This is for our house. Time for something for us!
Looking for detail/drawing for the above. The exterior wall is 11.5" thick. Would like to use white oak for the threshold.
So your suggestions would be great!

The very wide walls will require a 2 or 3 piece jamb to make the width, to panelize it to limit movement. If 2-3 pcs, then T&G them and set with a gap so they can move a bit. The sill should be no more than 8" or so, and even flatter, with exterior a down step to porch or porch surface running right up to the sill. If it is pocketed in the masonry, isolate the sill with lead or copper or aluminum or something to keep it dry.

Our drawings typically have the bronze interlock for door bottoms, but you may have other preferences.

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I would make the jamb in two pieces and hide the ship lap joint behind the stop. It would be an applied stop like an interior door.

White oak is the best choice for a sill. To limit the movement. Follow I would coat all 6 sides with epoxy resin wait a week(oil based finishes do not stick to epoxy for a week as it cures)and varnish it.

Use a high quality polyureathane adhesive/sealant like Sika or PL to waterproof the jamb/sill joint rot will start there if any wear. Follow David’s basic sill design. I would keep the sill above the flooring to gain height. Cutting out subfloor is not a great idea. 10 degrees is ideal to shed water. That is a lot of height in a 12” wall. I would also rip many saw kerfs on the back hoping it will limit warpage.

Adam has it right, but I disagree on two points:
Saw kerfs on the back to limit cupping. If this is needed, then there is far too much water moving under the sill - and that is the real problem. I have never seen a sill warp - cup or belly - in all my working life.
Finish: Epoxy coat is fine, until it fails. And it will fail. When that happens, watcha gonna do? Chip the stuff off? Remove the unit?
I suggest that since all finishes fail in the weather, leave the White Oak to weather.

It takes a bit of talking to the new home owner about why this is best, or you can let them finish it every 3 months, and still strip it every year. They won't rot, and the maintenance is an easy sell. They just need to get over their desire to have everything perfect.

Must be a regional thing but Dave's drawing wouldn't meet code in our area as a trip hazard.

Thanks for the replies. How about the sill width? I would like the door flush with the interior wall. So I am still needing a "wide" threshold.

tb - Yes, I wondered when or if that would ever come up. It hasn't. We do more and more flat sills low to the floor - 1/2" max height by request.
However, that makes a walk off rug impossible at the door. Once they hear that, they want the high threshold.

Tom - Run your exterior stoop materials right into the frame opening and a 7-8" wide sill. You will have jambs out there with no sill under, but they are sitting in/on something, so paint the ends with epoxy.

I do my epoxy and finish alittle different so you don’t have it “chip” as David says. I thin West System epoxy with acetone to allow the epoxy to soak into wood, rather than just lay on it. This does two things, it acts as a good preservative and it hardens the wood. While white oak is both hard and a good weather resistant wood, the epoxy makes it that much better. The down fall is epoxy is UV sensitive so you must topcoat with something to block the sun.

David, I continue to think through the ideas and was wondering. Have you ever fabricated a 2 piece threshold in order to accommodate a thick wall?

Yes, probably. But I would object since a two piece implies movement, which implies a gap, which implies a water trap, which assures us of problems. Also at 10 degrees or 5 degrees, it is a slippery slope to be treading upon.
I would encourage a masonry sill, or even wood, but let the exterior stoop materials make up the width.

I will post a pic showing the situation. Maybe this might explain a bit better.

I don’t think wide one piece sills are good construction. I have had good luck with 2 and 3 piece sills. The joint is not glued but siliconed. Also a good way to attach sill horns.
Please ignore the odd laminations on the green pine sample. Just made from some odd scantlings I had.

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Hi Joe,
Like your sill extension. Do you use any fasteners to back up the silicon joint?
Do you apply it on site after the jamb is installed?

Thank you,
Fred

Hi Fred,
Usually a screw on the siliconed ones. See crude drawing. We glue a lot of them, just depends on the situation.

The sill horns or extensions we usually leave loose and let the carpenters install. Sill horns attached are a pain to transport and store in the shop.

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John,
Thanks for this drawing! This kind of what I had been thinking. My idea would be to create the correct angle on the joists and rim joist in order to provide the proper support.