Very common issue here. If you've ever noticed, I often include seperate wall, and floor segments in my packs. That solves this problem when building levels, and mixing various floor, and wall pieces together.
You can use FPSC Segement Editor, to easily create seperate wall, and floor segemens. Or you can use a program like
Firma (free) by DarkFoil,
Signs (if you managed to get a copy before it was discontinued), or
Magic FPS to create rooms with the textures you want to use.
Firma: Recomended!
http://forum.thegamecreators.com/?m=forum_view&t=116899&b=24
Magic FPS: $15.00- but includes added features like custom segment pieces such as arches, and ramps, as well as the ability to create custom doors.
http://www.uberdroid.com/
FPSC Segment Editor: Recomended!
http://www.thegamecreators.com/?m=view_product&id=2001&page=Files
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