Jamb Kits Save Time
For most applications, you'll find that purchasing a jamb kit is a real timesaver. You'll free yourself from the tedious work of selecting the stock, cutting it to width and length, and machining the rabbet or dado at the top inner edge of each leg to receive the head jamb. None of the individual steps is particularly difficult, but when you take a hard look at the price of clear softwood lumber, you may discover that making your own jambs literally doesn't pay.
The components of a standard 2x4 interior wall covered with 1/2-inch drywall add up to a thickness of 4-1/2 inches. The jamb kit is minutely oversized at 4-9/16 inches wide. If the walls are both covered with 5/8-inch drywall, the computed thickness matches the jamb stock width of 4-3/4 inches.
If you're going to paint the woodwork, you can save a considerable amount of money by choosing paint-grade jambs. The finger-jointed stock may look a little weird while you're installing it, but the patchwork appearance disappears under the primer and paint. Stain-grade jambs may be defect-free solid lumber or veneers of top-quality wood adhered to a plywood backing. A few purists may cringe at the concept of veneered jambs, but they actually have several advantages over solid stock. Plywood construction has high dimensional stability, meaning that it resists changes in width and shape (such as cupping) that can plague solid lumber. Price is another distinct advantage. The oak veneered stain-grade jamb kit in the photo below costs a fraction of the solid hardwood you'd need to purchase, and you would still have to do all of the machining steps yourself.Rabbet or dado?
Some jamb kits feature a rabbet at the top of the jamb legs; others will have a dado, which is a groove that's cut across the grain. In a jamb made from solid wood, a dado does a better job of restraining potential cupping of the head jamb. If you purchase the rabbeted style, you may want to add some blocking or shims between the top end of the jamb leg and the header to counteract cupping forces.Tapered jamb edges
You'll notice that some jamb edges taper a few degrees, flaring inward toward the stud to which it's nailed. This helps ensure that the edge of the casing will fit tightly along the jamb. If you make your own jambs, you can create a similar angle by tilting the tablesaw blade about 5 degrees for the rip cuts.Pro Tip: Use setting-type joint compound for strength
You'll find joint compound packaged two ways: premixed in a bucket or the setting variety in a bag. The setting type is stronger, but the premixed version is easier to apply and sand. After the premixed type cures, smooth it with a sponge, which is faster and creates less mess than abrasive paper.
The other formula is setting-type joint compound, which comes as a powder that must be mixed with water. The bag will be labeled 90, 45, or 20. The numbers indicate how many minutes it takes for the compound to set up. The safest approach is to use the 90-minute mix. If you want it to set faster, mix it with warm water.
Once set, setting-type compound cannot be softened with water. It is harder to sand flat than premixed is, but it is much stronger. Professional drywall finishers often use setting-type for the first coat of mud, especially at outside corners, which are vulnerable to damage. It is also useful when patching drywall, as it helps reinforce the repair. Because this compound sets up so fast, you may be able to apply two or more coats in a day.
When you mix setting-type joint compound, mix only what you can use in the time indicated on the label. Otherwise the compound will set before you can apply it. Be sure to use clean water and a clean bucket. Pouring water into the fine powder raises a lot of dust, so put the proper amount of water in the bucket first, then add the powder, stirring as you go until you reach a consistency (similar to oatmeal) that spreads easily but sticks to the drywall without running.
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