Three Questions for Building a Roundwood Timber Frame on Boulders

A recap of the video:

1 – How can I stably raise one side of a boulder so the surface is flat? What's the most stable thing to put under? Can I get away with putting rocks underneath, if so should I stabilize them somehow?

2 – How do I transfer the surface of the boulders to the legs of the crucks, as both of them are separate up until I finally raise the crucks? Are there any special tricks, like clay and paper mache?

3 – The crossbeam of the crucks, can it also support heavy loads or is its purpose primarily to stabilize the crucks?

Maruf, here's two of Mr. Chickadee's videos https://youtu.be/YBhgge17vw4 and about setting foundation stones and scribing to them. The second one shows setting the stone in more detail, I think. It's slightly different size than your boulders, but seems like a stable footing.

In both cases he's putting the timber in place to scribe it. His posts are just vertical/plumb and easy to locate without being assembled first.
I could imagine two scenarios for you: doing the same... directly scribing to the assembled frame requires raising it twice, once to scribe and once again to erect the building. Or, an indirect method of making a negative cast (mold) of BOTH stones in relation to each other(connected by a board?), then a positive cast (again maintaining the relationship/connected) that you could use to scribe the posts to while laying on the ground. The second method seems like a lot of extra work and materials. You will already need a way to raise and brace your bents while you erect the structure, so why not use it for scribing as well.

I'm not sure how "heavy" your load is? a water tank? sleeping loft?

A couple of things come to mind;
- Crucks is the term for either of a pair of curved timbers extending from ground level to the transverse beam or ridge of a roof and forming a structure frame in a medieval timber-framed house.
- I believe your application is using poles.
- 25mm pins inserted into the rock and also inside the pole base may be a good idea, with the pole base carved to fit the rock a bit better.
- I need more information about the crossbeams to understand your question

Thanks for sharing your thoughts. I've seen this video, however, I don't see how to apply his solution to my situation, hence I'm leaning towards making molds. The frame raise requires a lot more people and risk, whereas the casts I could make myself.

@John, those round wood poles in the video are indeed crucks, according to Ben Law's Roundwood Timber Framing book, that I'm following.

The second floor would support a sleeping loft, basically a bed, maybe some drawers for clothes, maybe a desk and what not

Mr. Chickadee's stones were chosen for their domed top, which would both allow moisture to drain away from the joint with the post, and to allow the socket cut into the post to "grip" or "cup" the top of the stone.
Your stones are much larger and flatter... If you were successful at levelling the tops, and all at the same elevation, you could just assume a flat/level plane and just make "level cuts" on your posts?
Maybe the stones are flat enough at the bearing points to ignore scribing the posts?
Two other alternatives to scribing could be to get the boulders close enough, and flatten the post locations with a chisel; or when raising the frame, bed the posts on a pad of mortar or epoxy.

I think the pin or bolt in the stone to fix the location of the posts is a good idea.

Another option would be to scribe your sill/floor framing system to the stones, and join the posts on top. (Assuming an elevated wooden floor)

The core function of the boulders is to solidly connect the frame to the ground, and you do not want them raised up on smaller stones as one seems to be in the video. Dig down into the ground as much as necessary to bed them firmly. (Edit: the small stones Mr. Chickadee uses are bedded deeply into the ground and form a continuous base for the boulder. Setting them on edge as he does would allow the boulder to push high ones down for even support, while using small stones flat would require the bedding to be nearly perfect to start with.) The cruck blades will thrust outwards, so you do not really want the tops of the boulders dead level; I would set them so the top faces slope inwards a bit, while draining away from the post bearing points as much as possible. You really don't want any rock surface to drain towards the post if you can avoid it. The post line of thrust should ideally point to the center of the bottom of the boulder, so that it is not causing any tipping force on the boulder but just pushing it straight into the ground.

You will need the cruck blades and the crossbeam assembled before raising, and it would be simpler to measure the base fit while assembled than before assembly. Your timbers are relatively lightweight, and I would thing preparing a gin pole or whatever raising mechanism in time to raise the cruck to scribe would be the most reliable method. With block and tackle (compound pulleys to reduce the effort needed) you should be able to raise the crucks yourself. Just be sure to put a stop frame in place so that the cruck does not continue moving and fall over once it reaches vertical. I would try a best guess to precut the cruck bases so that they are somewhat close when you raise the cruck for scribing. That would make the task easier and more likely to be accurate the first time.

The crossbeam you show seems to have a series of deep notches in it, and would have very little strength to carry any load. If you use a timber of that size without cutting out those big notches, it would be plenty strong enough to hold a sleeping loft. Notching into the side of the timber for joist seating would be fine, just leave at least half of the top surface intact.

A point of terminology for those new to cruck framing: The traditional form of crucks in English medieval construction (one of the most highly developed regional traditions) was a pair of curved timbers extending from the base of the wall to the ridge, reducing the amount of interference of the timbers with the interior space. In Eastern Europe, it seems there were mostly softwoods which grow very straight, and curved timbers were not available enough to develop a tradition of curved cruck blades. Ancient frames that survive use A-frames more than what I would call crucks. Nevertheless, "cruck" is the term for the slanted poles here.