here's the problem, the stonetile-roof bitmap which is outside my wooden house (alt wallset) shouldn't be seen :
usually, walls at front1 hide the part of bitmap of flooritems on side1 (left or right), but the flooritem i use for stonetile-roof is displayed over the ceiling because its side bitmap rises above the wall bitmap...
I can see what you're saying, but I don't understand what you think the correct behaviour should be!
The problem seems to be caused by human interpretation of the graphics rather than any issue with the engine. For example, if your side "wall" graphics were of tall bushes instead (like the DM-II thicket, only bigger), then you would expect them to poke out from above the front 1 bitmap...