How to Build a Block Arched Window

Check local building codes prior to beginning construction. Often, municipalities regulate wall height, footing size, materials that can be used and other factors.

Plan the size of the window and transfer this measurement to two sections of 3/4-inch plywood to create sides for a form. Determine the length of the radius that the window arch will have, make a length of string this size and use this piece of string and a pencil to make a circle or two half-circles on 3/4-inch plywood.

Cut the plywood circle or half-circles out of the plywood sheet using a jigsaw. Draw a line across the center of the circle and use a jigsaw or circular saw to cut the circle in half.

Measure and cut several lengths of 2-by-4s that will brace the plywood half-circles. Calculate the length for the 2-by-4s by subtracting the width of the plywood from the width of the desired arch, typically the width of the wall.

Build the body of the form. Attach the two half-circles to each other by placing lengths of 2-by-4s every several inches between the half-circles along their curved side and attach each 2-by-4 to the plywood using two 2-inch wallboard screws on each side.

Measure, cut and secure a section of 1/4-inch plywood to the arched top of the form, bending it gently to fit and attaching it to the 2-by-4s using 1-inch wallboard screws. If the plywood is not sufficiently flexible, dampen it first.

Plan the positions of the blocks as they will be mortared with the form flat on the ground, arch facing up. This positioning step is very important if the blocks are not uniformly sized. Place one or more shims between each block around the form with the narrow, 1/4-inch end of the shim on the interior against the plywood arch. Use chalk to write a number or letter on each block and a corresponding number on the plywood form where it should be positioned.

Set the form on top of the existing wall where the window will be located. If the form cannot rest securely on the wall, build impromptu scaffolding out of 2-by-4s or other boards that spans the wall and rests on the ground.

Prepare a batch of type M mortar, mixing it so that it has a fairly thick consistency.

Lay the first block. Place mortar for this first block on top of the wall, creating a mortar bed about 1/2 inch thick and wiggling the block slightly as it is set.

Lay the lowest block on the opposite side in the same manner that the first block was set. As the arch is built up, alternate placing blocks between sides so that the weight of the blocks is balanced on the form.

Place blocks and mortar to almost entirely complete the arch. Build up an unevenly thick mortar bed on each block to accommodate the next block. The mortar should be only about 1/4 inch thick on the inside but thicker on the outside. If mortar oozes out when a block is placed on top of fit, place a wooden shim in the space to provide temporary support.

Install the keystone, the top stone at the center of the arch. Make sure there is adequate mortar on either side of the keystone.

Smooth all of the visible mortar joints. If temporary shims were inserted earlier, make sure that they are removed and the voids filled in with mortar.

Mist the mortared blocks regularly for about a week to encourage proper curing. After about a week, the form can be removed.

Remove the form. Push it out, pounding lightly, or somewhat dismantle the form, taking the plywood sides off so it is easier to pull out.

Clean up the mortared joints on the inside of the arch. Remove any excess mortar, then mist the joints lightly and tuck-point joints using a stiff mortar so they are all equally filled. Tool the mortar with a jointing tool.

Mist the mortared block arch regularly for another five days.