Running a dryer vent underneath a deck is one solution when the exterior vent is in the immediate area of the dryer vent. There are a few reasons that you will want to consider doing this: preventing lint buildup, preventing moisture buildup on the wood (which can result in mold and mildewing of the wood), and avoiding damage to the wood due to these two.
This is a reasonable option when relocating the dryer vent is not a viable option. Relocating the vent can be difficult and will require work to the exterior of the home. Optimally, the dryer should be vented by the most direct route, and manufacturers have recommendations on the maximum distance a dryer vent should be located. Overly extending this distance will increase the risk of lint buildup in the dryer vent (increasing the fire hazard) and will decrease the efficiency of the machine. Here is what you will need to complete the job:
Step 1- Assemble Vent Pipe
Measure the needed length of vent pipe. The pipe usually comes in three to four foot lengths, so there will be some joints. Join the pipes together to get the needed length, but do not fasten together. Mark the last pipe where it needs to be cut and disconnect the pipes. Cut the aluminum piping to the needed length, then reconnect them all and secure the joints with the duct tape. Place the metal draw bands around the joints and draw them tight. These will also help to secure the joints. Leave a bit of excess on the ends, it will be needed.
Step 2- Install the Pipe
Insert the venting pipe in the exterior dryer vent. It is helpful to have one person to hold the pipe while the other secures it. Secure the vent pipe to the decking, at a slightly downward angle. As condensation builds in the pipe, you will want it to be able to drain out. Use the ends of the metal draw bands to secure it to the decking using screws and drill.
Step 3- Install Vent Cover
You will want to consider how you want to finish the vent pipe once you have routed it underneath the decking. If the deck is fairly low, you may just want to leave it hidden beneath the deck. Keep in mind that warm, moist air will be coming out of it, and should be properly exhausted to avoid mold and mildew buildup. It would be easy to build a small frame on the exterior edge of the deck, through which you can route the pipe and attach a vent cover. This is ideal.
This can be as basic of a frame as needed. Simply cut the wood to form a 3-sided frame, with the vertical pieces slightly longer than needed. Affix these to the deck on the underside edge of the rim joist, with the frame extending about 6 inches from the bottom of the rim joist. Route the pipe through the frame and attach the vent cover.