OK, this is technically not a deck skirt, but I think the general principles apply.

I have a room and deck built out on pilings. It wasn't built with any sort of enclosure around it. We get some pretty serious winds here, and in the winter time it got pretty cold underneath (don't get me started on the insulation issue). Some years ago I went underneath and tacked up tarps all around the perimeter. On side is enclosed by the foundation, and on two sides is a deck, so the tarp was tacked onto the beams between the room and deck, closing off that space but invisible from the outside. On the fourth side, the tarp ran along the outer perimeter of the room.

It's exposed and ugly, but in a relatively un-trafficked space, but it's starting to fall apart so I decided to put something more permanent in its place.

My basic approach will be to put in a couple of 2x4's horizontally, for a place to screw on vertical 1x6's.

The space is about 18' wide. The only existing vertical supports are three pilings, one approximately at either end and one not quite in the middle (109" from one piling and 89" from the other). I plan to put in an access door near the further end of the larger space.

There is no face board (it's covered by siding) so the 2x4's will have to be the whole support, meaning I'll need at least two, top and bottom. I'm going to use two of the pilings as part of my vertical support for the horizontal 2x4's; the pilings are not perfectly in line and the leftmost is close enough to the edge of the joist that I won't be able to use it.

I'll need additional vertical supports--both as a place to attach the door, and for additional back support (again: high wind area), especially since I can't use that third piling.

I was going to use 4x4s: for the added rigidity, and because I can then use more screws to toe it in at top, and because I have a bunch lying around anyway. I was going to put a 4x4 right next to that unusable piling (I can't put it right up against it because it has bolts sticking out), have the door hinged to that component (1x6 on 2x4 on 4x4), then another 4x4 to screw the 2x4s onto and latch the other side of the door to. Maybe a third 4x4 in the 89" space.

With that long-winded intro, here's my main question: I was going to bury the 4x4 a couple of feet down. Digging the hole will be an issue: since it's underneath the room I can't use my post-hole digger. Will coming in at an angle from the front weaken it? And at least as important, will I have a problem with frost heave? The top will be attached to the thick joists so it's not as if it can push the whole thing up... I don't think.

The only other way I can think to attach it is to screw 2x4s to the back of the face board, but then I'll have to shim way out on top of them.

Attached are four pictures. The first is the opening, with the tacked tarp. The second is the underside of that lip; you can see the slightly curved surface in the blue tarp where it's tacked onto the piling. The third is that piling that's too close to the edge of the lip (long story why the beam boards are on TOP rather than the side of that piling).

The last picture: last year I made a deck skirt (decorative, to hide the opening). I'm going to model the design of this one on the other one, though the existing one is purely decorative, while the main purpose of the new one is a wind block.

Any thoughts about the vertical supports or anything else I've missed?