Here's my question - I am turning a friends small garage into a guest room and I need to place a wood subfloor on top of the cement floor. The room is 7.5' wide by 12' long and it slopes at a slight angle longways. I need to basically cut the 2 x 4 x 12's into 12' long triangles but do not have a table saw. What is the best way to do this?Can I purchase them already cut? Do I need to take them to a specific place that will cut them for me?
If the joists are all the same, layout a master on some 1x material then use a pattern maker bit to rout off your pieces (cut off majority of excess with skill saw first). If they are all slightly different use a straight edge or chalk line to establish your line, then cut off with a circular saw. If your floor is really wild scribe the bottom of each piece, then cut with a band or jig saw. Then lay back in place and mark the top line using a laser to create a global level, or measure and and mark off of a reference point if you want flat but not necessarily "level".
Here's what I've done. I wanted a flat surface that needed no over-layment. Using 2x6's and my level I found my slope then took that information and my materials to my bandsaw and ripped the boards across the width of the board so that the result were long wide wedges. Several of the wedges were 8' long. Then I made a fixture out of a long piece of 3/4" ply with some blocking set in place along the length to support the wedged lumber and ran it through my planer just to make it nice and even. Then I took it out to the location for installation and installed each piece with a few screws right to the old floor. These wedges were about 1" at the thick and nothing at the thin end (they were rather chippy at the thin end). The room, as I remember it, was was about 9'x10' and needed most of its floor leveled. There was a bit more to this installation but this description gives you the gist.
You can do this yourself with nearly any cutting tool you do have that's capable of making rip cuts, though the farther you get from the optimal tool (a table saw) the harder it will be.
If you have a circular saw, you could make the cut freehand, or use a ripping jig.
If you have a jigsaw, you can do it freehand or construct your own ripping jig by nailing a long straight piece of stock to the 2x12 such that the jigsaw's base is always touching the jig.
If you have a handsaw, you can do it freehand if you're okay with the 2x surface not being totally flat.
Make a jig. Tools required: skill saw with Rip blade and edge guide. Dry wall screws,metal straight edge or string.
If the floor is flat, and you have a metal straight edge, you might get a perfect straight cut with two timbers tacked onto plywood with finishing nails on the inside or power-drywall screwed to plywood. Separate the 12' timbers by the floor slope and verify the timber has a flat edge with a long metal straight edge.
Then use the skill saw edge-guide to cut one piece but positioning the wood to be cut just hanging over the parallel edge of your plywood ( 3/4" plywood can be elevated for blade clearance or have blade protrude half way between plywood thickness.
Plan on using the factory straight edge down so any imperfections can be gapped by filler to plywood, rather than concrete.
If the floor is not very flat, the other answers may be preferred. Contact to concrete with frame should continuous for a solid floor but with 2" shims every 16" will work but have more vibration. Shims can be bonded with dots of sub floor adhesive or PL400 if there are large gaps to prevent vibration..
If the concrete gets moist in spring, lay down heavy-duty poly. over the concrete and if really cold in winter, add fibreglass..