More Energy-Smart Details
With wall assemblies that include continuous exterior insulation, porch and deck connections can be a challenge for designers and builders alike. When making a structural connection, we need to minimize disruption to the four control layers and keep the details straightforward, buildable, and cost effective.
At Just Housing, we came across this challenge when designing an up-down duplex for which we wanted both units to have porches that would be sheltered from rain (see rendering below). In this post, I’ll explain the approach we developed and how it keeps occupants, builders, and structural engineers happy.
For context, consider the standard Just Housing wall assembly, which uses 2×4 or 2×6 studs with plywood sheathing sealed as the air barrier. Over the structural sheathing nailbase panels are installed for the fastening of continuous insulation and the siding. We put the drainage plane to the outside of the nailbase panels and specify a water-resistive barrier (WRB) sheet good, installed shingle style and integrated with the window, door, and base flashings. We also specify a class III vapor retarder at the interior face of the drywall.
Thermal and vapor controls
In many ways, the porch-to-house connection is straightforward. The porch joists are hung off a 2×10 ledger that is screwed through a layer of polyisocyanurate rigid insulation and into the house’s floor framing. The differences come with integrating the thermal, air, and water control layers. I’ll walk through the sequence of assembly to explain how we manage each of these layers.
After the sheathing is up and the air-sealing is complete, the nailbase panels are installed with a 9½-in.-tall gap to accommodate the 2×10 ledger to follow. This gap is then filled with 1 in. of polyisocyanurate foam insulation. At this point, we integrate the water control layer; flashing tape is applied at the…
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9 Comments
Leah,
Nice buildable detail. Having the ledger taller or shorter than the joists both make things a lot easier.
Thank you, Malcolm.
Malcolm,
I'm considering a detail similar to this and was wondering if you could provide a little more info on why a ledger taller or shorter than the joists makes this easier?
Rob,
The problem with a flush ledger is if you put the flashing over the deck boards (the way Leah has), water can get behind the ledger. If you put the flashing under the deck boards, they create a dam so debris and water have nowhere to go.
Leah has cleverly solved that problem by having a taller ledger, which blocks any water intrusion.
I go the other way and use a shorter ledger. The flashing runs along the top, down 1" or so, and gets sandwiched between the ledger and the ends of the deck joists. The gap creates a drainage path between the bottom of the first deck board and the flashing over the ledger for debris and water.
Malcolm,
Thanks for the clarification. I see that Leah's detail with a ledger taller than the top of the deck boards keeps the top of the ledger above any water draining back toward the ledger, blocking water intrusion behind the ledger. Your approach, keeps water away from encroachment behind the ledger by flashing directly over the ledger rather than the deck boards. It took me a few minutes of thought before I was able to see the advantages. Thanks again!
Malcolm, thanks to you, I have been a short-ledger fan for many years now. It's an elegant way to deal with the inevitable debris.
Michael,
Doing the rough sketch I was reminded of how we detailed ledgers when I first started in an architect’s office almost four decades ago. They simply didn’t warrant a drawing, and were lucky to get a bead of caulking at the joint between them and the #15 building paper they were mounted on. No wonder they subsequently became such a problem.
Malcolm, that's about when I started building things on summer and winter breaks, as a teen. My first couple of employers were pretty sharp and I remember using bituethane, which they called "bitch," behind the ledger, and/or using standoffs. We first used Maine Deck Brackets in 1996. But I still see other builders using nothing but a short Z-flashing over a ledger that's flush with the joists.
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