Wall Openings....

With concrete block home construction, window and door opening accuracy is much more important that in wood frame construction. With wood you can cut out a little if necessary, or move a wall stud when required. A surface bonded block wall can't be easily changed. You must be sure where you want that window and door opening early in the planning phase.

Update (03/10/10) Using a jig for square window/door openings.
This additional information is provided to make your building experience more successful. As described in Chapter 14 in the eBook, you want the window tops to be nearly the same height as the door height for asthetics reasons. Notice the header and window sill locations in the drawing below. This new section will explain how to get square window/door wall openings and also recommends a specific sequence for pouring the window sill and window header.
We would suggest that you build a jig which exactly represents the installed window/door rough opening width and height plus the framing material. These dimensions must also include any wood framing dimensions that you desire around the window/door (possibly to match the 2-1/2 block opening). The preferred solution is to use 2x4 framing of the jig such that the inside of the 2x4 jig matches the window/door rough opening requirements. Then brace the jig with plywood to keep it square. Cut lift holes in the plywood to faciliate jig handling.

We now recommend that you stack the wall rows until you reach the block row which will have the window sill (as shown below, that is row three). Now place at the correct wall location and shim the window jig so it is level on that row of blocks and vertically plumb and brace the jig to the ground. Now stack the blocks against the jig. Carefully place the first block on each side of the jig so the jig is not moved horizontally. The closing blocks for each row can compensate for any window opening variance. Remember that one side of each window/door block row will most likely have a half block.With a door opening, just place the door jig on the floor between the door-opening mortared blocks as they are mortared.

When the wall is completed, and the wall cells are poured and cured, pour the window sill before setting the window header forms. Don't forget to place a concrete stopper (Chapter 15 in the eBook) under block row three in this example. The window sill height should place the top of the windows nearly in line with the top of the doors. Now place the window jig on this overnight-cured window sill, and build the window header forms for exact height. Place window header board props on the poured window sill (Chapter 14 of the eBook), so that the jig fits into the window/door wall opening, and pour the header. This will result in a window/door opening that is square, plumb, and makes later window/door installation a breeze. Cut a couple of large holes in the jig bracing plywood to allow for easy handling of the jig. The cheapest jig can be made from OSB (just keep it dry), cut to be exactly square and of the desired wall rough opening dimensions.

Window Rebar Details
Just to ensure a good understanding of the window opening requirements, the following drawing shows the reinforcement requirements in detail. The bond-beam rebar is shown as part "A". The window header bottom rebar is shown as part "B". The upside down "L" shaped window wall cell rebar (required on each side of a window opening) is shown as part "C" and must overlap the bond-beam rebar by at least 24-inches. The "L" shaped slab cell rebar is shown as part "D" and must overlap part "C" by at least 24-inches. Part "D" must also be embedded into the foundation concrete by at least 6-inches.

This diagram does not indicate whether one or two rebar are required in each location. Check your local building code for exact dimensions and requirements in your area. You will find mention in the code about the rebar being 2-1/2 inches above the bottom of the header pour. This reference is with respect to the minimum header height as defined by the empirical design tables.

Since my empirical design requirements call for a 12 inch high header, then my bottom rebar has to be 9-1/2 inches below the top of the header. Since my lower rebar is about 15 inches below the top of the header I have exceeded the strength requirements of the header span. The additional concrete below the lower rebar does not matter since it does not contribute to header strength and only serves to establish the top of the window opening. Refer to the door framing section below for door opening concepts.

Door Openings
The door openings are designed quite similar to the window openings. You simply pour a custom bond-beam/header to achieve the door height, and use the same wood framing techniques as described below. The required door threshold is the greatest difference in detail.


Window Framing
This picture shows the inside of a garage window. For a house, you would need to allow for insulation and wallboard installation before installing the window inside trim. In this picture you can see the 2x6s which frame the top and sides of the window. These pressure treated 2x6s are concrete screwed and caulked to the block window opening. For a home, use treated wood that does not cause any health concerns, or place plastic between untreated wood and the wall opening. Check with your local building code for details. Then the window is fastened to these 2x6s. The inside surface bonding cement (SBC) wraps down over the top 2x6 in this picture. This is a result of my mistakes in the building sequence for the garage. At any rate, I installed 7 windows in this garage, and only had to plane one side 2x6 to get precise window installed fit for all seven windows. I made a wooden jig which was placed upright on the wall and braced at the top for each window opening. The dry-stacked blocks were butted against this jig for perfectly square window openings. This jig certainly made all my window openings the same as well as square, since the jig was built very square.

In this same picture, you can see that the bottom window sill is also concrete. I used precast window sills in the garage because I just didn't know any better at the time. For the house, I will use poured in place window sills for a couple of reasons. In the house construction, the poured in place window sill can be height adjusted to exactly match the required window vertical wall opening. Once the wall is SBC'd, then the final window opening will look very professional and will accept the window rough framing quite easily. SBC doesn't stick to precast concrete without a special additive. By pouring concrete window sills, they can be SBC's right into the wall without resorting to a special SBC mix. You could also use narrower window framing boards like I did in the garage, if you want the window recessed into the wall. I used 2x6s in my garage to have outside recessed widows that are flush on the inside.

Rebar Supports
In the picture shown below, notice how the bottom doorway header rebars are perched upon metal stands. The camera angle and lens distorts the wall sides which are plumb. These stands ensure that concrete can fully envelope the rebar for good strength. I made these stands by bending some truss tie-down straps. These stands are then wire-tied to the rebar so they can't move during the pour. This stand is also wire-tied to the vertical cell rebar so that the header bottom rebars remain centered within the doorway header. This was done for all door and window headers. Once again, I used a cut in half full block in this picture because I ran out of half blocks. The cut in half block sticks out on the left side of the door header opening in this picture because it was cut after the center web, which makes it about an inch to long. The poured header concrete will butt up against this and fill in the differences. After SBC, you will no longer be able to see this staggered block joint in your home construction project.

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