Flashing and trim details for doors and no-fin windows with board-and-batten siding over furring and exterior foam board
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Hello GBA,
After looking thru a lot of details here, and at FHB and BSC, I haven’t found one that’s quite right for my situation yet, and I hope you can help me.
I have a garage that was made to look like a small barn (22′ x 32′, 2 stories, gambrel roof). It was built in the early 1990’s, with 2 x 4’s 16″ on center, 1/2″ OSB (plywood at the corners), no insulation, no WRB, and lap siding that seems like particle board with a stamped grain pattern.
The siding has deteriorated (presumably due to the unfortunate combination of the lack of a single flashing anywhere on the building, and my failure to keep up with recaulking and repainting).
I’d like to embrace the barn “look” by replacing the siding with traditional board and batten siding (1 x 10, and 1 x 3). The plan is to install it over horizontal strapping every 2′. (leaning toward ripped 5/4 x 6″ treated deck boards, but might use 2 x 3s or 2 x 4s. I suspect that 1x boards won’t provide sufficient holding power for the siding nails.)
In the relatively near future, the second floor of the garage could become a home office- or kids’ hangout- and it would be a shame to not take advantage of this rare and inexpensive opportunity to add exterior foam board insulation. I’d like the insulation to serve as the WRB, and I’m leaning toward 2″ foil-faced polyiso, as it seems (to this novice) that tapes and/or adhesive flashings will stick to it better.
I’m struggling with the details for the trim and flashings (for both doors and windows). I’d like to reuse the existing doors and wood windows, which have brickmould instead of a nailing fin. I plan to replace the brickmould with 1 x 4, as that would look more appropriate on a “barn” with B&B siding. Lap siding usually butts up to brickmould/ casing, but in the case of B&B siding, shouldn’t the door and window trim be OVER the siding (outboard of it)- so that the casing is in the same plane as the battens? If so, then how does one flash the window back to the WRB?
In response to another window head flashing question, Martin Holladay referenced a sheet goods siding manufacturer’s detail that showed a scrap of siding behind the head casing- which served to get the head casing in the same plane as the other casings- and provided a location for a Z-flashing back to the WRB. I can apply that concept to the head casing- but I don’t know what to do at the sides or bottom.
If I can get this figured out, I suspect that with some jamb extensions, it will be adaptable to the service door and the overhead door.
Thanks for your assistance,
Ben
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Replies
Ben, The doors and windows need jamb extensions to take them out to the exterior of the siding boards. The head flashing is taped to the WRB (insulation) and extends out to the jamb extensions. It should have 3/4" end dams. The trim around the openings is applied leaving a small gap at the flashing for drainage. A small kerf is cut in the head trim to allow it to sit down on the end dams.
Edit: To clarify, while the sides and bottom 1"x4" trim is fastened to the window jamb extensions, the top one is not, and a gap left, to allow it to be flashed.
Does this help, or is something else troubling you about the detailing?
Ben,
I'm not sure that I'll be able to answer your question fully, because I'm not sure of your exact situation.
In general, when installing new siding on a building, making sure that the water management details are correct with the existing windows is tricky. The necessary steps vary depending on how good a job was done by the original window installer.
Ideally, the original installer flashed the rough opening before the window was installed, and all of the flashing at the rough opening directed the water to the WRB. If that wasn't done, you have two choices: (a) remove the windows and start from scratch by flashing the rough opening, or (b) compromise with improvised details, as best you can, knowing that you are working with a rough opening that was never properly flashed.
Malcolm and Martin,
Thank you both for your willingness to help me. Unfortunately, despite it's excessive length, my original manifesto omitted some relevant info:
My plan is to keep the original windows in their current locations (when looking at an elevation) but to remove and reinstall them as "outies". There are no interior finishes, so there is access to tweak the ROs, if needed. And, I can properly flash the ROs.
As they say, I picture is worth a thousand words. I'll post some sketches soon. Hopefully that will make it easier to help me.
Again, I truly appreciate your time and expertise.
Ben
Ben,
Q. "How does one flash the window back to the WRB?"
A. First, flash the rough opening.
Second, install the window without any window casing. (Presumably, the window will be secured by screws through the window jambs into the jack studs.)
Third, install flashing at the window head. The WRB above the window laps over this flashing.
Fourth, install the siding.
Fifth, before installing the head casing, cut a horizontal kerf in the board siding, at the level of the top of the head casing or a bit higher. Then install the head casing. Insert custom-bent Z-flashing in the kerf, with the drip edge of the Z-flashing hanging over the head casing.
Sixth, caulk the kerf.
See attached head, jamb, and sill details. I should have posted something like this in the first place- but I couldn't even wrap my head around it at first. (Martin, I drew these before seeing your last comment.)
The windows are small (i.e. light), around 2' x 3'-6". They were only fastened thru the factory-applied brickmold. I was thinking of doing the same thing- but with a 1 x 4 instead of the brickmold. Note that with the insulation, the furring, and the thick siding, the windows will be outboard of the jack studs, so screwing thru the jambs into the jack studs isn't an option. (Although I could screw thru the jambs and into the blocking.)
Head: (Martin, I think the flashing I drew is similar to your recommendation.) Am I creating a home for insects? Should I just raise the blocking under the flashing- and lower the strapping above the flashing- so that the volume of the cavity is as small as possible?
Jamb: I'm showing the RO with flashing- but the window isn't really flashed to the WRB- like it would be if it had a flange. Is this ok- or is there something more I should do?
Sill: Same question as at Jamb.
Any other impressions?
Thanks again. I really appreciate it.
Ben
Ben,
That looks like it would work just fine. The one very minor weakness I can see is that there is no path for moisture that gets to your sill flashing to drain to the exterior. You might consider cutting a few kerfs in the blocking at the sill to allow this.
I don't think it's an issue, but if you are worried about the jambs of the window having increased exposure by being out beyond the sheathing, you could apply a liquid or peel and stick membrane to them.
Roll the butyl tape onto the replacement style door , seal edges with silicone for added security