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This bathroom needs an exhaust fan as the only current ventilation is a small window in the shower wall (which does not stay open). enter image description here

My project planning is blocked by my limited understanding of the structural components in this space. I've had a few electricians walk through to quote the job, but didn’t share details on how they would do it.

I do not want to cut the tile because I lack experience with DIY and I think that would be too complicated with the wiring, etc. So I’ve ruled out venting through the tiled wall directly outside. Also, code requires these vents be installed a minimum of 3 feet away from windows.

There is roof directly above the bathroom ceiling, which may be an option. enter image description here However, as a new DIYer I am disinclined to cut holes in roofs.

I have considered venting through the wall right next to the tile. This may work, however the vent would likely terminate very close to the wooden support beams of the stairs which go to the upper unit and would face the underside of the stairs. It's worth noting that the staircase structure is not well protected from moisture and is already compromised (showing evidence of rot). Perhaps this would work if I could control the direction of the vent.

I was hoping it might be possible to install the fan in the ceiling and route the ducting out through this soffit. I would not vent into the soffit, but through it. I am concerned that this would introduce moisture into through the vented portions of the soffit, but I would look for a vent cap shaped to direct air away from the house rather than straight down. Venting through the soffit is technically permitted, however, it seems that this is not considered best practice. enter image description here

But I am not sure if this protrusion in the ceiling is part of the construction of the stair landing above (which leads to the upper unit in this duplex) or what it is? I don’t want to just start cutting into a mystery spot in the ceiling. It was suggested that I purchase a bore hole camera and I am researching that right now. enter image description here

The stairs are set back quite a bit further than where this ledge thing in the ceiling is enter image description here

But it doesn’t make sense that it’s part of the roofing structure either as the other side doesn’t have one. enter image description here

The bathroom floor joists run the opposite direction from the rest of the house, they run parallel with the window wall. However, the pitch of the roof indicates the rafters run perpendicular to the window wall so I assume the ceiling joists do as well.

I’ve been pouring through articles on all the major DIY websites and watching videos, but this is situation specific so I was hoping someone on here may be able to identify that ledge thing for me so I can decide if it can be used for ducting exhaust.

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  • could be part of the support for the the floor above, it's pretty had to guess. any reason why you can't vent directly through the wall?
    – Jasen
    Jul 28, 2022 at 4:03
  • It is OK to vent the exhaust fan through the exterior soffit vents. We really have no way of knowing what is in the drop ceiling area. It does not look like it is just an aesthetic addition- chances are something is in there- pipes or ducting or your stairway framing as you mentioned. Also, PLEASE, rotate you photos properly before posting.
    – Kyle
    Jul 28, 2022 at 4:03
  • @Jasen do you mean vent above that mirror on the wall right next to the tile/shower door? I didn’t think of that - it would wind up venting from the corner, along the edge of the house and directly facing the underside of those wooden stairs. It would also then vent toward the window partially hidden by the staircase addition, so I will have to recheck the code. Jul 28, 2022 at 4:38
  • if needed you can add exterior ducting, or put the vent in the the wall above the window, but you may need an SELV vent in that location check electrical code.
    – Jasen
    Jul 28, 2022 at 4:50
  • I don't believe venting through the soffit is recommended or allowed in most places in the US nowadays. An exterior wall is the best place, IMO, though the roof can be used (nothing wrong, just a bit trickier to prevent water intrusion).
    – SteveSh
    Jul 28, 2022 at 12:39

2 Answers 2

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There's a stair landing? that'll be what it is.

enter image description here

It's hard to guess how this load is supported. it may be possible to run ducting between the joists either towards the window wall or parallel to it.

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    This does not necessarily preclude using that space for ducting for the vent, however. It just means that the vent would need to run in the direction of the joists below that landing (not boring through them for the huge hole necessary for ducting).
    – FreeMan
    Jul 28, 2022 at 12:01
  • the question was "what's in the box"
    – Jasen
    Jul 28, 2022 at 13:04
  • Very fair point, Jasen. Your update is helpful, too.
    – FreeMan
    Jul 28, 2022 at 13:10
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    It's unlikely that the entirety of that box is structural. With just a little luck you'll find a path through it, which would be your best approach. But there's only one way to find out. Break open the drywall. Maybe a borehole camera can give you some info first. They are cheap so try one.
    – jay613
    Jul 28, 2022 at 15:41
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If your ceiling joists are parallel to the window, you will not be able to run any ducting thru the joists. However, there may be sufficient space above the joists to run your proposed duct? Don't cut large holes in the joists thru which to run the duct. If the ceiling joists are perpendicular to the window, it should be relatively easy to add a fan/duct a couple of feet in front of the vanity, run toward & thru the wall, and to the vented soffit in front of the window. No need to make a 90 to the side.

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  • "If your ceiling joists are parallel to the window, you will not be able to run any ducting thru the joists". Sure you can. Just need to be cognizant of the size of the hole and where it's located along the joist. Just look at the way engineered floor joists are made. They're mostly open space.
    – SteveSh
    Jul 28, 2022 at 12:43
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    @SteveSh The problem is that ducting is typically 6" across. Taking a 6" hole out of an 8" beam will split the joist in two. Most of an engineered joist is open space - but the bits that aren't are critical. Jul 28, 2022 at 13:07
  • @Martin - But you don't need a 6" hole for a bathroom exhaust. 3", may 4" top?
    – SteveSh
    Jul 28, 2022 at 15:14
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    @SteveSh inspectapedia.com/ventilation/Bathroom_Vent_Duct_Lengths.php suggests you can only use smooth-walled 3" duct for 5' and then only if you only need 50cfm air flow. Anything more and you are looking at at least 4". UK building regs say that a hole in a joist can only be 1/4 the depth of the joist - so even a 3" hole would need a 12" joist. It appears that US building codes allow 1/3 the depth - but that's still a 9" joist. Jul 28, 2022 at 15:35
  • So OP would be OK with a 3" duct and a 2x10 joist, in the US?
    – SteveSh
    Jul 28, 2022 at 16:04

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