Plataforma de procesamiento Seismic Unix (SU )
4.9. Recuperaci´ on de amplitudes
2x block nai led flush with tops of joists
Two 1 6d nails at each end
angle. Keep the nails % in. to 1 in. below the top edge of the joist so that you won't hit them with a sawblade when you cut the sheathing. After you have finished nailing in all of the regular-length blocks, cut and nail in the specials.
Scattering sheathing
It is more efficient to scatter all the sheets before be ginning to nail them in place. One person can easi ly carry a sheet of Sis-in. plywood or OSB - the trick is to carry it with one hand underneath and one hand on top for balance, with much of the weight resting against the upper body. Use caution when carrying sheets in windy conditions, as they can act as a sail. Scatter the first row across the joists a few inches in front of the control line, leaving room be tween the line and the edge of the building to ap ply an adhesive.
Sheathing panels should be carried with one hand on top and one on the bottom, with most of the weight resting on the shoulder.
For structural strength, four sheets shouldn't join together at one corner. They should be staggered by beginning every other row with a half-sheet. Stag gered sheets add to the horizontal shear strength of the floor, making it structurally stronger.
Installing sheathing
These days many builders fasten down the sub floor to the joists with an adhesive, in addition to nails or even screws. As joists begin to dry out in a house, they can shrink away from the subfloor and cause floors to squeak, which is tough on a teenager trying to sneak in late at night. Squeaks can be greatly re duced, if not eliminated, the entire floor structure strengthened and the quality of the house increased
Construction adhesive is applied to joists to bond the sheathing to the floor system.
The first row of sheathing should be aligned with the groove edge on the control line (left side) and the tongue along the edge of the building (right side).
by using a good construction adhesive. To get the best value for your money, buy the large 32-oz. tubes and dispense the adhesive with a caulking gun. One large tube is enough to secure about three sheets of plywood. When you are ready to sheathe, apply a Yt -in. bead on the joists and on any edge blocks that will fall under the first sheet.
If you are using tongue-and-groove sheathing, treat it with care so as not to damage the edges. Damaged edges make it hard to fit two sheets to gether. Begin laying the first row with the groove right along the control line, hitting the middle of a joist at each end. Tack it down with one 8d nail near each comer to hold the sheathing in position until it can be finished with a hammer or pneumat ic nailer. In a dry climate, lay only a few sheets be fore nailing them off so the glue won't dry out.
In areas of high humidity, leave about
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in. be tween the ends and edges of the sheets to allow forexpansion. This gap can be gauged by eye or by using an 8d nail as a spacer. Sheathing panels sized
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in. smaller in each direction are available in some areaSj these allow you to space sheets and still main tain a 4-ft. by 8-ft. module.If the jOist layout isn't totally accurate, some sheets will have to be cut to have bearing on a joist. T&G plywood has to be cut as it is laid down. No measuring tape is needed to mark it to length. Just hold the chalkline in line with the center of the joist and mark the sheet for cutting. Hold one end of the line with your foot, pull it across the sheet, and snap. You can also hold one end of the line on the mark with your little finger of one hand, extend it across the sheet with the other hand and snap the line with the thumb and index finger of the first hand. Once the line is snapped, set the circular-saw blade to the thickness of the sheathing and make the cut.
Let the sheathing run over at the edges of the building. The edges will be snapped with a chalkline and cut at one time after all the floor has been laid. Just be sure to make the cut before nailing so that no nails get in the way.
If you are not using adhesive, it will be quicker to nail off the floor once it is entirely laid. After each row is tacked down, make a mark with pencil or keel on the leading edge of the sheathing to mark the lo cation of every joist. This takes only a moment (it can be done while walking back to begin the next row) and makes it easy for the floor nailer to find the joist.
Once the first row of sheathing has been laid the length of the floor, begin the second row with half a sheet. These 4-ft. pieces can be cut right off the material pile. It's faster to measure and mark half sheets with a drywaller's T-square than with a tape and chalkline.
Sometimes it takes extra persuasion to unite T&G sheathing. One way to do this is for one person to stand on the sheet, holding it flat. A second person can drive this sheet into place by hitting a 6-ft. long 2x4, laid against the groove, with a sledgehammer, as shown in the photo at right. Don't hit the sheath ing directly. Two or three good licks on the 2x4 is usually enough to bring the two sheets together.
Don't nail within 6 in. or so of the leading edge until each succeeding row is laid. A little flexibility on this edge will make it easier to unite the T&G when laying the next course. Sometimes a joist will be bowed at midspan, which does not allow the end of the sheathing to break directly over the joist. Push the joist to the right or left by hand or with your feet until the end of the sheet aligns with the center of the j oist and stick a nail through the sheathing into the joist to hold it in place. This saves you from having to make a cut on the sheath ing. If the joist can't be moved, a 4-ft. piece of 2x nailed to it can give bearing to the end of the sheet. As you sheathe up to where the joists lap over a girder, you may need to nail on some 2x scrap pieces to extend the length of one joist and provide a nailing surface. Be sure to mark its location with keel to help keep the nailer on track.
Fitting tongues into grooves sometimes takes extra persuasion. With one person standing on the sheet, another can give it a couple of raps with a
sledgehammer. Lay a 2x4 along the edge to protect the groove.
The last row
Many codes call for the last row of sheathing to be at least 24 in. wide because a narrow strip can re duce the shear strength of a floor. For example, if a building is 2 1 ft. wide and you have laid 5 sheets of sheathing, the last piece would be only 1 ft. wide. If your code won't allow this, snap a chalkline back on the sheathing 2 ft. from the outside of the building then cut along this line with the circular saw. The last row will now be 2 ft. wide and up to code. This row may need to be edge-blocked because there will be no groove to unite with the tongue.
Cutouts
When plumbing pipes are to be installed later there is no need to make any cutouts in the sheathing; sheathe the entire floor and the plumber can drill through it. When the pipes are installed before the sheathing is laid, holes have to be cut in the sheath-
ing. The quickest way to make these cuts is to mea sure from the edges of the sheathing already in place to the center of the pipe and transfer these measurements to the sheet ready to be laid. These holes do not have to be cut out to the exact size of the pipe. Make the cut about
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in. larger all the way around. So, for a 3-in. pipe, cut out a 4-in. by 4-in. square, 2 in. on each side of the center mark, with four quick plunge cuts (that is, by dropping the spinning blade of your circular saw onto the cut line). With just a little practice it is easy to make this cut by eye. Lift the sheet up and drop it over the pipe.When sheathing a floor that has a stairwell in it, let the subfloor overhang 2 in. or 3 in. where the stairway will land. This overhang will make part of the last tread on a set of stairs. It can be cut to exact length later when the stairs are being built.