Flashing in a tiny gap
I’m a first time poster so I hope this is an appropriate question for your board. I’d really appreciate any help you can offer.
I’m hoping to get advice on how to add flashing to a small gap. I’m remodeling an attached greenhouse in Maryland. The “roof” of the greenhouse will sit just below the soffit of the main house. It will have a 3/4″ gap that is set back about 12″ from the top and 21″ from the bottom (please see attached drawing). I am leaving this air gap to deter humidity from the greenhouse from entering the attic. I do not want this gap to be any bigger to maintain growing space in the greenhouse and simply because it looks much nicer without a large gap. I’m planning to have the roof of the greenhouse be cement fiber panels (As I will probably have a lot of extra material) . The roof will have a slight pitch to shed water away from the main house.
Options I’m considering are:
A sealant that can be pumped or sprayed in with an extension hose (Maybe something like “Great Stuff”?). Then take a tiny corner squeegee on a stick to seal it against the seam.
Or, attaching peel and stick flashing to the back side of the greenhouse roof material and overhanging it by an inch.
Then angling the panel as it is positioned to lay the overhanging flashing against the back / vertical wall of the main house.
Then using a 3/4 “ square pole to reach in and press the flashing firmly against the house?
Thank you for any advice!
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Replies
David,
EDITED: Because my reply contained a fundamental error.
Whether partially covered or not, that's a flat roof abutting an end-wall. They need to terminate in the roof material running up that wall at least 3", not relying on a slight slope to stop water driven by wind, or dammed by debris or snow, moving back to the wall.
Whether there is a small gap, (or as I would do you use a vapour barrier on the underside of the soffit to stop moisture moving that way), the flashing over exposed section of the upper greenhouse roof needs to run up behind the existing fascia.
Thanks for the reply. I'm not opposed to a vapor barrier so I'll look into it more. An air gap seemed like the simplest and most full proof way to prevent moisture infiltration especially considering my minimal understanding vapor barriers etc.. I'm starting to see an air gap is not so simple.
before you add the greenhouse roof, you should make the building soffit enclosure as sealed and maintenance free as possible. if the greenhouse structure obstructs access, then you should dismantle enough to provide access to the building structure. make the air, vapor, and thermal control layers as robust as possible.
MT is treating the roof under the soffit as part of the water control layer. As an alternative I'd consider moving the water control layer from this blind area. The water control layer would be from the face of the fascia above over the exposed sky-facing greenhouse glazing. i'd look at field fabricated expansion joint details and counter flashing. otherwise you could just install a small foam expansion joint like emseal or willseal and hope for the best with snow.
vapor control layer from the green house space would be best addressed from inside the greenhouse where you can see and monitor [and repair] over time. if you feel like you must vent the interstitial area between these separate areas then I'd consider some kind of passive wall vents on each return wall with some kind of hood protection from wind driven rain.
i would not make the transition as a roof to roof b/c then you need to worry about venting the shed roof above. flashing roof to wall is always easier.
Stamant,
"i would not make the transition as a roof to roof b/c then you need to worry about venting the shed roof above. flashing roof to wall is always easier."
I agree. Dropping the roof say 3" to 4" would make the whole thing simpler and better.
Thank you for all the advice. I'll look into what you've suggested. Those ideas make sense to me.
In our area, every insect with wings and a stinger would be competing for a place inside that gap to make a home.
Good point. I'll put bug screen up.
I have dealt with a similar installation of a greenhouse, but the soffit formed part of the greenhouse roof (no gap). There was a major problem with moisture going up into the soffit and rotting things out. The solution was to rework the soffit in the greenhouse with PVC trimboard (the plywood-like sheets of PVC about 1/2" thick), and sealing the edges with polyurethane sealant. Moisture proof materials are a must, and they have to be sealed to keep the moisture from getting up into the attic space above.
If you have a gap, then you want good air movement. I would try to just use a drip edge and not seal things in that case, so that you have a way for air to circulate. I'd keep an eye on the underside of the soffit anyway, just to be safe -- you may still find it's prone to moisture issues, even with that small gap.
BTW, by "drip edge", I mean two things: 1- put a thin piece of PVC trimboard along the top of the greenhouse slightly inwards from the outer edge of the soffit. The idea is to make a sort of dam to keep water from flowing back into that gap. Now put something on the outer edge of the soffit, such as a wider fascia board, so that any runoff from the roof above runs down onto the greenhouse out past the "dam board". This way you have a drip edge and a way to keep water from pooling back into the gap too.
I agree with the other posters that this narrow gap complicates more things than it solves.
Bill
Thank you for the reply. I had read your past post about moisture infiltration caused by a greenhouse a while back. It's a big fear of mine. I'll go with your solution instead of an air gap. I'd love to hear any more details on how you solved this, or if you have any pictures. Any more advice would be greatly appreciated.
All I did was to remove the original plywood under the soffit and replace it with PVC trim board sheet, then seal the perimeter with polyurethane sealant. The primary concern was to keep moist air from getting up into the attic through that soffit. Obviously we removed all the vents in that part of the soffit when the PVC sheet was installed. You want to air seal things, use a vapor barrier material, and make sure anything you are building with on the greenhouse side is impervious to moisture and rot. PVC trim board materials are perfect for this, and readily available.
Bill
I don't understand why the little piece of flat roof, I think I would delete that.
I built a canvas roofed screen room over my deck. I have a 2 pitch rubber roof and the seam between the wood framed canvas roof and the actual roof would leak, so I slid a piece of regular drip edge under/behind the EDPM drip edge, upside down and backwards if that makes sense, and it does not leak a drop, even with 10 inches of snow on it. [what kind of idjit leaves a canvas roof up all winter?]
So...if that piece of flat needs to be there, I would flash all the way from under the existing drip edge, or pull the fascia and flash behind it