Answers to some basic Framing questions.
We do a lot of things in construction that can be confusing if you're just learning how things work. Here are answers to some of the questions a lot of new carpenters have.
Why Is Framing Spaced 16, 24, or 19.2 Inches On-Center?
Sixteen inches seems like a strange number to choose for stud, joist, or rafter spacing. It's not an easy number to add. In fact, to save carpenters from having to add 16-inch spacings together, most tape measures highlight the numbers that fall on 16-inch centers in red. The reason for spacing framing 16 or 24 inches on-center is so that an 8-foot-long sheet of plywood or OSB sheathing will land (or "break") in the middle of a stud, joist, rafter, or truss–leaving room for the next sheet to start from the same piece of framing.
Sixteen-inch spacing is common, and most of the time it's close enough to provide strength without using more materials than necessary. Twenty-four inches on-center spacing is also common and can save framing materials in walls, floors, and roofs, but only if the house is designed to work with the wider spacing.
Another spacing that is sometimes specified, for use with I-joists, is 19.2 inches. How do you measure 19.2 inches using a tape measure that's marked in 1/16-inch increments? Most contractor-grade tape measures have black diamonds at each of the 19.2-inch centers.
Why Do We "Crown" Framing Lumber?
Unless you are only using I-joists, finger-jointed studs, or other engineered lumber, lumber used for framing is never completely straight. Yet we are expected to frame flat floors, walls, and ceilings. To make the walls and floors appear flat, we "crown" joists and studs. First, we look along the edge of a piece of framing to see if it is bowed or crooked. Usually, you'll see a slight bow, which is called the crown. One of the first things a framing crew can do to save time is to "crown" your lumber piles as you sort through them getting ready for framing. Mark a "V" on each piece showing which edge is the crown–and then re-stack the pile with the "Vs" all pointing the same direction.
Installing framing lumber with the crowns pointing in the same direction is the best we can do to ensure flat surfaces in our framing. Crowning joists and installing the crowns pointing up is most important. If all of the joists in a floor are installed crown up, the weight of the furniture and people in the room tend to flatten out the joists. If the joists are installed with crowns facing down, it will make the floor sag.
With studs, it doesn't really matter much which way the crowns go, as long as they all point the same way. If you've got two studs next to each other with crowns going in opposite directions, it will cause a noticeable bulge in the wall once the drywall is installed. Having studs face the same direction is also important when it comes time to install cabinets. Save your best studs for cabinet walls in kitchens and bathrooms.
Why Leave a Space Between Sheets of Subflooring?
If you read the printing on subfloor sheathing, it usually tells you to leave a 1/16-inch space between the panels along their long edges and a 1/8-inch space at each end. That's because plywood and OSB can swell and expand if they get wet. If the panels are installed tight together and expand, you'll get ridges along the edges of the panels that you can feel even through carpet. Spacing the panels apart avoids this.
Spacing the ends of the panels, however, can lead to another kind of trouble. On a long floor deck, spacing sheets of subflooring apart 1/8 inch can start to force the plywood off of the framing layout–and it won't be long before the subflooring doesn't land on the middle of a joist anymore. I install the panels tight together, set my circular saw depth equal to the thickness of the subflooring, and after the subfloor is installed I cut a saw kerf between the panels to create the expansion space they need.
Why Are Rough Openings Bigger Than the Windows and Doors?
We make rough openings in the framing for windows and doors larger than they need to be to allow room for proper installation later and to allow for the fact that new buildings can move a little as they age. Rough openings let us frame quickly, and worry about the more precise work later when we're installing the windows and doors. Your crew leader finds the rough openings from the blueprints, and marks them on the plates so you can frame the walls.
Rough openings for doors are usually 2 inches wider and taller than the door alone, to allow for the frame and for shims to plumb the door unit. Window openings are usually 1 inch wider and about 1/2 inch taller than the window unit.
–Andy Engel is a writer in Roxbury, Conn.