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Question DetailsAsked on 12/30/2013

What's the best way to vent for a dryer-through the roof or through the crawl space?

I just purchased an older house and the previous owner never had a clothes dryer. Currently, there is no way to vent out for a dryer so my options are to vent up and out through the roof or through the crawl space to the outside wall or through the roof and out the side of my house. Not sure what option is best or who to contact for this repair. Thanks!

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5 Answers


I'd go with the crawl and out the wall. First it is easy to check for lint build up, and dryer vents should be be cleaned every two or three years as lint buildup causes longer drying times and increased energy use, can even cause a "dryer fire".

Through the roof being vertical,tends to build up lint faster, plus you can't easily feel the discharge air or see lint build up at the discharge.

Most all dryer can only have 25 feet of vent duct, elbows count as 5 feet,over 25 feet takes longer to dry and it may cycle on the dryer's high temperature limit.

Call around, appliance store/repair, HVAC contractor, or last choice a handyman.

Google dryer fires to be convinced.

Google dryer box, and learn about venting and some accessories for venting. One is an elbow that equal just over one foot of duct ,instead of five feet. Very helpful on long runs.


Answered 6 years ago by BayAreaAC


Like BayAreaCool, I recommend always going down and out versus up wherever feasible to facilitate lint flow and prevent blockages, with the proviso that you should avoid runs over 15-20 feet if possible, and try to have NO bends other than where the dryer duct goes down into the dryer from the floor or exits through the wall. If you have to have bends, try to put them at an accessible place so you can easily snake a vacuum hose through them without breaking the duct loose - for instance, if you need 3 bends (not advised in any case), have one right at the dryer, intermediate one where needed (like floor to wall), third at the exit so 2 of the 3 are readily accessible and a cleaner can snake his vacuum hose around the bends by reaching into the duct with his hand (one reason 4 inch is recommended over 3" duct). Check you dryer manufacturer manual on requifred duct size and maximum length of run allowed.

Ideal situation is one or maybe two 90's as needed from the dryer down into the floor, then a horizontal run between floor joists straight to an outside wall - which it sounds like you can do in your case. On a retrofit like this, to avoid tearing up flooring or ceiling to install the ducting if you do not have exposed floor joists on the underside, one thing that works is making the outside hole slightly oversize so you can install the duct, pre-joined, on a piece of angle iron or 2x6 clamped to the duct and supporting it underneath - this is then screwed at the dryer end to existing or newly installed blocking or bracket, and at the outlet end with screws - this "tray" then supports the duct so it does not need intermediate support or maybe only one or two intermediate pieces of blocking, hence you do not have to open up the floor or ceiling to install the duct, which will not adequately support itself for more than about 3 feet without sagging and risking kinking. IF doing this be sure to put foam padding between the duct and the tray or banding, to prevent rattling.

Also - use longest pieces of ducting as you can - generally 5 feet per length is commonly available and occasionally 10 foot, which means only 1-2 joints in mid-run (excluding the elbow joints), as joints are where a lot of the lint accumulation occurs. To avoid midrun leakage, the straight seams can be sealed with seam sealer, but to allow future removal the 90's should be sealed only with duct tape or a non-hardening silicone caulk so they can be yanked apart. Be sure joints are aligned so the upstream piece fits INSIDE the downstream piece at each joint, so there is no exposed joint edge facing the airflow for the lint to hang up on - and NEVER use sheet metal screws on dryer exhaust pipe - use seam sealer and external waterproof (because of the dryer air moisture) duct tape or metal banding only, because any screws penetrating the duct will catch and build up the lint VERY quickly - commonly blocking off the entire diameter with a large clob in a very short time. Also, don't forget a cover at outlet to prevent cold air infiltration (use louvered vent coer/hood) and insect entry (louvers or screened outlet). All support points should be paddeed with foam padding to avoid rattling or humming duct pipe when the dryer is running.

Since you said you have a crawl space, presumably open joist), this would actually be a fairly easy do it yourself hjob if you are up to it. Otherwise, probably about $50-100 materials and about $200-300 labor to get it done if a straight shot between floor joists to outside wall in reasonable distance - up to $500 if a more complicated run including up through walls. A handyman if you have one you trust to do the job right, otherwise an HVAC contractor.

Search the List (in green banner bar) for local contrasctors and their ratings and reviews.

Answered 6 years ago by LCD


Both above answers have very good advice. Code states that your dryer duct can not run more than 25'. Less is always better. Elbows and upward runs reduce the overall length of allowable run by a multiplied factor. What that factor is depends on the elbows used and how low the turbulence created is. Proper duct elbows typically count as 2 or 3 feet worth of run, each, and upward runs of pipe are usually multiplied by 2. So if you have 8' of upward pipe and 2 elbows you have already used at least 20' of your allowable 25' length. At that point you need a booster to help keep the air and lint moving. That's more money and more to maintain. You should always run dryer ducting sloping slightly downward to the nearest point of exit.

I have a different take on whether or not to use fasteners to hold joints together. If there are joints that will be inaccessible I use short sheet metal screws to hold the joints together. The reson is that I have cleaned out a few dryer vents that the pipes came apart when the snake was run through them. The tape broke down for whatever reason or just wasn't sealed well. Anyway, a simple duct cleaning turned into drywall work because the the joints had to be accessed to be put back together. A $150 minimum call turned into $500, not including repainting. I've also come across countless dryer vent pipes that were apart without a snake being run through them. Yes, screws will grab lint but the screws should not protrude more than about 1/4" into the pipe. Also, I recommend cleaning the dryer pipe every year. If it is heavily loaded with lint at 12 months then reduce that to every 6 months. I've had customers with small children who wash cloths constantly and every six months the pipe is almost completely clogged with lint. Make sure you clear the lint screen on your dryer with every load. This will make a huge difference. If you are at all handy around the house the big box home improvement stores sell a snake you can use to clean the vents yourself easily, saving a lot of money and ensuring a minimized risk of fire.

Answered 6 years ago by Todd's Home Services


Good point by Todd on joints coming apart - on my personal house I solved that potential problem by welding the joints - but not something that would normally be done, though I did have a hotel job I was on once where that was part of their standard spec for ALL sheet metal exhaust duct for dryers and kitchen ducting - but that was to prevent any duct fire from releasing flame into the surrounding area.

I guess my preference, though about $5 dollars more expensive for each joint, would be to use matching size sewer pipe rubber clamp type flex joint at joints where you might worry about it coming apart - the kind used to join ABS or PVC to cast iron, where there is a continuous rubber gasket around both sides of the joint, held on with stainless band or screw clamps - that way there would be no fasteners protruding inside the duct. Second alternative - use barely long enough pop rivets (which leave a pretty blunt protrusion), or even better pan head screws installed from inside the pipe while assembling segments (if using 3' or less sections) using a 90 degree drill, to the inside would be fairly smooth with no screw end sticking up to catch lint. It is amazing how fast a lint buildup can occur on a protruding edge or fastener, and once it starts it just builds and builds.

On te buildup and cleaning frequency issue - I try to get all the bends at the dryer if possible (easy access to clean as you can reach a few feet into a duct with standrad vacuum hose), and then a straight run t the outside, so it is easy to look in from the outside with a strong flashlight on a scheduled basis (preferably every 3 months) to look for accumulations.

One thing I forgot to mention on the crawl space ducting - if in a cold climate, insulate the duct pipe so the moisture condensation does not freeze into hoar frost in the pipe, which then thawes as the dryer starts and become a major lint collector, as the lint sticks to the wet surface, promoting fast buildup. In areas where it is allowed by code, using COEX ABS pipe (an ABS used for sewer pipe that has a lot of air bubbles in the plastic so it acts as an insulator) helps too. I have it from the house to the outside face of my 10' wide deck so the dryer moisture does not accumulate in the deck wood and ice the deck surface - even at 30 below there is very minimal frost and lint buildup because the plastic surface does not conduct heat as well so frost buildup is minimal, and the lint does not stick to the very smooth plastic surface as well as to metal duct.

One thing none of us said - avoid using flexible plastic duct if at all possible - banned by code in many jurisdictions as it has no resistance to a lint fire in the duct, and it also pulls apart and separates and kinks easily, which are all bad things.

Answered 6 years ago by LCD


+1 to previous answers.

Crawl space, straight runs, shortest distance, no flex lines, no screws, and perhaps a clean out in place to assist with season cleaning of the line.

Answered 6 years ago by WoWHomeSolutions

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