The opposite would be the building foundation, or the basement floor. (Most thesauruses simply say floor.)
As the roof is the top of the building, it follows that the floor would be the opposite, as it is the bottom of the building.
The opposite of tongue could be palate or roof of the mouth.
Floor
on the roof, opposite letter, and something else
Stalagmite. The opposite, growing downwards from the roof, is a "stalactite".
You would not have to add a ridge board, particularly if you are planning to build the addition roof directly over the existing roof.
The message by the Docks is "on the roof" (that's where the door is to the warehouse). The message about Spyglass is "opposite direction" (when you take the eye test, pick the symbol that faces the opposite way from the one he points to). The message at the BAD Bistro says "fingerprint", which is what you need to get, by posing as a chef to steal the wine glass from the BAD bigwig. yx dro byyp = on the roof kllkoepa zenapekj = opposite direction lotmkxvxotz = fingerprint
Stalagmite. The opposite, growing down from the roof, is a stalactite. (G for ground, C for ceiling!)
the roof of your mouth... like the top part opposite your tongue
I'm not sure if anyone can give a definitive answer but the rate at which the feed water leaves the roof may be a factor.
Fire
If the room is of square or rectangular shape then the diagonal is the shortest path it can choose to walk on. If the room is of triangular shape then it can walk along the side of the triangle to reach the opposite corner.