i am lowering my basement floor. the existing concrete slab is only barely 2 in thick but i will make the new 3 in.
Q1: how much gravel should i use under a 3 in concrete slab ? i was thinking 1 in.
Q2: a matter that puzzles me is setting the frame so that is absolutely 100% level. i will be doing it in stripes so the frame will actually be only one-dimensional (from one wall to the other, i.e. not a box). should i first set the stakes and then nail the 1x4 to it or make the structure first and set it in all together? the former sound easier to do and make it flush but i am afraid of screwing up the stake position while nailing the board to it.
Q3: should i get a 48 in level for this or is a 24 in one good enough ?
Q1 the deeper the gravel the better. All concrete will crack given time that is guaranteed, but the better gravel, and proper tamping (use a tamping machine) will prevent it from cracking to often
Q2 I would use 2 x 4 minimum, you should have at least 3 and a half inches of concrete. use the metal form stakes every 4 feet to prevent bowing of the concrete. Spread a thin layer of Kerosene on the 2 x 4's before placing to prevent the concrete from sticking. Once you've poured the concrete tap the outside of the boards along the framing to settle the concrete to keep it from creating air pockets.
Q3 I would definetely use a 4 foot level and double check it with your 2 foot level to make sure that it is not bent by placing it on top of it. Both bubbles should read the exact same. If not buy a descent level (Stabila make preferred)