How to Lay CMU Around a Window

Preparation

• 1

  Using a pencil and paper, create a drawing of the CMU structure you want to build. Include the exact placement and dimensions of windows you intend to build.

• 2

  Using your first drawing as a template, draw the outline of each CMU you plan on using for your structure, based on the dimensions of your structure and the size of CMU you're using. CMUs rarely, if ever, fit perfectly around a window or door; you must cut them to size.

• 3

  Put on your work gloves, work boots and safety glasses.

• 4

  Based on your drawing created in Step 2, cut concrete blocks with your circular power saw to fit around your window. For instance, if you want a window 20 inches from one end of the wall made of 8-inch long CMUs, cut 2 inches off of two blocks so you have one 8-inch block and two 6-inch blocks, for a total length of 20 inches. Avoid using short blocks, such as two 8-inch CMUs and one 4-inch CMU.

• 5

  Cut your 2-by-4 planks of wood with your circular saw to fit the dimensions of the window frame specified by your drawing. Ensure that one fits the dimensions of the bottom of the window, and two fit the dimensions of the sides of the window.

• 6

  Place a CMU on top of the board you plan on using as the bottom framing element of your window frame. Use your pencil to sketch the placement of the openings in the CMU onto the board. Repeat this process across the entire board.

• 7

  Use your drill to affix your 3-inch long deck screws to the 2-by-4 you're using as the bottom framing element of your wall. Drive the screws no more than an inch into the wood, and place each one within one of the areas sketched in Step 6.

Laying CMU

• 8

  Begin laying your CMUs. Start at the corners of the structure and work toward the middle. Smooth mortar across the top of each layer of CMUs to bind them to the subsequent layer.

• 9

  Using your drawing from Section 1 as a guide, build a standard CMU wall until you reach the height of the bottom of your window. Use your measuring tape to check the height with that specified on your drawing.

• 10

  Use your measuring tape to determine the width of the window frame. Using your pencil, mark the edges of the frame on the last row of CMU you completed.

• 11

  Use the blocks you cut in Step 1, Section 4 to create sections of wall to frame the window. Build from the left and from the right toward the window opening, stopping exactly where you marked the edge of the window frame in Step 3. Apply mortar in thin, flat layers and even quantities. Use your level.

• 12

  Stop building these partial walls upon reaching the height of the top of the window. Measure each partial wall to ensure that it meets the dimensions of your drawing and that the walls are precisely the same height.

Finishing

• 13

  Apply a layer of mortar, using your trowel, to the top of the CMUs forming the bottom of the open area you have in your wall for your window placement. Use your level to ensure flatness and straightness of the mortar.

• 14

  Use your scissors to cut your plastic sheet so it fits perfectly over the mortar you spread in Section 3, Step 1. The plastic prevents moisture from penetrating concrete and damaging wood.

• 15

  Place the wood board you prepared in Section 1, Steps 6 and 7 on top of the plastic sheet. Push the board down so the exposed screws penetrate the plastic sheet and the mortar.

• 16

  Install your headers at the top of the window frame. The number and type of headers you use depends upon your local building code. The residential building code of Anne Arundel County, Maryland, for instance, requires two headers for each window, though the type of header used depends upon the size of the window. The header sits above the frame and will integrate with subsequent CMU rows built over the window. Attach the header to CMUs with concrete screws and your drill.

• 17

  Apply weatherproof caulk around the sides and top of the wood in the window frame to protect against moisture penetration.

• 18

  Continue building your CMU wall above the level of the window frame as you did in Section 2, Step 1.