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Q: I want to convert my garage into a family room. I'm thinking of paneling for the walls, carpet and a suspended ceiling. Is there anything I should know before I fill in the large opening where the garage door used to be?
Terminology:
A header refers to a single beam or assembly of smaller members which spans over the top of an opening. There are door headers, window headers, crawlspace headers, etc.
A jamb member refers to a single member or multiple interconnected members which form the sides (or edges) of an opening.
Where more than one member is used, the individual studs are referred to as either cripples or through-studs. The cripples do not run full height. They start at the top of the bottom plate (which may be sitting on a foundation or on top of the 1st or 2nd level floor sheathing) and stop at the bottom of the header. They provide support for the header. The through-studs do run full height. They provide lateral stability for the cripples and resist the out-of-plane forces (due to wind or seismic loads) which are received from the header.
A: Before you start, it is important to know the general framing of the original garage door opening. Of interest will be the method of connection between the door(s) and the header/jamb members. If your garage doors are mounted on rails and wheels and slide left and right, allowing access to one side of the garage at a time, you may skip to Part II. If this is not the case, I assume that your garage doors are supported in some fashion by the jamb members.
Part I - When the wind blows on the existing door, the wind forces are transferred either: #1 - through the segmented door panels into the door panel guide tracks then into the jamb members; or #2 - through the framing members into the levered spring mechanisms then into the jamb members. When you replace the door with your new opening infill, you will be installing vertical studs which will redirect that wind loading to the header and to the garage's floor slab. Chances are the existing header is not sufficiently connected to the jambs each end to transfer this new load. This can be corrected by installing metal framing angles and/or additional nails from the header to the jamb member. For double wide openings (the typical two car garage), you should consider using a 2 x 8 double top plate on your infill wall, nailed to the bottom of the existing header and connected to the jamb members each end to reduce the out-of-plane deflection. The exterior face of these 2x8's aligns with the face of the existing wall studs. The interior face will project into your remodelled space a ways, but can be concealed behind a ceiling cove or trim piece.
Part II - The garage walls are typically built on top of concrete curbs which hold the wood sill plate above grade level (to reduce the chance of dry rot and termite infestation); a good water barrier. But the area where you garage door used to be has no such barrier. You will install one in the form of a new 6" tall, 6" wide concrete curb, bonded to the existing garage slab with a structural epoxy. Please follow the manufacturer's directions regarding the cleaning of the existing concrete surface so the new curb concrete achieves a watertight bond when poured. Drill and set a few 1/2" diameter x 8" long machine bolts with the threads embedded in the garage slab about 3" to anchor your curb in place. Keep the spacing under 24" and use the same epoxy to hold them firmly in place. Wire two 3/8" diameter reinforcing bars just below the heads of the embedded bolts to give the curb strength and prevent cracking. Your new sill bolts (for the infill wall) can be held between these reinforcing bars, which helps keep them in the proper position.
Part III - Be aware that, like your driveway, your garage slab is probably cast directly on compacted native soils. This is fine for a garage, where you are not concerned about the occasional seepage of moisture up through the slab (capillary action). But this moisture can have a deleterious effect on certain vinyl tile adhesives and self-adhering tiles (causing release), can lead to mold growth in carpet pads and can encourage dry rot in subfloor plywood (which is attached directly to the slab prior to installing a wood floor). Water may also seep up through the gap between the garage slab and the existing perimeter curbs. A liquid sealant should be applied prior to installing your floor covering. Again, follow the manufacturer's directions as best you can regarding preparation and application. This may involve high-pressure detergent washing or even sandblasting in difficult cases.
Part IV - If you plan on installing windows in the old opening area, you should make provisions for the flexibility of the garage structure. For orientation purposes, we'll label the old door area the "front" wall, the wall opposite the front the "back" and the other two walls the "sides". An unattached garage is really a three wall structure. If the garage did not have a roof, the side walls would certainly fall over in a high wind. As it is, all of the wind forces blowing perpendicular to the side walls must be resisted by the rear wall...and the whole garage twists as a result. When you plan for your windows, you may wish to allow extra space around the panes to provide for this twisting motion. If your windows arrive in fabricated frames, increase the thickness of your window stops to compensate. Tell to your window supplier where his/her product is to be used and see if they have any other recommendations.
Scott McVicker, S.E.