Things You'll Need
Clamps or tape
Sandpaper or power sander
If you don’t have a jig saw, you can make curved cuts with a reciprocating saw, or use a circular saw to make short straight cuts that approximate the curved line. You can clean up rough lines quickly with a router (always be careful when free-handing) or a belt sander. To give the shelf the same shape as the wall on both edges—so it has the same depth along its entire length—use the template to mark both long edges of the shelf, making sure the marked lines are spaced consistently apart.
Mounting a shelf flush to a curved wall (or any wall that just isn't flat enough for a nice fit) calls for a traditional carpentry technique known as scribing. Scribing is a simple trick of using the surface itself—the curved wall, in this case—to mark the cutting line onto a template or the workpiece itself (the shelf). To scribe a line, you hold the template in place against the surface, then drag a compass (or other marking device) along the surface, letting the pencil mark the cutting line onto the template as you go. This transfers all of the contours and irregularities of the surface directly to the template and is much more accurate than conventional measuring and marking.
Scribing a Shelf to a Curved Wall
Cut a piece of butcher paper or thin cardboard a little longer than the shelf dimension. Make two marks along one edge of the paper to represent the ends of the finished shelf.
Lay the paper on the floor directly below the planned location of the shelf. With a convex wall, make sure the paper is equidistant from the wall at the shelf-end markings.
Set a compass (the kind you used in elementary school to draw circles) to span a little beyond the widest gap between the paper's edge and the wall. Keeping the compass perpendicular to the wall at all times, move the pointed end of the compass along the wall while the pencil end marks the paper. If you don't have a compass you can use a pencil taped to a block of wood.
Cut the paper along the scribed line with scissors. Check the fit against the wall and make any fine adjustments to the paper as needed for a tight fit.
Clamp or tape the template to the shelf board, and trace along its edge to mark a cutting line onto the board. Cut the board using a jig saw. Test the fit, and refine and smooth the cut as needed with sandpaper. Install the shelf as desired.
Philip Schmidt is author of Install Your Own Solar Panels, The Complete Guide to Treehouses, and 18 other home-related how-to books. A former carpenter, he has been a full-time writer and editor for over two decades, teaching DIYers about houses and everything we do with them.