Traditionally, adobe homes used wood lintels. However, today the building codes in your area may not allow wood lintels, specifying a concrete or steel beam instead. The code has concern that wood lintels may decay over time due to weather or bug infestation.
As a result, my solution is to provide for both strength and aesthetic by casting in place a concrete beam and later bolting a mesquite wood face plate. See photo example of the finished wood facing and trim in an adobe out-building I constructed a few years back (below).
In the picture above, you can see I scuplted the 'cob' walls on each side of the window so they can be filled with concete and span the window opening. I used two pieces of vertical rebar that were placed when the stone stem wall was being constructed, to secure a horizontal piece of rebar as reinforcent of the lintel. The lintel is 4"-high and 5"-wide and spans the cob wall about 12" in each side of the window. Note that I made cob walls at the base (of the form), about 3/4" higher than the top of the window frame. That way, the poured concrete in the form will not be directly resting on the window frame, thus defeating the purpose of the support in the first place. To account for the 3/4" gap above the wood frame, I cut a strip of insulating foam board (see photo above and below). The concrete will rest on the foam instead.
In the photo to left, observe one of the two anchor bolts I added. The threadede end extends through the wood form. These bolts will 'anchor' the mesquite wood face plate that will be added later as a finishing touch along with the trim.
The photo below shows the poured concrete lintel with the wood form removed. Notice the two anchor bolts and foam between the lintel and the wood window frame. The rocks embedded in the concrete on top of the lintel will give the 'cob' that will be placed on top of the lintel something to grab onto.
You may have noticed that the width of the lintel does not cover the approximate 2-foot width of the massive cob wall; only the width of the window frame. I need to address this and will present a solution in a future blog entry.
Photo at right show an example of the mesquite wood face-plate and trim on another finished adobe I completed (click on photo to enlarge for a better view).