How much does a beam actually need to sit on the post? Thinking about a corner joint I saw

I notched my beam seats on my own deck about eight years ago and I made myself a rule: either the notch fits or the hardware carries the load, but I never want to be guessing which one is doing the job. On my 6x6 corner posts I used a circular saw and chisel to get the seat flat, and I checked the beam acutally sat full on the bearing before I bolted anything. A notch that only catches part of the beam would have bugged me enough to pull it apart and redo it.

Your brother-in-law's setup sounds like the worst of both worlds. If the beam is resting on part of the notch, the load is squeezing into a smaller patch of grain than the cut was meant to give, and the gap on the side means the bolts are prolly what is keeping things from shifting. That can stand for a long time, gravity is patient, but it is not what I would call a clean load path. If it were mine I would either shave the notch deeper and wider so the beam seats square, or scrap the notch idea and hang the beam off proper hardware sized for the post. Half measures are what I would lose sleep over, not the notches themselves.

I'm firmly in the hardware camp, and I stopped cutting notches altogether years ago. When I rebuilt my back deck about six years ago I used face-mounted post caps and carried beams on the hardware instead of on a cut seat. The big selling point for me was that a notch takes away wood right where the post is doing its most important work, and if you cut it a little deep on a 6x6 you can't un-cut it. With a cap or bracket the post stays full section and the beam sits on top where it belongs.

That said, your brother-in-law's corner sounds like the worst of both worlds, a partial seat plus bolts. The gap on the side would bug me too, because that means the beam can rock slightly under load and the bolts take a pounding they weren't really meant for. The ten-years-standing thing cuts both ways. Wood is forgiving and decks sit unloaded most of the time, so plenty of marginal details survive. But a corner post on a deck is usally carrying more than any other post, and if the beam ever gets loaded hard, say a hot tub, a big snow, or forty people at a cookout, that's the joint I'd want to be boring.

If it were mine I'd jack the beam up slightly, shim it square into the seat or add a proper cap on the outside face, and stop wondering about it.

This is helpful beacuse I've got something similar on my own back steps landing and now I'm second guessing it. I stood there with a flashlight last night trying to see if the wood is actually touching under there or if the bolts are just holding it in mid air, and honestly I couldn't tell. Is there a trick to checking that, like sliding a piece of paper in the gap, or do you have to loosen a bolt and watch what moves? I'd rather know now than find out the hard way someday.

One thing I'd add about the ten years standing part, wood keeps shrinking across the grain for years after a deck goes up, so whatever contact that beam has today is probably less than what it started with. I checked my own beam seats with a feeler gauge last summer, the thin wire gauge I use for spark plugs, and found one side had opened up maybe the thickness of a couple sheets of paper since the original build. Also worth watching the bolt holes, if the connection has been doing its job through shear all this time the holes can wallow out oval and you'll feel play when you push on the beam end.