First Half Round
If you’re working offline (in the forest), you can download the instructions here: Lesson 7 First Half Round
A Firm Foundation
With any new building project regardless of the style of construction, it is important that we start with a properly designed and installed foundation. There is much material already available in print and online that deals with conventional foundation options and construction therefore we will only touch on foundation and subfloor details that are particular to log construction. We have erected log buildings on many types of foundations and subfloor combinations. Examples include wood subfloors, Sonotubes, screw piles, full basement foundations and crawl spaces, as well as concrete slabs on grade with and without poured concrete or wood framed curb walls. As long as they are properly designed for the loads that they are to carry, meet necessary building code requirements for your district, and properly serve your required design needs, any of the above are suitable choices.
Most of the log buildings that are built by professional log building companies are first constructed off site at their log yard then labeled, disassembled, and shipped to the client’s final location. For varying reasons, you may also choose to build your log shell at a temporary location then move it onto the subfloor when it is completed. Some common reasons may include:
- you are starting the shell in the winter and the foundation did not get installed before freeze up,
- the foundation budget wasn’t in place yet,
- the final site is more awkward to work at then an ulterior temporary site,
- you want to minimize large machine costs such as long reach cranes,
- you want to minimize site impact,
- the final site is simply an inconveniently far distance from where you currently reside,
- you may not own a piece of property yet but you have access to a location where you can temporarily build the shell.
Building a dovetail log shell allows you to start building up sweat equity that will go towards your overall property value before you own the property, even if you haven’t found your dream location yet.
Setting the Blocks
If you decide to build your dovetail shell and then move it, it is important that you establish a square and level temporary foundation to work on. We do this by laying out appropriately spaced blocks of timber or log stumps that are leveled individually and then shimmed to the same height. The size and complexity of the dovetail shell to be constructed will dictate the number of blocks required.
Each corner will require a support block as well as a mid span block to stop the logs from sagging over its length. If the wall length is over 24’ a second intermediary support block should be added. It is good practice to have a block located under a door location to help carry the point loads transferred down either side of the opening.
Setting up the blocks for the Willowbank course cabin.
Time to LOG-In Now that we are familiar with the different parts of the dovetail, how it functions, as well as how to lay out and cut the receiver and the connector, it’s time to get started! Whether you choose to build directly on a finished subfloor or on temporary blocks, let’s get on to our first logs.
The First Round
Typically, in modern log building the foundation and subfloor are constructed and the log shell is then assembled on it. This requires that all the logs of the first round be flatted to sit firmly on the subfloor as well as to be able to make a tight seal against the flat subfloor. We will look at best practices for sealing and installing first logs to the subfloor later in this section.
The first round of logs that you lay is critical as it sets the pattern and template for the remaining logs that are to be fitted above. A round of logs, also referred to as a course of logs, is one full wrap of logs that includes one log on each wall of the building. If the house is a simple box, then this will be 4 logs. If the house design is more complex such as an L shape or T shape, this could be 7 or 10 logs.
Start with a Half Log
The first round of the building is unique because in order to create the proper starting pattern we can not simply begin with all the same sized logs. Let’s look at this idea on a simple 4 walled building. It is important to understand the concept that the first two opposing walls of the log building must start with ‘half logs’. Afterwards, all of the walls will continue with full logs for the remainder of the stacking process. This results in a pattern where the two opposing walls are always half a log higher or lower than the other two perpendicular walls. It is also essential that we start with the right sized half logs to ensure that we ‘leave up’ enough wood at the notch, but not too much that the next log will not be able to cover it.
First ½ Round and first full round.
As mentioned earlier, the approach to dovetail building described in these pages will use logs that have been sawn into dimensional uniform material. This means that we do not have to take the taper of the log into consideration. Therefore, as long as we start with the right sized half log for our first two logs, we will have created the proper pattern for the rest of our logs to follow. The term ‘half log’ is a little misleading; to create the half-log it isn’t simply dividing our chosen wall log size in half. We must consider not only the size of our wall log but also the chinking gap that accompanies each log that we lay.
Looking at the log wall from a mathematical standpoint the true size of each round is actually the amount of solid wood plus the size of a chink gap. For example, if we are building with 12” timber and using a ¾’ chink gap between each round of logs then mathematically we will gain 12 ¾” in height with each round that we add to the wall. I find it most helpful to picture it as each log having a half of a chink gap below, and a half chink gap above, stuck to it and thereby making up its full height. We’ll explore this idea further after we look at the formula to calculate the half round.
Calculating the Half Log Size
There is some Information you will need to know and decide on before you can calculate the required size of the half log:
- Firstly, you’ll need to know the height of the log material that you are going to use to build the walls. Using the same size of material throughout makes the building process simpler as there are no more log selection calculations to be made once the half round is calculated. Different log sizes can be mixed into the wall, but this requires a calculated process each time we introduce a different log size. For now, we will assume that we are building with like sized material throughout the walls. We will use the size example from our course cabin, the Willowbank Cabin: 12” tall logs.
- Secondly you will need to decide on the size of the chinking gap that will be left between each course of logs. Leave a significant enough gap that it allows for efficient chinking install without being so large that it is difficult to chink (chinking material is not cheap). I find ¾” to be an optimal size. Prepared with that information you can complete the formula below:
Half Log Height = (Height of Next wall log / 2) – (Chink Gap / 2)
5 5/8” = (12” / 2 ) – (¾” / 2)
5 5/8” = 6” – 3/8”
For a 12” tall log, the half log size required is 5 5/8”. A smooth way to remember it is: Half of the Next Full Log minus Half of the Chink Gap. This formula can be used to calculate the required ½ round dimension to suit any sized material that is going to be used for wall logs.
If we begin with a half log size calculated by the above formula, this will establish the correct pattern. The proper first round pattern is achieved when the centre point of the chinking gap is at the same height elevation as the midpoint of the perpendicular wall log. Another way to say this is that the centre of the chinking gap between the two logs on the side walls should line up with the middle point of the log faces on the front and back walls and visa versa. By establishing this on round one, this pattern will carry on throughout the height of the walls.
The midpoint of the chinking gap should line up with the mid point of the face of the full height log that is running perpendicular to the half log.
Now that you have your temporary foundation blocks set or subfloor installed and you’ve calculated the required size of your half logs you are ready to start on the first round.
Fair and SQUARE
The first round of logs will be the template that we stack the remaining logs onto and therefore we need to be sure to notch these logs to the exact building widths and ensure that the walls are square. An accurate way to ensure that we are starting with a square layout is to check the diagonal measurements from corner to corner. If the diagonal length measured between two corners of the box reads the same as the diagonal measurement shown between the remaining two corners, then this proves that the four corners are square to each other.
You can do this by trial and error, but that means muscling heavy logs in and out until the measurements read the same. It is much easier on our backs if we use math. If we can determine what the diagonal should be based on our building dimensions, then we can simply lay it out properly the first time. To do so we use:
Pythagoras Theorem A2 + B2 = C2.
As log builders we use this formula frequently around the log yard to set up temporary blocks, to layout the first and last round, when building stairs, to calculate knee brace lengths for timber frames, and to calculate rafter lengths for different roof pitches (just to name a few instances). Of course, today we can just google it; however I still recommend that you learn it as it is a traditional builder’s skill.
Use the values of the A wall length and the B wall length to calculate C, the diagonal length from corner to corner. The values shown are for our 16×20 Willowbank Cabin project.
Set your first half rounds in place on your blocks or subfloor so that they are paralleling each other: one log on the front wall and one on the back wall. For our Willowbank plan we will be referring to the wall with the entrance door as the front. Place the half logs on the blocks or subfloor so that there is roughly an equal amount of additional log length running past at each end. Manipulate the logs until they are the precisely desired distance apart at both ends and based on your specified building dimension. For example, your half logs would be 20’ apart and parallel for the Willowbank Cabin.
Check that the exterior faces of the logs are sitting plumb. Wedge to correct subtle variances if necessary and ensure that the widths remain true after shimming into plumb. Re-adjust as necessary. If you are building directly on your subfloor, ensure that the logs are lined up properly to suit the floor size. At this point the half rounds are not yet trimmed to length. If there is a difference in rough length between the two logs start the following layout on the shortest of the pair.
Do not trim your half round logs to rough length. Leave them as long as possible until after the receiver layout is completed. This will provide more room to land a square layout with out having to physically move the timbers.
Laying Out the Half Logs
- Mark out the Building Width Place reference marks that centre the building width on the rough length of the untrimmed log. Mark this clearly on the top face and closest to the exterior of the log. Do not draw plumb or square lines from this reference. These marks will be our building width reference that we measure the diagonals from. We will add additional trim to these lengths shortly. If you are building on a subfloor, use the actual floor edge as your reference to transfer up your appropriate building width marks.
- Layout the Diagonals Using your calculated diagonal measurement, measure diagonally from the building width refence mark on the first log to the opposite corner on the parallel log. The tape, with its desired measurement in view, will be able to swing across the top face of the log. The correct location is where the diagonal reads the desired number as it intersects the precise exterior face of the timber. Mark clearly on the top face of the log where it meets the corner of the exterior face. Repeat this step for the second diagonal. Check that these two new reference marks are indeed the proper distance apart to match the desired building width. Remember that these diagonal measurements will only be accurate if the two half rounds are set exactly parallel and precisely the correct distance apart.
We now have 4 square layout points marked on the top of our half logs. These points represent the outside corners of our building and the exterior face locations of our perpendicular logs and they will be used as a starting point for our receiver layout. As mentioned earlier we build our walls allowing for an additional 2” trim at each end of all logs. Add and mark this additional 2” of trim now to each end of the logs beyond the building width. Plumb this mark down the exterior face and square across the top face. These will be your trim lines. (The trim lines should be the building width + 4” apart).
Trim the half rounds to construction length (including 2” of trip on each side). Once trimmed, it is a good practice to temporarily secure the half rounds down to your blocks or subfloor to prevent them from moving around when we introduce the next logs. Tack the logs by running toenail screws through the end grain into blocks or from the outside of subfloor up into the underside of half logs.
Tack the logs by running toenail screws through the end grain.
- Layout and Cut the Receivers Follow the same description and steps outlined in Lesson 5, Laying Out and Cutting the Receivers. Remember to first select the next two logs that you are going to use on the wall in order to obtain the width measurements that will correspond to each receiver. The only difference in the layout for the first half round receiver is that there is no log below to measure up from to locate the horizontal tail height. The height is still determined the same way: 1/3 of the combined full log height + a chinking gap. You can simply measure up from the subfloor or the temporary block to locate this point.
Remember to score your lines prior to cutting the receivers. Follow all the steps as outlined in Lesson 5 Cutting the Receivers. To view a full demonstration of laying out and cutting the first receivers please refer to video 7.
If you’re working offline (in the forest), you can download the instructions here: Lesson 7 First Half Round