Liquid Flashing The Threshold of a Large Patio Doorway
Hi GBA Folks,
Bit of a last minute preparation question. I have very large mulled Loewen patio door units that will be installed on a concrete slab out to a covered patio. The transition is all on the same level. The patio concrete hasn’t been poured yet. I have 4″ of XPS foam continuously under the slab, however the foam tapers as it meets the stem wall of the house. At doorway threshold opening you can see where the slab edge ends and meets the foam/vapor barrier taped to the stem wall. (Theres a 1/4 gap where the slab shruk away from the vapor barrier.
Can I safely prepare this threshold by slathering Zip Liquid Flashing over and into the area in question? Essentially coating the entire threshold area of the concrete and up the sides of the RO doorway 3-4″ inches. I have some Prosoco Joint and Seam filler as well as Zip Liquid Flash.
Bit of a mad dash to prepare before the doorways thresholds before door/window installers start on Monday.
Thanks You!
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Replies
If I'm picturing what you're describing, you're asking about creating a sill pan using liquid flashing, and making it continuous across the stem wall, the slab, and the 1/4 inch gap between the two.
Are all of those things on the same horizontal plane, or does the slab horizontally overlap the top of the stem wall at the threshold (meaning the gap is on the vertical outer face of the stem wall)?
In general terms, it's a good idea to have some sort of vertical backdam when you're making a sill pan using a liquid flashing product. That's what stops water intrusion under the interior side of the door threshold. Some sort of L-metal works fine for that (extruded aluminum, brake-bent sheet metal, etc); usually you would lay down some liquid flashing on the concrete, bed the long horizontal leg of the L-metal into the liquid, and then apply more liquid over top of that. There should be a small gap between the vertical leg of the backdam and the back edge of the door threshold, which should be caulked for air sealing (or something like Prosoco AirDam). You should apply liquid flashing a few inches up the sides of the rough opening too, yes.
You will need to know some door threshold dimensions to place the backdam in the proper location, which is simple if you have the door on-site.
If you don't use a backdam, you're relying on a caulk/sealant joint under the door for water sealing, with nothing to stop horizontal water intrusion if that sealant fails. Maybe not a huge concern if the covered patio slab is well protected from rain, etc.
I'm unsure if the Prosoco Joint & Seam is a proper product to use for the 1/4" concrete joint (it may be), but regardless of what you use, it's a good idea to use a backer rod in the joint. This controls the depth of sealant versus the joint width, and prevents 3-sided adhesion. This is important to keep the sealant bond from failing at the concrete instead of just stretching as needed with thermal expansion/contraction.
I have used this method a few times on monolithic slabs (no stemwall joint like you have) with Prosoco Fast Flash for entry doors, and it's worked great.
Hopefully some of the pros will add some experience too.
Thanks Chris,
That was super helpful. I was able just get the threshold liquid flashed before the window/door installers came. Yes the entire area was at the same height so the flashing was mostly to seal the gap between where the slab shrunk away from the vapor barrier. It was really only around 1/16 gap, so no backer rod was necessary. It’s a bit messy until you get used to it, but the liquid flash is a really nice/tough product.
Door/window units are under a covered patio, and the finished floor is the concrete slab, so we went with a caulking backdam. Didn’t have the time or materials to get the L-flashing setup, but that sounds like a good weatherproofing detail.
Thanks again