Copyright Goodheart-Willcox Co., Inc. 322 Section 3 Plumbing System Design and Installation will go through the roof. This can be done by extend- ing a length of pipe through the opening in the double plate at the top of the wall until it touches the under- side of the roof sheathing. Check the pipe for plumb and mark the location of the pipe on the sheathing. If roof framing prohibits vertical installation, the vent will need to be offset to avoid cutting framing mem- bers. It is generally easier to cut the opening from above the roof using a reciprocating saw. Drive a nail through the sheathing from the underside to transfer the loca- tion of the center of the hole to the topside of the roof. Cut the hole approximately 1″–2″ larger in diameter than the vent pipe. Carefully raise the shingles around the hole, using a flat pry bar. Remove enough nails to allow the shingles to be lifted. Place the flashing over the hole and mark the shingles that need to be trimmed. Make the neces- sary cuts and check the fit. Once the fit is correct, slide the flashing under the shingles into the position shown in Figure 21-12. Renail the shingles beginning with the highest course. No roofing nails should be exposed. The bottom edge of the flashing is not nailed. Once the flashing is in place, determine the length of the stack by extending a tape measure from the roof to the sanitary tee. Two people are needed to install the stack. One pushes the pipe through the flashing from below, while the person on the roof holds the flashing in place so it will not be damaged in the process. Most stack flashing is fitted with a plastic or rubber collar that seals around the pipe. If this is not the case, apply caulking at the joint between the flashing and the stack to prevent water from entering. 21.3.2 Installing Horizontal Drain/Waste Piping The key concern when installing horizontal runs of drain/waste piping is maintaining the required fall for the entire length of the run. If there is too little fall, the waste will not flow through the pipe. If there is too much fall, the liquid will flow so fast that the solids will be left behind to clog the pipe. A slope of 1/8″ or 1/4″ per foot of run is generally required by code. Excavat- ing the trenches for the horizontal runs of the drain/ waste piping must be done carefully so that the correct fall is produced. It is often easier to dig extra deep and use fine gravel or sand fill to provide solid, properly sloped support for the pipe. The slope can be measured with a builder’s level or transit, but a 4′ or 6′ carpenter’s level is adequate for most resi- dential installations. Figure 21-13 shows how to check a level to determine how much the “bubble” should be off-center for a given fall. Once the level has been checked, it can be used (without the block) and the pipe adjusted until the bubble is in the desired position. For a lengthy run of pipe, lay a length of pipe into the partly dug trench and check the fall with the level. Re- move the pipe and continue digging and checking until the needed fall is achieved. This process is repeated until the trench is completed. A trial assembly of the pipe and fittings will usually reveal some need for ad- ditional digging and filling of low spots. The wye and “½ bubble off” “Bubble off” Block 1. Place the level on a flat surface. If the reading is not level, shim one end of the tool until it reads level. 2. Place a block under one end of the level to simulate a fall of ¼uni2033 per foot: Length of Thickness level of block 1uni2032 ¼uni2033 2′ ½uni2033 4uni2032 1uni2033 6uni2032 1½uni2033 3. Check position of the bubble. This is where the bubble needs to be for the pipe to be at a fall ¼uni2033 per foot of run. Goodheart-Willcox Publisher Figure 21-13. The bubble of a level can be used to indicate the degree of fall of a horizontal pipe run.