I'm just curious - in reading more about subflooring I see that a lot of sources are saying to use plywood with tongue and groove. Why? What is the benefit of tongue and groove plywood as opposed to plywood (of the same thickness) with flat edges? Does the interlock help the subfloor be more stable? Does it protect against leaks?
The way we lay flooring is to use subfloor adhesive and lay a bead down the whole length of the groove. You wedge it together till the gap totally closes. Doing this in addition to using tongue and groove plywood/OSB accomplishes two things:
You have an airtight floor membrane. This gets rid of heat loss and also the intrusion of moist damp air possibly mold contaminated from the crawl space.
You have a much stronger floor that is seamless which can carry more weight and not have bounce because the edge is supported by and locked into the adjoining sheet.
Why you shouldn't use standard plywood -> if you just butt plywood up to create the subfloor, you get this neat sawing action from the unsupported edges that slowly fills the crawlspace with sawdust as the wood eats away at itself, causing the seams to open up. It also groans nicely if you have the right amount of frictional contact and leaves you with a spongy floor because there is no support from the mating plywood sheet along the length of the seam to prevent differential motion between the two sheets.
Tongue and groove products offer extra stability at joints. It reduces movement and helps avoid uneven points in the sub-flooring where two pieces come together.
Not sure what you mean by "Does it protect against leaks"
The biggest reason I can think of is due to the weight that it may have to withstand. Tongue and groove gives added support at the seems and makes for a more solid floor. If it was not tongue and groove, you could possibly step hard enough around a seam in the plywood and see the seams separate a bit. Plus, tongue and groove will help force a more level floor. Leaks have no bearing on why they are used.
Well let me say this...
Plywood of the same thickness is the same. As far as the grove tongue and grove, you are to leave a 8th inch gap between the sheets, this is done with a 8d nail. now if your going to insulate the floor, then there won't be a problem... The problem is this, companies have to many people fooled in to following what ever they say, that is way tongue and grove cost so much more. look at the walls of a house built now a days, that sheathing is NOT tongue and grove 99% of the time, so they tape and cover with a layer of something, or use the new water proofing sheathing...
If you have ever lived with a non-tongue/grove plywood (or chipboard) subfloor, the answer will be obvious: it is to prevent squeaking, creaking, and grinding from people walking over it.
Squeaking may be a symptom though, even though it can be darn annoying. The squeaking caused by the plywood rubbing is wearing the edges of the sheets and not re-distributing the weight. A heavy piece of furniture pushing down at the edge will bend the floor and eventually remove its installed flatness.