Wood: Board Foot

Wood: Board Foot

Wood: Board Foot - Full TranscriptionThis video covers a couple of terms and concepts that pertain to timber construction. Here I took my students to a lumber sawmill and everything is computer controlled. As soon as these logs start to be processed, they are immediately scanned by a computer and optimized how to cut it to get the most out of it. Typically, nothing is wasted. The lumber is debarked. There are several machines that do the debarking. And then they cut the bark into wood chips and mulch and what have you. And then these saws are programmed immediately by the computer automatically to be cut to a certain length to maximize the number of pieces that they can get out of that log. And then they start cutting it into whatever was on that screen. So it's all computer controlled, and they cut it. And then it runs through a bunch of conveyor belts, and they do visual grading. They look at the lumber, and they see how many knots, wanes, imperfections, and they give it a stamp. For example, Douglas fir or Southern pine, number one, number two, number three, utility grade. Anyway, so, and then it's processed and shipped out to Home Depot, et cetera, and to construction sites. I need to talk a little bit about sizes in wood and classification of lumber. We have two basic categories, dimensional lumber and timbers. And most mills in the United States are producing dimensional lumber, which is 2x or 4x, versus the larger pieces. Those are the timbers. So let's look at these two categories based on size. Dimensional lumber, as we said earlier, is 2x and 4x. And dimensional lumber comes in two classifications, a light framing and joists and planks. Light framing is 2x4 inches by 2x4 inches. So a 2x2, 2x4, and a 4x4 fall under light framing versus the more rectangular pieces. a 2x6, a 2x8, 2x10, 4x6, 4x8, 4x10, etc. are called joists and planks. So both these categories are under dimensional lumber. So dimensional lumber has joists and planks and light framing. Versus timbers. Timbers are 6x8x, the larger pieces that very few sawmills produce. And pretty much the larger pieces have been replaced with engineered lumber, such as glulam, LVL, etc. So under timbers, under the classification timbers, again we have two categories, beams and stringers, or posts and timbers. Beams and stringers are 6x8x10x. If there is such a size, 10x is pretty big, that's a whole tree. Anyway, beams and stringers are more rectangular in proportion versus posts and timbers are more square. And so, based on classification, beams and stringers are going to be 6 by... Let's start with posts and timbers. Posts and timbers are the square pieces, 6 by 8 by 10 by, and they're square. So, 6 by 6, 6 by 8. That's square and square plus. Those dimensions fall under posts and timbers. 6 by 10 is a dimension of 4, so that goes here. So 6 by 6, 6 by 8 is posts and timbers. 6 by 10, 6 by 12, 6 by 14, if such a size exists, that's extremely expensive. But so square or square plus is under posts and timbers. Clearly a rectangular is under beams and stringers. So we have 10 by 10, 10 by 12. 10 by 14 would be on this side. So that's the difference between these two. And we go back here and we say dimensional lumber is two classifications. Light framing, 2 by 2, 2 by 4, 4 by 4. Joists and planks are 2 by and 4 by. And then beams and stringers are rectangular pieces that are 6 by and larger. Posts and timbers are square pieces or square plus, starting with 6x6, 8x8, 10x10, etc. So 2x and 4x members are going to give us type 5 construction. And type 5, as you know, could be A, which is protected, maybe 5-8 inch sheetrock on each side of the studs. And that would be a 1-hour exterior wall and 1-hour on structural frame, one hour for the ceiling, the floor, and the roof, versus type 5B, which is unprotected. Timbers, on the other hand, will give us type 4, or HT, construction, as long as the members are 8 pi or larger. And that means two hour exterior walls, one hour for the structure, and heavy timber floor, etc. So we're not seeing a lot of that in new construction. Instead, they're using glulam and other structural members, engineered lumber. But still, in historic structures, you might have heavy timber or solid sawn wood. So that's the classification based on size. And then we typically have Douglas fir and southern pine. Those are the two most common species in construction. And we look at this classification, and again, we see dimensional lumber, 2x or 4x. And then we have timbers, which are 6x8x10x, the larger pieces. And then we have a grade, a number 1, a number 2, a number 3, or a dense number 1, or a select structural. This is all based on how many imperfections, knots, wanes, cups. The imperfections, so the better the piece of wood, then it's going to be select structural or appearance grade because there's very few imperfections in there. So once the size is determined and the grade is determined, then we can look up pretty much all the allowable stresses and how much that piece of wood of that species and that grade can handle in bending, in tension, in shear, in compression parallel to the grain or compression perpendicular to the grain, and its modulus of elasticity. So all the properties of the piece of wood is known. Once we see that grade stamp, then we know everything about the piece of wood, and that is called allowable stress design. Okay, so moving on, I'd like to look a little bit at how lumber is sold. Typically, it's not by cubic foot. Concrete, for example, is sold by the cubic yard. Steel is sold by the ton. Well, lumber for construction is sold by the board foot. And one board foot is basically 1 inch times 12 inches times 12 inches. And so if we take one of these pieces and we stack 12 of them, we get one cubic foot. So one cubic foot is the equivalent of 12 board feet. So that's the measure for lumber, dimensional lumber typically. So typically the width is called B, the depth is called D, and L is the length in feet. So I'd like to calculate some board footages for you as an example. So I have this little formula that says the width times the depth divided by 12 times the length times the number of pieces gives me the board footage. So these dimensions are nominal dimensions, and L is in feet. You take the width, you take the depth nominal, you multiply them by each other, you divide by 12 times the length in feet times number of pieces, gives you board footage. So the best way to show this is with an example. If I have some 2x4s that are 8 foot long, and I have 60 pieces, Then I leave the length in feet. I leave these dimensions as nominal. 2 by 4 is actually 1.5 by 3.5. I don't deal with that. I just use the nominal dimensions. I divide by 12. That's my formula. Take the width times the depth, divide by 12 times the length. So this gives me 320 board feet. And let's say I'm buying also some 4 by 4s that are 10 foot long. and I need 30 pieces. So let me divide by 12, and let me run the numbers here, 4, 3, 30, 90, 400 board feet, and let's add to those some 2x10s that are spanning, let's say, 12 feet, and I need 50 pieces of 2 by 10. So let me divide by 12 here, and I get a total of 1,000 board feet. And so now I can add these together, and I can place my order for 1,720 board feet of lumber, and that's the way it's priced. It's priced by the board foot when you're buying a large quantity of lumber. I'd like to also mention something here as far as spans. Rule of thumb for residential, for wood frame construction, rule of thumb, if you are trying, let's say I have a 2x10, if it's at 16 inches on center for some kind of floor framing, I can very simply, as a rule of thumb, without running the numbers or anything, I can add 4 to the depth of the piece, and I end up with 14. So a 2 by 10 will go roughly 14 feet when 16 inches on center as floor framing. Likewise, a 2 by 12 will go 16 feet, and a 2 by 6 will go 10 feet. So just a quick rule of thumb, add 4 to the depth, you can get the span. Or otherwise, if you're trying to span, if span needed equals 12 feet, then subtract 4 and you get 8, which means probably a 2 by 8 will do the job. Likewise, if you want to span, let's say, 10 feet, subtract 4, you get 6. A 2 by 6 will do the job. Speaking of nominal and actual dimensions in lumber, I prepared this table to help us understand the actual dimensions from the nominal dimensions. When it comes to dimensional lumber and smaller depths like 4 and 6, then the difference in the width and the depth is 0.5. So a 2 by 6 is 1.5 and 5.5. And a 4 by 6 is 3.5 by 5.5. When it comes to the bigger pieces, the deeper pieces in dimensional lumber, the difference is 3 quarters on the depth and 1 half on the width. So a 2 by 8 is 1.5 by 7.25. A 2 by 12 is 1.5 by 11.25, and a 4 by 10, as an example, is 3.5 by 9.25. When it comes to timbers, the larger pieces, the difference is 0.5 on either the width or the depth. So a 6 by 6 is 5.5 by 5.5, and a 6 by 12 is 5.5 by 11.5. The same for 8 by, the same for 10 by, 12 by, etc. So an 8 by 12 would be 7.5 by 11.5. Hope this helps.