What is technique for framing arch top windows?

Answer 1

For arch top windows you need to figure the peak(top) and spring point(height at which the arch starts on the sides. If you have the window where you can scribe the contour on the plywood that will be much easier. Typically the window flange will be slightly smaller on the inside of the house, which we will account for a bit later. Usually arch top windows will be a half round. To draw the arch you will need to mark out the plywood to fit inside the window frame. after you have done this, mark the canter of the width on top and bottom if the plywood. Use the measurement for the outside of the window. secure the plywood to the ground with some concrete nails or what have you. Snap a line with a chalk line starting at your center mark on the top of the plywood and extending several feet past the bottom of the plywood making sure that it lines up with the center mark on the bottom of the board. From the peak of the arch, extend a tape measure along the line you have just snapped. Take another tape measure and extend it from the spring point mark on one of the sides of the plywood. The point at which the tape measures intersect at the same number will be your pivot. Mark and tack a nail at this point or have a helper hold the end of your tape measure or chalk line on the pivot point. Take a pencil and start at one spring point marking while holding the pencil against your tape measure on that same point as you scribe the half circle. Now cut it out using a jigsaw or walking your circular saw around the contour if you have the skill. Scribe another arch by either repeating the process or scribing from your cutout. Cut 2x4 blocks at the width of the framed wall minus 1 1/8 inches to account for the plywood assuming you are using 1/2 inch of course. Cut boards the length of the top of the plywood and the length of the sides of the plywood minus roughly 6 inches to allow space for the blocks on the bottom. Rip these deadwood boards to a width that matches the length of your blocks. Using a staple gun, staple the plywood to the deadwood starting with the top and then the sides. Place your blocks under the arch portion and staple them with the contour leaving 1/8 inch gap to account for expansion cause by fluxuation of heat and humidity. If the window is already installed, set the arch in place and use a framing nailer (3" nails) to nail the deadwood to the window frame while making sure that the plywood side is flush with the other framing. Set the other plywood side in place and staple it to the blocks. You should have a gap on the inside flange that will need to be filled to meet code. Rip down a piece of 1/4" birch plywood 1/2" wider than the distance from the outside of framing to the window flange. Bend the plywood into place and staple it to the blocks. Cut holes in plywood on the inside of the arch that you can fit your hand into so that it can be insulated. Wrap the exterior plywood with tyvek or a similar vapor barrier. Using Great stuff or a similar spray foam specifically for windows and door, seal any gaps between the window and framing. You should now be finished with the framing stage.