First Floor Additions: More Room More Happiness
(Part Three– Framing Facts)

by: Liz Roberts

Now that you have the info down pat on your foundation, it’s time to think about the next vital steps of your addition – subflooring and framing. It’s important to correctly construct these otherwise there could be disaster. Be precise and exact with everything from framing to simple hammering. Keep these in mind and you will have a sturdy, professional looking addition.

What wood do you select for framing and subflooring? That depends on you and what was originally used in the building of the main house. Lumber woods are divided into two types softwood and hard wood. Softwood’s origins are from coniferous trees such as pine, yew or cedar. Hard woods come from the hardier trees such as elms, maples, hickories, oaks, sycamores and poplars. The lumber is then graded on how many knots it has(Word of warning, some lumberyards have their own wood grading systems. Be savvy before you buy and always look for the least flawed). Be eagle eyed when spotting defects. A board that is heavily twisted, bowed, cupped or crooked will not be good building material. Knots themselves can become loose and pop out. Be on the lookout for this problem. If you can, visit several lumberyards to see who has the best quality wood. Choosing inferior lumber may save you money (and even time) but it could make for disaster later on.

The first item to start on is the subflooring. You’ll first have to measure all the locations of the anchor bolts and then transfer these to the sill plate which is made from 2" x 8" treated lumber. Using a speed square, draw a line across each mark, then measure and mark each line’s center. Drill out each center using a ¾” spade bit . Countersink the holes the same size as the washers that will be later used. Set the sill plates in place over the anchor bolts, also tightening washers and bolts together where they meet the corners. Toenail (driving the nail with the initial surface to allow it to penetrate down below) the boards together where they meet at the corners.

This is now the time to install the floor joists. Joists are a series of parallel beams, usually two inches thick. They are used as supports for floor and ceiling loads. For your addition make sure they are 2" x 12" . If you are building a great room, then go with what’s known as engineered joists, joists that can cover the entire subfloor. Install the joists perpendicular to the rest of the house. Doing it this way will make later installation of heating and cooling ducts much easier. Measure the joists, remembering to subtract the rim joist’s thickness (the rim joists are the beams that cover the joists’ exposed ends). Attach joists to the sill plates that are perpendicular to the house. The remaining joists will also face in the direction. Now secure the joists to the sill plates and rim joists of your addition. After attaching the joists on the sides of the rooms you can begin attaching the intermediate joists using joist hangers. Set the joists 16" on center and nail them into place. Rest one end in the hanger and the other on top of the sill plate. Attach the rim joist after all other joists have been put in place. Now you can begin to attach the ¾’ tongue and groove subfloor. Stagger the seams to prevent the floor from having weak spots. Your subfloor should be lined up flush with the edges of the joists. Once the subfloor is in place, snap a chalk line to show where the inside edge of the addition’s outer walls should line up.

Now it’s time to start building the walls. Mark the placement of wall openings and studs. Lay the top and bottom boards (also known as the sole and top plates) besides each other, temporarily securing them to the floor. This allows you to use your speed square to mark both plates at the same time. The center of each wall opening now has to be measured and marked. You will have to do this for the outer edges for the opening too. If you’re planning on a 48” window, measure out 24” from the centerline. Mark the boards and measure another 1 ½ “(this will be the board’s thickness) from that mark to allow for framing studs. Mark the inner line to indicate that this will be used as a reference point placing the supporting studs. Next mark the outer line. This will indicate that this will be a full length stud. Also mark the positions of the partitions where the interior wall will join the outer wall. This area will use three studs. Two will be perpendicular to the room while the third will face the room. This one will be sandwiched between two outer studs. Dry fit the wall together on its’ side, then nailing the studs in place from underneath and above the top plate.

Headers have to be cut. They are a vital part of your addition because they support the weight of the roof over the windows and door. Always check (usually when the building permits are being given out) with your local building inspector to determine the proper thickness of the headers. In most cases they are made from 2" x 12" pieces of lumber surrounding a ½” plywood board. These will usually meet or exceed local codes. Don’t skimp by omitting the plywood because it gives the needed thickness for the header to be flush with the studs on either side. Lap studs will be the next to be installed. These are partial length and will be nailed next to the full length studs. Install sills for your windows, using the same method you used earlier for installing the laps studs and header.

You’ll also need to put in cripple studs that are short support pieces beneath the sills. After the wall is assembled begin attaching exterior sheathing made up of low grade plywood. This should not be attached to the studs of the final wall that you’ll be raising. The sheathing will extend past the corner, covering the sheathing from the adjacent walls. Now that the sheathing is in place, it’s time to raise the addition’s walls. Start with the walls at the ends, then raise the one that will be in the middle. Make sure each wall is plumb, bracing it with temporary supports. Attach a second top plate (also known as the connecting plate) to tie together the wall supports. Now adjust the walls bottoms, aligning them with the earlier drawn chalk lines. Nail the sole plates into place. Attach 2" x 4" blocks to the outside top corners of each wall, stretching a string between them. Adjust the wall’s tops so the string is 1 ½ " away from the wall down its’ entire length. Secure the wall with 2" x 4" braces that are set in on angle. Nail these in place.

Once the subflooring and framing are done, you’re almost there in completing your addition. Remember to follow the above instructions to the letter. You want your addition to be sturdy and strong as well as long lasting.

Next part of the series, building the roof