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For the floor assembly on our solar kiln, we chose to use pressure treated wood since the skids will be in contact with the ground.
But when it comes to the walls and roof structure, we'll be constructing it much like a typical stick-framed building - in this case, with 2x4s.
Dimensional Lumber - Actual vs Nominal
2x4 (pronounced: "two-by-four") boards are so-named for their cross-sectional dimensions - 2 inches by 4 inches. The length is arbitrary, but 2 foot increments starting at 6 feet long are the norm, with 8 foot long pieces being common for wall studs.
Except if you go into a big box store and buy a 2x4, you might be surprised to learn that its dimensions are not, in fact, 2 inches by 4 inches. Rather, the piece will be 1-1/2 inches by 3-1/2 inches.
Why such a difference?
The reason is somewhat historical. When boards were originally cut, they were in fact 2 inches by 4 inches. But between shrinkage during the drying process and material removal during planing, the final product was smaller by about half an inch in each dimension.
Modern drying and planing processes result in far less dimensional loss, but the boards are also cut smaller such that the end result is still 1-1/2 inches by 3-1/2 inches. So, ironically, a modern 2x4 is not and has never in its life been 2 inches by 4 inches.
The terminology used here is that the board is known as a nominal 2x4 but has actual dimensions of 1-1/2 inches x 3-1/2 inches - it's important to be clear about nominal vs actual.
Fun fact: In the UK, the equivalent lumber is still often referred to as "four-by-two" (sometimes colloquially pronounced "four b' two") even though it's been sized metrically for many years as 100mm x 50mm. It wouldn't be uncommon to ask for "three metres (sic) of four-by-two".
The same is also true for most dimensional lumber - e.g. 1x4, 2x8, 4x4, etc. All have actual dimensions less than their nominal dimensions. For nominal dimensions equal to 6 or less, the difference is 1/2", which rises to 3/4" for nominal lumber 8 or more. So for example a 2x6 is actually 1-1/2" by 5-1/2" but a 2x8 is actually 1-1/2" by 7-1/4".
We decided to mill the 2x4s for our solar kiln out of pine. Given the spruce trees on our property, spruce would typically be a better option for framing, but we haven't felled many spruce trees yet (and can't until our Current Use Exemption is approved) while we have an abundance of pine logs from our driveway clearing.
But the big question is, what dimensions do we mill to?
After some debate, we eventually settled on cutting them to full-dimension 2" by 4". They will shrink a little as they dry, but will still be larger than 1-1/2" by 3-1/2" afterwards.
We will probably use them as they are, and just build with the rough sawn lumber. But by milling them a little larger than needed, we'll have the option to joint and plane them (to smooth their surfaces and ensure they're straight and square) if we want to.
A material take-off from our SketchUp plans for the solar kiln told us we'll need around 90 2x4s, about 20 or so of which need to be a few inches shy of 9ft long and the rest 8ft long.
It's commonplace to buck logs to a little longer than the end board length you're after to account for potential cracking. Some places will cut the logs just a couple of inches long, although many sawyers prefer a buffer of 6" or more. We decided to play it extra safe and add an extra 12" in most cases.
As a result, we have a good selection of 9ft long pine logs to choose from. As long as we don't have too much cracking in the ends of our 2x4s as they dry, these will be long enough for our needs.
In general, it's advisable to add about 10% to your material take-off to account for wastage - damaged boards, mistakes, mis-calculations, etc. Because our plans are still a little in flux, we decided to aim for 120 boards, about a third more than the 90 boards suggested by the take-off.
Plain Sawn vs Quarter Sawn vs Rift Sawn
There are three common ways to cut boards of out a log - called plain sawn, quarter sawn and rift sawn.
Plain sawn involves simply cutting slices of the log starting at the top. It's the easiest and quickest method, and creates the least wastage. However, the grain will run differently in different boards creating variation between the boards, and this approach has the most opportunity for the boards to cup.
For quarter sawn, the log is first cut into 4 quarters, and then each of those quarters is then plain sawn. The resulting boards are more structurally stable than straight plain sawn, but it's slower, more difficult and creates more wastage.
Rift sawn is the slowest, most difficult and most wasteful, but produces boards with completely striaght grain from end to end. This is achieved by cutting boards out in a radial pattern around the center of the log. As this is the most expensive way to cut boards, it's typically reserved for applications where the grain pattern is critical - e.g. table legs.
For our purposes, we chose the simple plain sawn approach. We're still novices with our sawmill and want to keep things as simple as possible!
Milling Cants & Boards
Through a process of trial and error, we settled into a process that worked for us. Essentially it boils down to milling each log to create a 3-sided cant, slicing the cant into 2" slabs, flipping the slabs 90° on their flat edge and then cutting down in 4" increments to cut the 2x4 boards.
First, we lifted the logs up onto the sawmill using the tractor. Some of the pine logs are pretty wide (our largest was 24" in diameter) and even at 9ft long were pushing the limits of our Kubota L3901 tractor.
It's important that the log is tight against the log stops, but also that it's centered on the bunks. We ran into problems on our 24" wide log because the log stops meant the log wasn't centered, and then it was too wide for the sawhead on our Woodland Mills HM126 to pass over. We learned to lower the log stops a little so that they would contact the log lower down and allow it to move further over on the bunks.
Once the log is centered, we would prop up the thin end a little if necessary so that the axis of the log is parallel with the saw.
With the log locked in position, we milled slices off the top until we had cut down to where we felt the first edge of the cant would be. One tip we picked up online was to cut off roughly 1/2" slices at a time, rather than one big intial cut - not only are thinner slices easier to manage, but the 1/2" boards are useful for lots of little side projects.
Once we had milled down as far as we wanted, we used the tractor to rotate the log by 90° so that our cut edge was now held tight against the vertical log stops. Repeating the 1/2" slicing, we were left with a second sawn face that is perfectly perpendicular to the first.
Rotating the log a further 90° leaves it sitting on one flat flace while the second sawn face is tight against the vertical log stops. Once again we would cut slices down until we had formed our 3-sided cant. However, the dimensions of the cant are important - it should be a multiple of 2", while allowing for the thickness of the blade kerf between the cuts.
We took some careful measurements and determined that the kerf on our saw is 3/32", so target cant heights for us are: 2", 4-3/32", 6-6/32", 8-9/32", 10-12/32", 12-15/32", 14-18/32", etc. Our sawmill came with a scale that already takes into account the kerf for 2" cuts, so we were able to calibrate that and use it instead of measuring every time.
Once we had our final dimension cant, we began cutting off 2" slabs from the top - removing each one after cutting and stacking it on the tractor pallet forks. After the final cut, we rotated the bottom two slabs so they were upright with the good edge down, then stacked all the other slabs we had removed alongside them. The end result was a row of 2" thick slabs stacked next to each other.
Again, accounting for kerf, we could begin cutting 4" slices off the top. We made a story stick using a piece of scrap and marked our cut heights: 4", 8-3/32", 12-6/32", and 16-9/32". Now it was just a case of lining up the saw blade with each of the marked heights, and then cutting down in 4" increments.
As we did so, we removed the freshly cut full-dimension 2x4 boards and stacked them back on the tractor forks until we had cut all our slices.
With a perfect cant, there would theoretically be no wastage - it would all be turned into perfect 2x4s. However, our cants were deliberately imperfect - we don't mind a bit of bark on one edge because we knew the boards in the middle would be fine.
As a result, some of the 2x4s we cut had imperfections on - usually sections of bark on one edge, or as was the case with one of our logs that had damage at one end, the 2x4s weren't full length.
Rather than throwing these on our offcut pile, we kept the vast majority of them. With some we'll be able to cut shorter lengths out that will still be fine - in some cases maybe just 3 or 4ft, but in other cases 6ft or more.
Others may have bark along an entire edge and while we may choose not to use them for framing our solar kiln, they'll still be perfect for side projects - maybe creating a saw horse, or a rack to support logs for loading onto the sawmill.
In general, our philosophy is to use as much of each log as possible. We've already used some of the 1/2" slices to create roofs for our firewood piles, and the bark offcuts to weigh these down as well as protect our stacked lumber.
Offcuts that we can't use we'll put through a wood chipper - we have a lot of uses for woodchips, from laying on the trails to mulching and composting.
We're also generating a lot of sawdust and are looking for ways to use that too - maybe for compost again, or perhaps find some neighbors who can use it for horses, etc.
Stacking & Stickering
Once the 2x4s have been milled they need stacking. We use a small brush to clear off most of the sawdust, then stack them neatly in piles using the stickers we had milled previously.
We chose to place stickers every 16", with the final one a little closer at about 12" so we could stack 8ft and 9ft pieces in the same pile (longer pieces at the bottom). That meant we needed 7 or 8 stickers per row, and we could fit ten 2x4s in each 4ft wide row.
Even though we milled about 200 spruce stickers before starting on the pine, it already feels like we'll never have enough stickers, especially once we have the solar kiln operational!
We found that each log, depending on size, produced somewhere around 15-25 perfect 2x4s, and usually some imperfect 2x4s too. In the end we milled 6 or 7 (we've already lost count!) pine logs, giving us about 120 usable 2x4s and almost 140 in total including the imperfect ones!
By the end of our final log we were settling into a good routine - each log taking about 2 hours for us to fully process, meaning it takes the two of us about an hour to produce ten 2x4s.
While this is a significant time investment, it's also incredibly satisfying. Knowing that we have not only the tools but the skills to turn a tree into a stack of dimensional lumber that we can build with is an awesome feeling!
We have a lot more lumber to mill for our solar kiln - particularly the 1x material for the board-and-batten siding - but I'm sure we'll continue to get faster and more efficient with practice.
Our sawmill has barely 12 hours on the clock, but already we're smitten with it and can't wait to get onto some of the more exciting woods too - like birch, ash, hard maple and cherry.