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Why a Square Line Matters More on Big Timber
On a 2×4, a line that’s 1 degree out is barely noticeable. On a 6×6 post (actual 5-1/2″ x 5-1/2″) or an 8×8, that same 1 degree puts one side of the cut almost 1/10″ higher than the other. That’s a visible gap in a pergola joint, a wobbly post base, or a rafter shoulder that won’t close. With large timber you can’t force it square with a sander. You need four lines that meet perfectly around the post, and they all have to be square to the same reference face.
The trick is not drawing one perfect line. It’s making four lines that connect and stay in the same plane all the way around. If they don’t meet, your saw will follow the error.
Tools That Actually Work
You don’t need expensive layout tools for this, but you do need one tool with a reliable 90-degree stock and a way to mark cleanly on rough-sawn timber.
A 12-inch combination square is the most versatile for posts up to 6×6. The 12″ blade reaches across a 5-1/2″ face with room to spare and the cast stock rides well on sawn timber. For anything over 6″, a 12″ square is too short to reach across in one pass, which is where a framing square or a large speed square earns its keep.
For marking, a sharp carpenter’s pencil is fine on rough timber, but a marking knife or a 0.9mm mechanical pencil gives you a line you can actually saw to. On wet or fuzzy Douglas fir, a thick pencil line can be 1/32″ wide — that’s your entire tolerance gone. Knife lines also sever wood fibers so your saw or chisel has a shoulder to register against.
| Tool | Best For | Where It Fails | Cost vs. Value |
|---|---|---|---|
| 12″ Combination Square | Posts up to 5-1/2″ faces, checking 90° on all sides | Blade too short for 8×8 and larger; small stock can rock on wane or rounded arris | $25-60 is the sweet spot. Cheaper ones often out of square out of the box. |
| 12″ Speed Square / Rafter Square | Quick square off factory edge, holds well with one hand | Lip limits use on second and third faces; not ideal for wrapping a line | $15-30. A good budget option if you already own one. |
| 16″ x 24″ Framing Square | Large posts 6×6 to 12×12, marking long wrap-around lines | Bulky, flexes if thin steel, hard to keep tight to post alone | $20-40. Don’t buy the thinnest stamped steel — it bends. |
| Paper / Card Wrap (No Square) | Emergency method when you left your square at the shop | Paper stretches, tears on rough timber, unreliable if post isn’t perfectly straight | Free but least accurate. Use only for rough cuts. |
If you’re buying one tool specifically for timber posts, the honest answer is: a mid-range combination square covers 80% of backyard pergola, mailbox, and deck post work. If you regularly handle 8×8 or reclaimed beams, add a framing square later. You don’t need both on day one.
Pick a Reference Face First
Large timber is rarely perfectly square or straight. Don’t assume adjacent faces are 90 degrees to each other — they often aren’t, especially with pressure-treated posts that have twisted while drying.
Sight down the post and choose the straightest, flattest face as your reference face. This is the face your square’s stock will ride against for the first two marks. Mark it with a big “R” in pencil. All your square lines will be pulled from this face, not from each face individually. If you keep flipping which face you reference, you compound the post’s own out-of-square error.
Check the reference face for bumps, dried sap, or stapled tags right where the cut will be. Pare off any high spot with a sharp chisel or hand plane for 2″ on either side of the line. A square rocking on a 1/16″ bump will throw your line off by more than 1 degree.
Marking All Four Faces So The Lines Meet
Work on a pair of saw horses or on flat ground with the post blocked so it cannot roll. You need both hands free. A post that shifts mid-mark is a ruined mark.
1. Measure and set your length on the reference face. Make a small tick at the cut length near the center of the face, then use the square to draw a full line straight across that face, stock tight to the adjacent face (the one 90 degrees to the reference). Hold the stock firmly with your thumb and slide the blade until it hits your tick. Draw the line with the square held in place — don’t mark the tick and freehand to it.
2. Without moving the post, roll the square to the next face. Keep the stock on the same reference face and extend the line around the corner. The blade now hangs over the second face. Make sure the stock hasn’t lifted — you should feel it flat against the timber. Draw that face. The corner where the two lines meet should be a crisp 90-degree corner. If there’s a step, stop and re-check.
3. For the third face (opposite the reference), you can’t ride the reference face directly because it’s on the other side. Here you have two good options: a) Keep the stock on the same reference face but use a longer framing square that reaches all the way across, or b) flip the post over and use the opposite face as a temporary reference, but only if you’ve verified those two faces are parallel with a tape measure. On a 6×6, measure face-to-face in two spots near the cut; if it’s 5-1/2″ both places, you’re parallel enough. On rough timber, option (a) with a longer square is safer.
4. Close the loop on the fourth face. This last line is your truth test. It should meet your starting line at exactly the same height. If it’s off by 1/16″ or more, don’t average it out. Erase and re-mark. A 1/16″ mismatch over a 5-1/2″ post equals about 0.65 degrees of error, which will show as a gap under a post base bracket.
Once all four lines connect, go over them with a marking knife or deepen the pencil line. Then make a small diagonal hatch or “waste” mark on the offcut side so you cut on the correct side of the line. On large timber, sawing on the wrong side wastes a $50 post.
Checking Square Before You Cut
Don’t trust a single square without checking it first. Test any square on a known straight edge: mark a line, flip the square over, and mark again from the same edge. If the two lines diverge, the square is out. Many imported combination squares are 0.5 to 1 degree out of the factory. Adjustable combination squares can be tapped true with a punch; fixed speed squares cannot.
For the post itself, check diagonals after marking. On a 5-1/2″ square post, the diagonal between corners of your cut line should be identical when measured with a tape from reference-corner to opposite-corner on the last face. If not, one line is high.
The wrap test is the most reliable hands-on check: cut a 3″ wide strip from a grocery paper bag or use a 12″ wide roll of brown builder’s paper. Wrap it tightly around the post, align the overlapping edges perfectly, and crease it. That edge is as close to square as the paper is straight. Slide it to your pencil line — it should sit exactly on the line all the way around. If the paper edge and your line separate on any face, your line is off.
Mistakes That Ruin the Cut
The most common failure is lifting the stock. If you press only the tip of the stock against the post, the blade droops. Keep full contact along the length of the stock, and keep pressure toward the post, not down toward the ground.
The second is marking from different faces. Each face you reference adds the post’s own error. Pick one reference and stick to it for at least three of the four lines.
The third is a dull pencil. A blunt carpenter’s pencil held at an angle will ride away from the square’s blade by 1/32″ on each face. Sharpen to a chisel point and hold it vertical against the blade, or use a knife. On pressure-treated timber, graphite also smears — a knife line stays put.
Finally, don’t mark in direct sun if the post is dark. Heat makes treated timber move, and glare hides whether your lines meet. Roll the post into shade for layout, then move it for cutting.
What to Buy If You’re Starting Out
If you cut large posts a few times a year, you don’t need a Starrett. A decent 12-inch combination square that you verify for square, plus a sharp marking knife or a mechanical pencil, will beat an expensive square you never check. If you build with 8×8 or larger beams, or you cut a lot of pergola tails and shoulders where every cut has to match, the extra reach of a heavy-gauge framing square pays for itself in fewer re-marks and tighter joints.
Whichever you own, spend five minutes verifying it and cleaning up the reference face before you mark. That prep is what makes four lines meet in one plane — and makes the cut that follows actually square.