Is my stringer connection to this shallow beam going to fail inspection?

The issue is the **effective depth** of the stringer. If that heel is hanging in the air, your hanger is only supporting a fraction of the wood grain. This ussually leads to a horizontal split right at the top of the seat cut because the load is being transferred trough the top half of the board instead of the whole member. Standard joist hangers are useless if the bottom of the stringer isn't fully seated on the metal flange.

Sistering a piece of 3/4-inch structural plywood or an LVL scrap to that beam is the most reliable fix. Iu2019d lean toward the plywood because it wonu2019t split or shrink like a standard 2x12 might. Glue it and screw it with a heavy staggered pattern to effectively drop the depth of the header. This gives the hangers enought surface area to actually do thier job. Itu2019s a common workaround when youu2019re dealing with the weird dimensions of an old timberframe where nothing is square or standard.

But you have to make sure that furring or plywood is properly tied into the main beam. Don't just use standard framing nails. Use somethig like **SDS screws** or structural lags that can handle the shear weight of the stairs and the people on them. If the sistered material isn't rock solid, the hangers are just going to pull the furring right off the timber.

Also, consider adding a 2x4 ledger or a kickblock underneath the stringers if you have the clearance. It adds a secondary fail-safe. Even if this is just for a storage area or a ping pong room, stairs are the one thing you never want to feel spongy. If the original specs are off because of on-site lumber reality, you're better off building it to be twice as strong as the code minimum anyway.

If I go the plywood route, how far below the timber should that plywood piece hang? Should it be exactly flush with the bottom of the stringer heel or is a little extra overlap better? I'm trying to figure out how much "new" surface area I acutally need to create for it to be safe. Also, those SDS screws you mentioned—are those the ones that look like mini lag bolts with the hex heads? I'm always worried about grabbing the wrong thing at the store and ending up with something that snaps. I've seen some labeled "structural" that look like normal wood screws, so it's pretty confusing for a newbie.

Drop that plywood about an inch or two past the heel. You want enough meat so the bottom of your hanger isn't right on the edge, wich prevents blowout.

Those SDS screws are the hex-head ones—way better shear strength than standard lags. Since it's an old timberframe, look for LSC adjustable stringer connectors instead of standard joist hangers. They’re designed for this specific angle and can be bent to match your rise. It’s a much cleaner connection than forcing a 90-degree hanger to work. Also, if you’re worried about it being solid, run a bead of subfloor adhesive between the plywood and the beam before you drive those screws.