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locating bath exhaust fan on double roof

2leftfeet | Posted in Energy, Heating & Insulation on September 22, 2006 06:51am

Hi all

This weekend I need to put in a bathroom exhaust fan. There has never been one installed and I am trying to figure out the best way to locate exactly where to put it.

The thing that makes it tricky is that I have 2 roofs. The first is the original flat roof, 1/2 inch drywall ceiling then 4×6 beams with 2x t&g, a layer of foam, then roll roofing. On top of that I have an added (about 10 years after orig construction) gable roof with prebuilt trusses and traditional asphalt shingles. There is no fiberglass or other insulation in the gable roof area.

Am I best off going up into the attic, locating the plumbing vent, then measuring over to approx where I want it, then adjust to miss the beams and trusses?

What should I use to go through the original flat roof? Drill bit to start then a recip saw?

I the shingles are old (15 years) but in fair condition I fear that if I cut into them to place the vent correctly that I will have a hard time with the brittle shingles, can I run the exhaust approx 8 feet (horizontal) to the gable vent? I am sure I am putting on a new roof in 2 to 3 years and could undo the hack later, it is just not in the budget to replace it this year. I can’t find anywhere in the building code that says I can’t do this but I cant find anywhere that says I can either.

Thanks in advance

 

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  1. piko | Sep 23, 2006 07:34am | #1

    Start by finding the approx location you want. Use a stud-finder to determine any beams in the area you've chosen. None? Good, now make the opening for the fan (BTW, how will you support it?). The duct would normally  go horizontally out thru the joist- or truss-bay to the outside. Here you'll have to go up thru the 1st roof. Cut in it a decent hole any way you can - from the top, if poss.  Now the duct will come up thru this hole, and from this point you will now run it slightly downhill to where it exits (this will stop condensation dripping back into the house. Oh, and wrap insulation around the vert bit).

    Not a good idea to go to the gable vent - wind pressure can blow moist air back into the attic space. better to go thru the wall some place else, or down thru the soffit. Unless you can guarantee that venting thru the roof will not result in another upward angle that'll collect condensate, then perhaps the roof vent will work ... don't make any bigger hole than you need there. Gently lift the tab to slide the vent under, or slip[ a bit of flashing under the tabs and over the top edge of the vent - and use a good sealant all round.

    All the best...

    To those who know - this may be obvious. To those who don't - I hope I've helped.

     

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