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Friday, December 17, 2010
Foundation Design For Log Homes
The building department in your area will have a lot to say about the type and design of your foundation and what they will allow.Before you go off and buy a log home kit, you need to know a few basics.
The first decision that you will be faced with is the foundation of course. Should you build on piers, on a solid foundation, on stilts or should you include a basement. The lot that you are building on will give you your first hint. If you have a lot that has a decent slope to it and it is perfect for a basement, you might want to consider adding one. The why of this is that a basement is going to be the cheapest square footage that you can get and will add more resale value than almost any other choice you are going to make. Typically a basement will cost you no more than thirty to forty dollars per square foot, depending of course on the market that you live in. If you want a solid foundation and the lot is suited to a basement, the cost will not be much more than the solid foundation alone. Then there is the pier or stilt method, this works well and cuts costs considerable, the downside to this is that the sub-floor is going to require extra insulation. Another drawback to these types of foundations is stability, when you leave an open air-space below your house, you are giving mother nature a toe hold on your house. Log homes are heavy and they need all the stability that they can get. In a storm or tornado, your house is more likely to shift if the wind can get under it. The biggest advantage of these foundations is cost. They are certainly cheaper and allow you to get to the meat of your project allot quicker. If cost is a deciding factor, you can always enclose the foundation at a later date. Another drawback to this type of foundation is the water and electrical lines. Of course you don't have to worry about your electrical lines freezing, but your water lines are five times more likely to freeze with an open foundation. There is heat tape that you can apply, but these products do die and they don't tell you when they die. If you decide to go this route be sure to check your anti-freeze systems every year before winter and monthly during winter.
Another newer type of foundation on the market is the pre-cast panel system. These are great because they do not require a footing and can be installed in a few days (in most cases). After your panels are set, the builder comes in and installs a termite shield and then the sill plate. Almost all pre-cast systems require that the sill plate lap at least four foot on to the next panel. This is what locks them together and keeps them from shifting. This is IMPORTANT, if your builder neglects to do this properly, your foundation can shift when back-filled and the warranty will be voided. This is not easy to fix because the way the system works you cannot back-fill until the sub-floor is on. You will not discover the problem until it becomes a very expensive process. After your sub-floor is installed, you can pour the basement floor and lock in the walls permanently, then and only then can you backfill.
Now for the newest type of foundation to hit the building world, The hollow foam block. These blocks are made of Styrofoam which interlock and are stacked very quickly. During the stacking process a series of reinforcement bars are installed along with plastic spreader pieces. (The plastic pieces prevent the blocks from spreading when they are filled with concrete.) While stacking the blocks, all of the openings are formed out, as a result these form boards become permanent bucks for your windows and doors. It is best to use pressure treated material for the bucks. Once everything is in place and a few kickers (which keep everything from shifting) are installed, it is time to pour the concrete. This is best done with a pump truck or a conveyor truck as trying to pour this much concrete with wheelbarrows and buckets is too time consuming. The concrete could set up in the bottom of the forms and cause you to have cold joints.
Whatever type of foundation that you choose, all of them will need a termite shield on top of them before the sill plate is installed. A termite shield is a continuous aluminum sheet (aluminum flashing comes in rolls from ten inches to two feet wide) at least two inches wider than the top of the foundation. Flush this material with the outside of the wall and let it hang over on the inside. The theory behind this is that termites cannot walk over the slick aluminum preventing them from building a tunnel up the inside of the wall up to the sub-floor. Do not neglect this process. This is not a costly procedure and can save your home in the long run. Do not let your contractor or anyone else talk you out of this step.
When building the foundation you will need some sort of method to attach the sill plate. (sill plates are almost always pressure treated two by sixes) In the case of the pre-cast concrete panels, they leave holes for you to bolt the sill plate on. Use every hole and the hardware provided as it will be rated for this application, this is extremely important when using this type of system and it could void your warranty if you don't. In some areas, such as coastal plains, your local codes will dictate how you must attach your sill plate.
The most common fastener for sill plates is the anchor bolt with a threaded nut and washer. These bolts are shaped like an L and are usually put in the wet concrete about every four feet or as required by your local codes. (keep in mind when you are installing your bolts that wherever one board ends you want two bolts so that the end of one isn't just laying there, this is especially true at the corners of the wall where you may need as many as three bolts, lay out where your boards are going on top of the wall to better plan for bolt placement) When you install your termite shield, just lay it out on top of the bolts and working from right to left, hit the flashing lightly with a wooden headed hammer on top of the anchor bolt, this will knock a hole in the flashing where you need it then you can drive the metal down flat on each side of the bolt. Be careful not to damage the threads on your anchor bolts, if you want to play it safe, put a nut on the bolt before you hit the metal and after you have a hole in the flashing lift it up and take the bolt off, this will straighten any threads that you may have damaged. Once you have the flashing on, you can bolt down the sill plate. Work from right to left and measure each bolt from the end of where you are starting. Make a straight line across the board with a speed square and then measure how far the bolt is from the outside edge. (when installing the bolts, it is best to stagger them alternating in and out while making sure the leg on the bottom of the bolt is covered well) Pre-drill each hole using a bit that is no more than one-eighth inch larger than the bolts diameter. Once the board is in place (you may have to work the board back and forth while hitting it with a two pound hammer) place the washers and finally the bolts. Tighten until the wood starts to compress, making sure not to over-tighten as you can pull the anchor bolt right out of the concrete. (allow the concrete around the bolts to cure at least thirty-six hours before applying stress)
If you have chosen a block foundation that you do not plan on pouring solid with concrete, you can stuff the cells where your anchor bolts are going with the left over mortar bags. If you will pre-plan and layout where your floor joists are going on top of the wall, you can place your anchor bolts in such a way that they do not interfere with the joists. You do not want to have to notch your floor joists before you even get started good.( if you neglected to pre-plan your bolt placement, you can always counter-sink the nuts and cut the top of the bolt off with a sawz-all) Most log homes have a double rim joist ( I like to call it a boxing band, that is the board that borders the outside of your sub-floor framing and your joists will be attached to it) This means that your anchor bolts must be at least three and a half inches in from the outside of your wall. ( I realize that this does not allow for much staggering of the bolts, but whatever stagger you can get will be better than having them in a straight line) Technically all of the pressure on a sill plate is down pressure which means there is no lateral force on the plate. This means that bolting the plate down only fastens the structure to the foundation and typically the weight of the house would hold it in place. However, during a freak wind event, there can be strong lateral forces on these bolts. These bolts also help keep the boxing band (rim joist) still while the builder is working on the sub-floor.
Another consideration when planning the foundation is the final appearance. If you are planning on using stone or river rock for the exterior finish of your basement then the foundation must be left back the thickness of your material to look right. Typically you will find that most log homes have stone covering the block foundation. Try to stay away from anything thicker than two inches as this is almost the limit of set-back that you can get due to how the first log course much be attached to the sub-floor. (that is why there is a double boxing band or rim joist) You have an unlimited selection of materials to choose from when covering a foundation. One of the cheapest methods (versus painting the block) is to stucco the exterior wall, (natural stucco is just two or three layers of colored Portland cement, synthetic stucco is an expensive material that must be purchased be a licensed professional) the advantage of stucco is that no set-back is required making pre-planning a little easier. The biggest drawback to stucco is that if your builder doesn't use a good bonding agent or it is applied at too cold of a temperature, the material can flake off.
Then there is brick, honestly I don't think brick belongs anywhere near a log home, (that is a personal opinion) brick offers the same challenges as stone as you will need to set the foundation wall back to allow for the brick. There is also a product which is face brick which is only about five-eighths of an inch thick and goes on much quicker than standard brick. With this material you will also need to set the wall back to allow for the difference in thickness. Whatever the choice even if it is fake brick (stucco applied to one-half inch thick and then mortar lines scratched out to resemble brick) the challenges are certainly the same and must be accounted for before you begin construction.
Decks, porches, stairs or any other exterior attachment that will eventually be attached to your home at the sub-floor must be planned for now. Wherever you plan on putting a deck etc. plan on using a pressure treated board for your outside boxing band (rim joist.) Even though you will install a vapor barrier and a piece of aluminum flashing to protect your sub-floor, use a pressure treated rim joist at all of these areas. When you eventually build your deck etc. make sure to bolt it to the rim joist with lag bolts at least every two feet and preferably every sixteen inches (through bolting is even more efficient but requires more work), making sure that the bolts will penetrate through all three layers of the rim joist. (in most areas of the country this is code due to the many deaths and injuries from decks failing during parties or other unexpected load events) You especially want to use these methods if the deck or stair is not covered by a roof (technically a deck with a roof is a porch) even though you will probably slope the deck away from the house, water tends to get in places where we don't want it. More often than not this area of the boxing band will rot out if not planned for in advance making the attachment unstable and unsafe for you and your family. It is also a good idea to start your decking about an inch or so away from the edge of the house rim. This will allow water to fall and not lay up against your rim joist. You can prevent this before you even start if you know what to look for and how to solve the problems.
For more information see LR Goddwins foundation articles.