When you’re sheathing a structure, it’s important to create crisp cut lines in the OSB walls for your rough openings. A clean, square rough opening guarantees a good solid surface for attaching window and door flanges, as well as trim. But it’s difficult and very time-consuming to accurately pre-cut the rough openings in the sheathing ahead of time.

Most framers just place sheathing right over the rough openings, and then cut the excess sheathing out of the way. That’s the best approach, but here’s a simple tip for obtaining accurate rough openings in your sheathing every time—and all you need is a circular saw and reciprocating saw, four screws, a screw gun, and a chalk line.

1. Sheathe the structure. Leave a 1/8-inch gap along all the edges of the OSB to accommodate natural changes in the panel’s dimensions, as per APA recommendations.

2. Working from the inside of the structure, drive four screws through the four corners of the frame of the rough opening. These screws must protrude entirely through the OSB with room to spare, so use a 2-inch screw just to be safe. These four screws poking through the other side will define a shape that has the same dimensions as the rough opening you want to create. Be careful to make sure the screws are exactly in the corner of the rough opening. To do that, use an extended bit on your drill so the screws are not angled, as that would change the position of the cut lines.

3. Move to the outside of the wall and string a chalk line between the four protruding screws. Snap the chalk line by pulling the string away square to the wall, not at an angle.

4. Use a circ saw to plunge-cut your lines to within 3 or 4 inches of the corners. You don’t want to run your circular saw blade right into the corner, because at that point the saw blade would cut into the framing beneath the sheathing. Once the circular saw cuts are done, finish the cuts with a reciprocating saw loaded with a long wood blade.

Upon completion, you’ll have a rough opening cut in the OSB that’s exactly the same dimension as the frame, without having to guess where to make any of the cuts.