How to Build an Arch in Block Walls

Plan the size of the arch and transfer this measurement to one or two pieces of 3/4-inch plywood to create the sides for the form. Determine the radius of the planned arch then cut a length of string to this size. Use the string and a carpenter's pencil to draw two half-circles or one full circle that can be halved later.

Cut the circle or half-circles out of the plywood sheet with a reciprocating saw. If you made a single circle, draw a line across the center of the circle and cut along this line to create two half-circles.

Measure the width of the wall or otherwise determine the width of the planned arch. Then cut several lengths of 2-by-4 board to the size of the arch width minus the thickness of the plywood. These 2-by-4 boards will make up the bracing within the form.

Build the body of the form for the arch. Use the cut 2-by-4 sections to attach the two half-circles to each other. Secure each length of board to the plywood with two 2-inch wallboard screws on each side. Place the 2-by-4 boards every several inches along the top of the form, with the thin side facing out and flush with the edge of the plywood.

Measure, cut out and attach a section of 1/4-inch plywood to the arched top of the form. Bend the plywood gently to fit and attach it to the 2-by-4 boards with 1-inch wallboard screws. If necessary, dampen the plywood prior to bending it to make it more flexible.

Test-fit the blocks around the form as they will be mortared. Keep the form on the ground with the arch up. Use wooden or composite shims against the form to separate the blocks where the mortar will eventually be placed. This step is especially important if the block size is not uniform. Mark the position of each block on the form. Use chalk to number each block and write a corresponding number on the form where it will be placed.

Set the form on top of the existing wall at the correct height once the wall has been adequately completed. For an archway or other situation where the form cannot rest securely on the wall, build impromptu scaffolding out of 2-by-4 boards that will adequately support the form, blocks and mortar.

Prepare the mortar. Typically, type M mortar is the best type to use for this application. Mix the mortar to a fairly thick consistency. Lay the first block in place on top of the existing wall and against the form. Create a mortar bed about 1/2-inch thick for this block then wiggle it slightly into place to force air bubbles out and check for evenness with a level before continuing.

Set the first block on the opposite side of the form in the same way that the other block was laid. Use this alternating method as the arch is constructed; place one block at a time on each side of the arch so that weight is placed evenly on the form.

Place mortar and blocks to nearly complete the arch. Place an uneven layer of mortar onto the top of each block to create a bed for the next block. The mortar should be only about 1/4-inch thick against the form and thicker on the outside. If mortar oozes out of position when a block is placed on top of it, set a shim in the space to provide temporary support.

Set the keystone -- the block at the top of the arch -- in place. Build up mortar on each side of the block before placing it in the opening. Then smooth out all of the visible mortar joints. Remove any temporary shims about two hours after they were placed once the mortar has hardened. Fill in the voids left by the shims with mortar.

Mist the mortared block work regularly for about a week to facilitate proper curing. If rain or especially dry or hot weather is in the forecast, protect the arch with plastic sheeting.

Remove the form after the mortar has cured for a week. Remove the scaffolding and push out the form by pounding it gently or partly dismantle the form by taking off the plywood sides so it is easier to pull out.

Clean up the underside of the arch and fill in mortared joints as needed. Remove any excess mortar. Mist any joints that require additional mortar and tuck-point a stiff mortar into these joints so that they appear uniform.

Mist the underside of the arch for about a week to allow the mortared joints to cure properly.