If I cast blade barrier, does it have to be anchored to the ground? Or can I have it float anywhere I want that it could occupy, such as cast it above me as a sort of roof of blades?
While there is no requirement that walls be anchored to the ground As @JeremiahGenest notes, there is no requirement that a wall be anchored to the ground unless otherwise noted. See Wall of StoneDDI for an example of a wall that does need to be anchored.
However your plan for a roof composed of a Wall of BladesDDI may not work as you hoped.
Walls (defined on pg110 of the Rules Compendium) can only be adjacent horizontally to at most 2 other wall squares. This means that you could not form the Wall of Blades into a 4x2 horizontal configuration above your head.
You could form a doughnut shape though.
XXX X X XXX
A wall fills a specified number of contiguous squares within range, starting from an origin square. (Player's Handbook 1, page 272.) As a Blade barrier is comprised of spinning blades of astral energy there is no need for it to be supported. So no, it does not need to be anchored to the ground.
(Asking cust service, my reading is that the "up to 4 squares high" wording suggests that it must be based on the battlefield, but I couldn't find anything on the forums to support either side.)
Here is the question I sent to them, I will update with their responses.:
I am trying to answer a question on Does blade barrier have to be anchored to the ground? about blade barrier.
Specifically, as stated, it occupies a zone of area wall 5, as per the normal wall rules, up to 2 squares high. Wall notes that I can "shape it as I see fit" within the parameters given. Does that include making a 1x1x1 square gap in the middle of the wall, or for that matter, eliminating the bottom row of the wall, so that it's floating 1 square above the battlefield?
A more modern text (as we're talking about a power that appeared in PH1 is Bulwark of stone from the most recent DDI earth domain update. It has a line in it about:
: You conjure a wall of rough stone that acts as blocking terrain. The wall can be up to 4 squares high and must be on a solid surface.
Customer service, in an e-mail response to me says:
Thank you for contacting us. I have pasted the rules for creating a wall below, you will notice the squares need to be contiguous (so no gaps) and anything outside of these rules (like floating walls) will be up to your DM to make a decision on as they are, strictly speaking against the rules, because the rules do not say you can do that.
A wall fills a specified number of contiguous squares within range, starting from an origin square.
Each square of the wall must share a side-not just a corner-with at least one other square of the wall, but a square can share no more than two sides with other squares in the wall (this limitation does not apply when stacking squares on top of each other).
You can shape the wall however you like within those limitations. A solid wall, such as a wall of ice, cannot be created in occupied squares.