walls the rafters will drop 104mm. My own house is 6 metres from dpc to ridge and so the roof dropped 132mm (5 inches) around the chimney. If you fix your rafters to the chimney you have a very serious problem. Where does that 132mm go? The roof will belly up by this amount and cause other problems.

The chimney stack must be free standing inside the log house and The Log House Company have designed a special flashing unit for roof to chimney detail. 

We calculated and accommodated this settlement from the beginning and because we now have a number of houses constructed over 8 years we are not just guessing. We have worked very successfully as consultants on supply and self build projects.

So how do we accommodate all this settlement? Apart from the chimney flashing unit designed by The Log House Company some manufacturers supply grooved and sliding brackets for rafters to wallplate and for stud to log walls. Stud walls must be min. 52mm lower than the ceiling and a cover board installed to cover the gap. In some areas the grooved brackets won’t work. For example, for kitchen units we groove 50x25mm timber with a jig saw and screw through the groove onto the log wall. Then we screw the units to the grooved battens. For tiling we use the same method using 100×25 battens, then screw plasterboard to the battens and tile. This is good because it leaves a 25mm air gap. Electrical conduits must be grooved also and piping loose clipped. Calculate and use your head.

Please Note: The Log House Company takes no responsibility for use of the above information. They are general guidelines and suggestions. Each project should be individually assessed.

Regards

Eamon Murray

Making Allowances for Settlement

By Jim Cooper

Wood shrinkage and the resulting settlement and movement are not problems but characteristics of working with a natural material. Shrinkage only becomes a problem when the manufacturer, builder or homeowner fails to recognize or respect it. I would much rather have a house made from logs with 30 or even 40 percent moisture content, that is designed and built with full recognition of that fact, than a house made from logs at 20 percent moisture content, designed and constructed in denial that any more shrinkage or settlement will take place.

How much shrinkage or settlement are we actually talking about? I certainly hope that no one expects a log home to drop down around their ears from settlement. While it is important and under certain conditions even critical, most settlement and shrinkage concerns are more for energy efficiency, maintenance time and expense and construction quality than for structural integrity. The basic engineering of a log home makes for a strong house that can withstand a great deal of design and construction abuse.

When I speak of settlement, I’m generally referring to a range of 1 to 3 inches in an 8-foot wall. What does this mean to the homeowner? Consider a rectangular log home 24-feet wide, with a shed porch on the front and a second-floor master bedroom. The logs surround the first floor, there is a cathedral ceiling over the great room, the gables are log, and the roof is framed with either conventional rafters or heavy timbers. There have been no provisions for settlement built into the home. Let’s say the log walls enclosing it settle an inch.

Now the second floor, which started out level, is sloping from the centre of the house toward the front and rear walls, at the rate of one inch in twelve feet or 1/12 inch per foot. In three feet, this works out to 1/4 inch. The master bedroom door, set in the framed wall crossing the house at the top of the stairs, is almost three feet wide. Originally the door had 1/8-inch clearance on all sides. Now the jamb closest to the log wall is 1/4 inch lower than the other, so the door won’t close. The remedy? Plane the door top so it’s no longer square and reset the strike plate so the door will latch, or remove and reset the door jamb so the door remains square in its opening, although the frame will no longer be square with the wall.

Over time, the homeowner may notice difficulty opening a window. Eventually the window may cease to function entirely and may even crack. The cause: settlement causes the weight of the logs and roof system over the window opening to rest squarely on top of the window buck, or frame. Pressure on the frame distorts the opening, eventually interfering with the functioning of the unit. The same thing may occur at door openings in the log wall. The remedy: Reset the door window, shortening the door, if necessary, or replacing the window unit with smaller one.

These aren’t major repairs and in the long run will probably cost no more to fix than the problems that can arise in a conventional home. If settlement is greater than half an inch, or the design of the house is more complex than a simple rectangle, problems and remedies may become more complex, costly and even risky.

I’ve talked with homeowners who insisted their houses hadn’t settled at all. However, they pointed out that gaps had appeared between their logs that required caulking. Their logs were shrinking, but their houses were not constructed to allow settlement as this shrinkage occurred. The result was spaces between logs. Had the houses included settlement provisions, the gaps would not have occurred.

So, what if a log home salesman has assured you that his company’s house will settle less than 1/2 inch. Consider this: Depending on the height of the logs, a typical wall has 16 to 21 courses of logs laid horizontally–the direction that produces maximum vertical settlement. In addition, there are 16 to 21 horizontal joints in the wall, often containing foam seals, gaskets or caulks. If each log shrinks in cross section by 1/16 inch or if each joint compresses by that amount, the settlement in the log wall will exceed an inch. In other words, even though the average movement may be small, the overall movement can be significant.

The important consideration is actually how to handle settlement, not whether you can avoid it. Settlement is affected by the kind of wood, conditions under which the trees were grown, season when logs were cut, engineering system, construction methods and even the type of heating and cooling system used in the home. So there isn’t a single simple answer. Engineering and building for settlement involves preparing for a range of possible movement.

Most log home companies include settlement spaces above doors and windows. This gap, filled with loose insulation, allows the logs to settle around the opening without putting pressure on top of the frame. For some manufacturers, that may be the only allowance made for settlement. My question to them is, “If you expect for the log wall to settle an inch at the window or door, where do expect that inch to go in the centre of the house?” The answers are sometimes amusing. “We put the settlement space in as a precaution, actually you don’t need it.” Or, “There’s enough flexibility for the interior framing to absorb it.” The first answer is like saying, “We put seat belts in your car but we don’t include buckles because our buyers don’t have accidents.” In the second case, I wonder when they last applied a few thousand pounds of pressure to the edge of a piece of drywall to see how well it was absorbed.

To accommodate settlement inside the house as well as in the log walls, there must be settlement space in interior framed walls. So-called “shrinking” or “settling” walls, used by a number of log home companies, place the weight of the second floor and roof on posts rather than bearing walls. Interior framed walls contain a space near the top that is concealed behind trim secured only to the top of this settlement space. The support posts rest on shims or adjustable jacks.

As the log walls settle, the shims are removed or the jacks are lowered. This lowers the centre of the roof and second floor system, keeping it aligned with the log wall. As the centre is lowered, the settling space, concealed behind trim, closes. Because the trim is fastened only at the top of the settling space, it isn’t necessary to remove it to make an adjustment. As a bonus, since there are no interior bearing walls, the house is a remodeler’s dream. You can knock out and rearrange walls to your heart’s content. Just don’t mess with those posts!

At this point, you’re probably saying, “Whoa, you’re telling me to lower my second floor and roof as my log walls settle? And this is simple? And easy?” Actually, yes. Just recently, I performed exactly this procedure on a house built almost two years ago. When we started, the second floor was 3/4 inch out of level and the door into a second-floor bedroom would not close. The adjustment required a wrench and took about an hour. (Actually it took longer because we made the adjustment in stages to minimize stress on the second-floor system.) When we left, the second floor was level again and the door worked perfectly. So the process is not difficult, time consuming or costly. But there are two big “ifs” attached. If the house is designed properly and if the builder followed the instructions and construction detail drawings provided by the manufacturer. Builders who understand the principles involved can usually build a fully adjustable log home without special drawings or instructions.

In addition to settling spaces in interior framing, it’s important that the logs be allowed to settle. This means that framing attached to log walls is attached by nails or screws driven through a slot in the framing. As the logs settle the nails or screws simply slide down the slot. The same method is used to secure window and door framing to logs. If this is not done, logs will be prevented from settling and gaps will appear.

There are three areas of special concern in making a log home fully adjustable for settlement: log gables, stairways and fireplaces. Log gables settle unevenly because of their shape. Consider a log gable with 20 courses of logs at the peak, tapering to none at the edges. If the log wall settles 1 inch, the bottom of the rafters will sit 1 inch lower while the ridge of the house will actually lower 2 inches (1 inch in the log wall and another in the gable.) As this happens, stresses on the roof and gables will change, perhaps causing additional roof and gable maintenance such as resealing, flashing and re-setting trim. Log gables may be pretty, but they are expensive, difficult and prone to problems. It’s much less expensive to frame gables and cover the exterior and interior with matching log siding or contrasting treatment. Many companies make log siding and tongue and groove especially for this purpose.

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