Copyright Goodheart-Willcox Co., Inc. Chapter 21 DWV Pipe and Fitting Installation 321 vertical pipe leading to the stub-out will need to be se- curely fastened to the blocking. It is extremely important that the fittings be correctly positioned so their outlets will align with branch pip- ing. To ensure correct alignment, make a trial assem- bly of the pipe and fittings and mark the position of the pipes at joints as shown in Figure 21-10. Marking per- mits the fittings to be repositioned after the joints have been prepared. Also, it will be possible to lay the pipe on a flat work surface or secure it in a vise while join- ing subassemblies. Extending the stack through the roof requires pipe and at least one coupling because the distance will most likely be more than 10′. Extend the stack through the ceiling and add an increaser and a larger pipe, Figure 21-11. The amount of pipe that must extend above the roof is specified by code. Be aware that walls extending above the roof near where the vent is to be located can affect the height of the vent above the roof. Because tall vents are generally undesirable, it may be necessary to offset the vent so it can extend through the roof at a point where it will be less conspicuous. In cold climates, it is desirable to use larger pipe above the in- sulated area of the building to reduce the possibility of the vent becoming blocked by frozen condensation. One of the most important parts of stack installa- tion is to make sure the flashing is correctly installed. Flashing is material (often a formed metal or plas- tic product) that is used to prevent water from leak- ing into the building around the stack. If the finished roof has not been installed, the roofing contractor can install the flashing during the roofing process. If the roofing material is already in place, the plumber may need to install the flashing. Procedures vary some- what, depending on the type of roofing material used. For residential work, where shingles are the most com- mon type of roofing material, the plumber can install the flashing. Care must be taken to correctly place the flashing between the courses of shingles and to avoid damaging surrounding shingles. The flashing must be installed before the stack is put into position so that it can be fit under the shingles, Figure 21-12. The first step is to locate where the stack Mark with permanent marker Goodheart-Willcox Publisher Figure 21-10. Pipe and fittings are trial assembled. If satisfactory, position of fitting is marked as shown. Alignment is then certain to be correct when joints are made. Roofing Flashing Rafter Vent increaser Ceiling Vent stack Sanitary tee Goodheart-Willcox Publisher Figure 21-11. An increaser is installed on the stack before it extends through the roof. Goodheart-Willcox Publisher Figure 21-12. Installing vent flashing involves cutting a hole through the shingles and roof sheathing and then trimming the shingles to fit around the flashing.