Guide
How to cut a bevel with a circular saw (and which angle to actually set)
A bevel is a cut where the blade is tilted, so the edge is not square through the thickness of the board. On a sloping-ceiling unit you need one wherever a piece has to meet another at the roof angle rather than at a right angle. The cut itself is easy. What catches people out is the number: the angle you measured on your roof and the angle you set on your saw are usually not the same angle, and the difference is what turns a board into an offcut.
What the bevel scale is actually measuring
Your saw’s bevel scale reads 0 degrees when the blade is perpendicular to the base plate, which is a normal square cut. The scale therefore measures how far the blade has been tilted away from square.
Roof pitch is measured the other way, from the horizontal. A 30 degree roof rises 30 degrees above level.
These two conventions meet at different faces, and that is the whole problem. Setting your saw to the number you measured on the ceiling is right in some cases and wrong in others, and the wrong ones are wrong by 90 minus the angle, which is a large mistake, not a small one.
The rule, and the question to ask first
Before setting anything, ask: which face of this piece is the cut measured from? That is the reference face, and it decides the number.
If the cut has to finish plumb, that is truly vertical when the piece is installed on the slope, set the saw to the roof angle itself. A top panel lying in the sloping plane whose ends must meet the vertical inner faces of the sides is this case.
If the cut has to sit against the slope, meaning the cut face lies flat on the sloping surface, that is also the roof angle, but measured from a different face of the piece. A divider whose top edge has to seat against the panel above is this case, and so is a cleat ripped to lie flat under the slope.
If the cut has to finish level, that is horizontal when installed, set the saw to 90 minus the roof angle. A shelf end that has to finish horizontal under a sloping ceiling is this case, and it is the one that most often lands outside what a saw can do.
Write the reference face on the piece in pencil before you cut. It costs three seconds and it is the single most effective guard against the flip.
Setting the angle on the saw
Unplug the saw, or take the battery out. You are about to put your hands near the blade.
Loosen the bevel lever or knob, tilt the base plate, line the pointer up with the mark on the scale, and retighten firmly. Then check it, because these scales are coarse and are often out by a degree or two. A small square or a digital angle gauge held against the blade and the base plate takes a moment and is worth doing every time you change the setting.
Lock the depth setting as well. Depth and bevel are separate adjustments and it is easy to nudge one while setting the other.
Cut a test piece from your offcut first, offer it up, and adjust before you cut anything you need.
The depth trap
Tilting the blade reduces how deep the saw can cut, by roughly 25 to 35 percent at 45 degrees. A saw that cuts 65 mm square may only manage about 45 mm at full bevel.
For 18 mm sheet material this is rarely a limit. It matters when you are bevelling thicker stock or a cleat on edge. Check before you start: set the bevel, retract the guard by its lever, and measure the blade projection below the shoe. If it will not reach through, cut from both faces rather than forcing a single pass.
Safety, and why a bevel is less forgiving
More of the blade is engaged in the material during a bevel cut, so there is more to grab and kickback is more likely.
Clamp the work. Both hands belong on the saw, not one hand holding the board.
Let the offcut fall free. A piece that sags and pinches the blade is the classic kickback trigger, and it is worse on a bevel because the cut is longer through the material.
Never wedge or tie the lower guard open. It is tempting on an angled cut because the guard can catch on the edge as you enter. The answer is to start the cut with the guard lifted by its lever and release it once the blade is in the material, not to disable it.
Eye and ear protection, sleeves not loose, and cut where you can stand square to the work rather than reaching across it.
When your saw will not tilt far enough
Most circular saws stop at 45 degrees, some reach 56. A level cut under a shallow roof needs 90 minus the pitch, which can easily exceed that.
Kamba assumes the same 45 degrees. When a setting passes it the plan says so instead of printing a number you cannot set, and on the shallower roofs most houses have it is the shelf-end cut that goes out of reach first.
If the setting is out of range, you have three options. Cut it square, if the piece tolerates it. Cut it with a jigsaw, which tilts further and is more forgiving in thin stock, at the cost of a less crisp edge. Or cut it by hand with a guide block set to the angle.
Which of those is acceptable depends on the piece, and that is the subject of the next section.
Which bevels you can skip, and which you cannot
This is worth being precise about, because "the bevel is optional" is true of some pieces and not others. The four below are how Kamba classifies them, and the plan you print says the same thing.
Cleats: optional. A cleat is a support strip screwed to a face. Ripping its top edge to the roof angle gives a top panel a continuous flat landing, but a square cleat set back a few millimetres from the slope carries the same load. If the bevel is awkward, set the cleat back instead.
Shelf ends: optional. A shelf’s span, and therefore its rated load, is measured to its top face where it meets the supports. A bevel on the end closes the wedge below that face. Leaving it square loses no strength; it leaves a gap you can see if you look for it. This is the one cut where the square fallback is offered outright, which is just as well, because it is also the one that most often falls outside a saw’s range.
Divider tops: not optional. A divider standing under a sloping panel has a different height on each of its two faces, and the difference is the board thickness multiplied by the tangent of the roof angle. On 18 mm board under a 30 degree roof that is 10 mm. Cut square at the taller dimension, the piece is 10 mm too tall on one face and will not go in. Cut square at the shorter one and it does not reach the panel at all. This edge has to be angled.
One exception, and it is a whole orientation rather than an edge case: where the roof falls across the depth of the unit rather than its width, the dividers run along the slope like the side panels. They are then marked-line cuts with no tilt at all, and this paragraph does not apply to them.
Top panel ends: not optional either. The top panel drops in between the two sides, and its ends have to finish plumb for it to get there. Cut square, its two faces end in vertical planes a little apart, so the panel measures wider across than the gap it has to enter and it jams on the way in. Nothing about strength: it simply does not go in. Kamba files this cut with the divider’s, not with the cleats’.
The general principle, and the one Kamba applies: a bevel that creates a bearing surface, or that lets a piece fit between two others, is structural. A bevel that only closes a visible gap is a finish. Read what the piece is doing before deciding to skip it.
Most of the angled cuts on this unit are not bevels at all. Here is that one.
In Kamba
An under-slope plan gives one roof angle, measured once, and derives every saw setting from it. Each cut in the list names the setting and the face it is measured from, so you never have to work out the flip yourself. Where a setting falls outside what a circular saw can reach, the plan says so and offers the alternative rather than printing a number you cannot use.
See the unit these cuts belong to
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