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Question DetailsAsked on 2/25/2016

Why is water leaking through 1st floor half bath light/exhaust fan during heavy rains usually with wind?

I had the pipe on roof fitted with cap so water can't get in. New roof 8-9 years ago. Had roof inspected for possible leaky areas. None seen. I think fan also vents out of side of house.Had that checked. No evidence of water running down pipe in attic. Still getting water into first floor half bath, not as much as previously, but still happening. Usually hard rain, possibly with wind in certain direction. Older house. (1911?) .

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1 Answer


I am presuming this has nothing to do with thawing after a very cold spell - which can cause frosting in the vent pipe, which then runs down into the fan unit when it thaws out as the ducting warms up to above freezing - usually noted beczuse it starts dripping down shortly after turning fan on, as the warm bathroom air melts the frost.

Depends on type of cap if on roof - if a vent hood like following, which is normal for this type vent, if the flapper door (just inside the front opening) sticks open because it is bent or rusty, then blowing and especially splashing rain can get into it and down to the fan/light. Or some types have no flapper door at all - only a bug screen, so splashing rain driving toward the opening can spalsh right up inside.

Otherwise, if this general type -

with openings the rain can drive into sideways, they can cause leaks like yours. That type are designed for exhaust ducts from furnaces and water heaters which will generally evaporate away minor rain inflow with their hot exhausst before it gets to the point of leaking out the bottom. If you have a vertical duct and it is open to the side like that, you need one with rain hoods similar to this with a positive rain cap over it which should come down below the lowest open part of the opening - and you need (and are required by code) to have a damper in the duct to prevent cold air blackflow too. For this applicatiom, plastic is a lot better than metal if in an area with winters, because a stick-up duct with metal cap cools down fast and promotes condensation and frosting in the cap and duct - which is why the first type above is commonly used for kitchen and bathrooms fans, to minimize the amount of metal exposed to air and wind for cooling.

Other possibility - leaking seal around duct, or under the roof jack - the metal flashing that surrounds the pipe and is supposed to waterproof the penetration. To see if that is the problem, you need to get into the attic during a rain (or use a hose spraying on roof) to see if the water is coming in around the opening cut in the roof sheathing (which could be from a roof leak or leakage under the roof jack or occasionally a damaged roof jack) or coming down right along the ducting itself - meaning it is leaking or entering somewhere above the roofline.

Since you say there was no evidence of water running down the duct, then presumably coming down inside the ducting. You can unscrew the center knob or pull down on the grating over the fan in almost all cases (or look for very small mounting screws and remove the grating or grill - of course making sure power is off to both fan and light. Then look inside with flashlight (may need small mirror to look around corner) and see if it is coming down inside the ductwork. If so, then it is getting in the cap.

You also said it might vent through the side of the house, not the roof - if so, a louvered type vent like this -

can leak in driving rain - might try swapping it out for this a pretty easy DIY job if not afraid of ladders -

Otherwise, last possibility - in rain check the attic floor around the bathroom pipe if it passes through there - see if maybe a roof leak elsewhere jsut happens to be flowing over the top of the dryhwall/vapor barrier and that bathroom fan is the first opening in the ceiling it comes to, so that is where it is leaking. Obviously, if 2 story house and fan vents through second story floor joist spaces to outside, this is not a viable case.

One last thought from rereading your post - you said "no evidence of water running down pipe in attic" - so this sounds like a roof-mount vent. Or if it vents through the side ofthe house, it is improperly tilted uphill to the outlet - should rise to above outlet vent level right above the fan, then tilt downward to the exit to keep any blown-in water or condensation/melted frost from getting in.

Answered 4 years ago by LCD

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