To retrofit a slider or French door, you have to knock a 6- or 8-foot hole in an exterior wall -- probably a structural, weight-bearing wall. To evaluate a suitable header, you must consider both strength and deflection. Strength is obvious: The beam must have enough meat to accept the stresses on the material. Out of experience, I arbitrarily limit the allowable sag to 3/32 inch. Greater deflection may affect door operation later on, while any less requires an excessively heavy beam.
For header material, I typically use steel or V24 glulams. (LVL or Parallam has about the same properties as a glulam for beams this size.) My suggestion for steel beams in residential construction is rectangular steel tubes with a 1/4-inch wall thickness, rather than angles or W-sections. Tubes are not too heavy to handle, and it's easy to attach 1-by blocking with powder-actuated fasteners. Plus, the tubes are not too deep to cause a headroom problem and are narrow enough to fit into frame construction.
You may want to keep the original window header in place. In that case, you could use a steel angle under the header, extended on either side to the full width of the new doorway. It will still take some chopping to fit in the angle, and because angles are extremely inefficient as structural beams, you'll need a fairly hefty angle (see the table by clicking on the link below). Still, in some cases it might solve your problem more easily than a tube or a glulam.
--Harris Hyman is a civil engineer in Portland, Ore. This article is adapted from an October 2002 article in the JOURNAL OF LIGHT CONSTRUCTION. To subscribe, call (800) 375-5981 or visit jlconline.com.
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