Guide
How to follow a cut line: marking and cutting a sloped panel
Most angled cuts in a sloping-ceiling unit are not bevels. The saw blade stays square to its base, and what makes the cut angled is the line you drew on the panel. This is good news, because a marked line is far more forgiving than a tilted blade: you can see it, check it, and correct it before the saw touches the wood. The two skills involved are marking the line in the right place and then cutting a long line straight. Neither is hard, and both go wrong in predictable ways.
When the blade stays square
A circular saw reads 0 degrees as a square cut, with the blade perpendicular to the base. That is the setting for this kind of cut. The angle lives in the line, not in the tool.
You get an angled edge simply by drawing a line across the face of the panel and following it. A side panel for a sloping ceiling is a trapezoid: tall at one end, short at the other, with a straight line between them. Nothing about that requires the blade to tilt.
Set against this, a bevel is when the blade itself is tilted so the cut edge is not square through the thickness of the board. That is a different operation, needed on different pieces, and it has its own guide.
The practical test: if your cut list gives you two lengths for the same edge, one for each face of the board, the cut is a bevel. If it gives you the two ends of one line on the same face, it is a marked line and your blade stays at zero.
Marking the line from measurements
If you have the two end measurements, this is quick. Measure and mark the short end, measure and mark the tall end, then join them.
Use a chalk line or a long straightedge for the join. A metre rule stepped along a two metre panel accumulates error at every reposition, and the line ends up slightly curved in a way you will not notice until the panel is offered up.
Mark on the face you will be looking at while cutting, and mark clearly enough to see through sawdust. A fine pencil line disappears under the shoe of the saw.
Scribing, when the surface is not straight
Measurements assume the ceiling is a plane. Real ceilings wave, and plaster builds up in corners. Scribing copies the actual surface onto the panel instead of assuming it.
Offer the panel up as close to its final position as you can get it, propped or packed so it stands still and is level. It does not need to fit yet.
Set a compass, or a purpose-made scriber, to the width of the biggest gap between the panel edge and the surface you are following. That setting does not change again.
Run the compass along the surface with the point riding the ceiling and the pencil marking the panel. Keep the compass at a constant angle for the whole pass. If you rotate it as you go, the offset changes and the line no longer matches the surface.
Cut to the line, then offer it up again. Small high spots are quicker to plane or sand off than to re-cut.
Cutting a long line straight
Freehand, a circular saw wanders. Over 30 cm you will not see it. Over 180 cm you will.
Use a straightedge clamped to the panel and run the saw shoe against it. Any straight rigid thing works: a purpose-made guide rail, a spirit level, or a strip of sheet material with a factory edge, which is straighter than anything you will cut yourself.
The step people skip is the offset. The blade is not at the edge of the shoe, it sits somewhere between about 38 and 55 mm in, depending on the saw. Measure that distance on your own saw once, from the side of the shoe to the side of the blade, and write it on the tool. Then set the straightedge that distance back from your cut line, at both ends, and clamp it. Setting it back at one end only is how a guided cut still comes out tapered.
Cut on the waste side of the line. The blade removes about 3 mm, and which side that 3 mm comes from decides whether your piece is right or short.
Press the front of the shoe against the straightedge as you start, and keep the back of the shoe against it as you finish. The two ends of a cut are where the saw is most likely to drift, because only part of the shoe is supported.
Supporting the panel
Support the panel on both sides of the cut line, as close to the blade as is practical, and let the offcut fall free.
If the offcut is unsupported it sags as the cut opens, pinching the blade. A pinched blade is the main cause of kickback, where the saw climbs out of the cut and comes back at you. If the offcut is supported but the piece is not, the same thing happens in reverse.
Rigid foam insulation on the floor is the cheapest good answer: set the panel on it and cut just deep enough to score the foam. The whole panel stays supported everywhere and nothing pinches.
Where to set that cut up when you have no workshop
Which face goes up
A circular saw blade cuts upwards through the material. The teeth exit on the top face, so that is the face that splinters.
Cut with the good face down. If both faces will show, score the line first with a knife to sever the surface fibres, and use a blade with more teeth. A 40 to 60 tooth blade leaves a noticeably cleaner edge in sheet material than the 24 tooth blade most saws ship with.
When to reach for something else
A jigsaw is more forgiving on a curved or scribed line and in thin stock, but it is harder to keep square through the thickness and slower over a long straight run. Use it where the line is not straight.
A hand saw with a guide is slower but it is the safest option for a single cut, and it is not as slow as people expect on 18 mm board. For one panel, it is a perfectly reasonable choice.
Some pieces do need the blade tilted. Here is which, and how to set it.
In Kamba
For an under-slope unit, Kamba prints the two edge lengths of each angled piece rather than a nominal size, so you can mark the line directly from the cut list without transferring an angle.
Which pieces are cut this way depends on which way your roof runs. Where the slope falls across the depth of the unit, both side panels and any divider are marked-line cuts. Where it falls across the width, it is the back panel. Each of them is labelled with its reference face, and the panel shapes page draws the trapezoid so you can see which way round it goes before you mark it.
See the unit these panels belong to
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