Why heavy timber tallies so much larger than it looks
A 2x10 is not two inches by ten inches — it is 1-1/2 by 9-1/4, dressed down from a green rough size that no longer exists by the time you buy it. Board feet are still billed on the nominal figure, which is why a 2x10 tallies about 44 percent more wood than it contains. Heavy timber does not work that way. An 8x8 is sawn at eight by eight and sold at eight by eight, so the tally and the tape agree. The consequence is that the board footage in a timber frame is honest, and enormous. A single 8x8 post fourteen feet long is 74.67 board feet. Four bents with two posts each is 597.3 board feet before a tie beam, a plate, a girt or a brace has been counted.
That is the number this page exists to produce, because it is the number a mill quotes against and the number a truck is loaded against. It is not a design. Which member is an 8x8 and which is a 6x8, how far a bay may span, and whether a tie beam is doing the job of a tie or the job of a beam, are questions with structural answers that come from the person who drew the frame.
How the schedule is built
The whole takeoff falls out of two counts. Bents are the cross frames — post, post, tie beam, braces — assembled flat on the deck and stood up one at a time. Bays are the spaces between them, so four bents give three bays, and the length over the plates is three bay spacings, not four. Girts, purlins and floor framing are bay-length members and get multiplied by the bay count. Plates run the whole length of the frame in one line and are therefore almost always longer than any stock a mill will cut, which is what makes splicing a routine part of timber framing rather than an emergency.
| Member | How many | How long |
|---|---|---|
| Post | bents x posts per bent | sill to plate, plus tenons |
| Tie beam | one per bent | the bent width |
| Plate | two | the whole frame length |
| Girt | bays x girts per bay | one bay |
| Brace | counted off the drawing | overall, tenons included |
Board feet for any of them is thickness in inches times width in inches times length in feet, divided by twelve. That divisor is not a convention: a board foot is 144 cubic inches, a foot of length is twelve inches, and 12 x 12 divided by 144 leaves the twelve. Twelve board feet make one cubic foot, which is how the weight column gets built — cubic feet times whatever density you entered.
The density field is the one to think about
Everything else here is geometry and cannot really surprise you. Density can. The same species can sit anywhere between roughly 25 and 60 pounds per cubic foot depending on how much water is in it, and a timber frame is frequently cut and raised green because green timber is easier to work and because a twelve-inch timber does not dry through in any useful span of time anyway. A frame that tallies 3,000 board feet is 250 cubic feet of wood; at 35 pounds that is 8,750 pounds, and at 55 it is 13,750. Nothing about the frame changed. If the figure is being used to decide what gets moved and how, it needs to be the density of the timber in the condition it will be in when it is moved, and that figure comes from a species reference or from weighing a piece, not from this page.
What it deliberately will not do
It will not size anything, will not check a span, will not tell you whether two posts per bent is enough, and will not comment on the joinery. It also stops short of a cut list: the same total board footage can come from timbers a mill has in stock or from timbers that have to be sawn to order, and that is a conversation with a sawyer. For fitting parts into stock efficiently once the cut list exists, the cut list optimizer does the packing.
Questions people ask
Why are board feet calculated on the full timber size instead of a dressed size?
Because heavy timber is normally sold at the size it is sawn. Dimensional softwood is dressed after drying, so a 2x10 finishes at 1-1/2 by 9-1/4 while still being tallied as 2 by 10, and the tally overstates the wood by about a fifth. An 8x8 sawn at eight by eight is eight by eight when it arrives. If your timbers are being planed to a finished size below the nominal, enter the finished size and the tally will follow it.
Does the takeoff subtract the wood removed by mortises and housings?
No. A mortise in an 8x8 might be 2 by 5 by 5 inches, which is 50 cubic inches out of a piece holding several thousand, and housings and shoulder cuts are smaller still. Left in, they act as a small allowance against the offcuts and mistakes the tally also does not include. If you need a genuinely exact volume, for a weight that matters, subtract them by hand.
How many braces should a frame have?
That is a question about the structure, and it is not one a calculator can answer. Bracing resists racking, how much is needed depends on the frame, the loads, the sheathing and the engineering behind the design, and the answer belongs to the frame designer or engineer. Count the braces off your drawing and enter that number.
The plate is longer than any stock I can buy. What now?
That is the normal case, not a problem. Plates get spliced with a scarf joint, which is a long tapered lap that keeps the run continuous. Every splice consumes an extra scarf length of timber on top of the run itself, so the material figure goes up even though the finished length does not. The scarf joint layout calculator works out the length, the count and the positions.
Why does the weight change so much when I change the density?
Because weight is nothing but volume times density here, so it moves in exact proportion. A frame is a big volume of wood, and the water in green timber is a large fraction of what you would be lifting. Twenty pounds per cubic foot of difference across 250 cubic feet is 5,000 pounds. It is worth getting that figure from the timber you actually have rather than from the middle of a range.