Connection detail: wood deck floor to wood floor system
Bosco1956
| Posted in General Questions on
Is there a recommended detail for flashings at the connection of a wood deck floor system ledger joist, to an existing wood framed residential floor system? House has 2×10 floor joists which are perpendicular to the exterior wall.
Question: is should there be a membrane flashing applied to band joist before attaching the ledger joist? Thanks, and direction would be appreciated.
Bosco1956
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Replies
Bosco1956,
There are two basic approaches to attaching deck ledgers:
- Fasten the ledger onto the rim-joist through a waterproof membrane that is properly lapped with the WRB on all sides, then flash the top and bottom of the ledger. When I do this I like to set the joist-hangers up 3/4" so the top of the deck joists are that far above the ledger. This leaves a gap for water and debris to drain off the cap-flashing.
- Use spacers - either washers, or a proprietary system like Maine Deck Brackets - to keep the deck ledger away from the exterior wall. If you do this make sure to clad the area behind the ledger with something that doesn't need maintenance, as it will be inaccessible once the deck is built.
Whichever way you go, remember the there are lateral load requirements for decks. The easiest way to meet them is with this Simpson hardware: https://www.strongtie.com/resources/product-installers-guide/dtt2z-installation
Here's a visual which is close to what Malcolm is talking about.
https://hammerandhand.com/best-practices/manual/9-structure/9-1-deck-ledgers/
The third approach is to not attach the deck to the house. The deck can be built free-standing, with a small gap between the deck and the house itself. I've found this approach can often be somewhat easier and cheaper than using the Maine Deck Brackets, and there is no penetration of the exterior shell of the house at all. Simple bracing handles the lateral loading. Note that his works best for low decks, but can certainly be done for tall ones as well. The economics start to shift in favor of attached decks when they get tall, though.
I'm a big fan of this. We did it when we replaced a 45 year old deck and fixed the rot at the ledger. I expected to see the deck move relative to the house and see the 1" gap grow and shrink seasonally, especially given our clay soil, but it doesn't seem to have shifted at all.
But do be aware that some code officials might have issues with it, and you might need an engineer to make sure you have the bracing done right or to convince the code official that it's OK.
I like and use the freestanding deck approach when possible. Maine Deck Brackets are useful for some situations but they often seem to be under-engineered--you probably need more of them than you think you do. I usually end up just using a conventional ledger, but my borrowed trick is to drop the ledger by 1/2" to allow for drainage.
Peter,
When I started out some decades ago, many deck ledgers were flashed with a bead of caulking at the top, and most weren't Pt lumber. I've looked at deck ledger failures both ar0und me and those that make the news, and have never seen one that didn't include either design or execution that wasn't obviously flawed.
I'm a bit mystified by how big a problem flashing a ledger is seen to be. If you use either of the ways I described with the amount of diligence you bring to other building tasks, it will perform well and never cause issues.
Part of what I like about freestanding is the complete lack of thermal bridging.
I also think that it's good practice to choose things that are less dependent on the builder getting the details right, even when you know that it's perfectly feasible to get the details right.
But I'm not dead set on it--there is some height at which the bracing requirements become difficult enough that freestanding is no longer worthwhile.
Charlie,
What you say makes a lot of sense. As long as the deck isn't too far above grade why not make it freestanding. I'm anti-deck in general (says the guy who just finished up drawings for a house with over 700 ft of deck). Too much maintenance, and usually a poor relationship to the surrounding landscape are my main complaints. If possible I opt for an patio at grade instead.
Thank you for all comments.