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I seem to recall at some point that a glass floor was not a candidate for mob spawning (which would be nice given that one cannot place torches on glass). However, I can't seem to confirm that information at the moment... is ittrue? (i.e. if I have a glass roof, and a glass loft, do I need to worry about somehow keeping it lit)?

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Mobs can now spawn on slabs as of 1.7 not sure about glass or stairs i am still doing tests. –  Halo49999 yesterday
    
@Halo49999 It depends on how you place the half slabs: Mobs can spawn if you place the half on the "top" side of a block. Mobs can not spawn if you place the slab on the "bottom half" of a block. –  IQAndreas yesterday
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2 Answers

up vote 18 down vote accepted

Mobs don't spawn on glass, ice and half steps, so your loft should be safe as per the minecraft wiki. Spiders can climb glass walls, so you may want to add a small overhang or a row of ladders if there's an entrance.

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...or soul sand if you get some of that. >.> –  Ullallulloo Feb 12 '11 at 19:34
    
What about stairs? As stairs are weird-heighted blocks like steps and soul sand, I would think they might be safe, but I don't know for sure. –  Kevin Reid Feb 13 '11 at 2:49
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From the wiki - "Enemy mobs can spawn on any fully-opaque cube block...". Since stairs are half-transparent to light, I'd say enemy mobs could not spawn there. –  Cyclops Feb 13 '11 at 20:17
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I love that to stop things climbing your walls, you put down ladders. –  Istvan Chung Oct 23 '11 at 17:58
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@Ullallulloo you are wrong, spiders can climb soul sand –  Nick T Jan 21 '12 at 1:44
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No mobs except for spiders (it seems like as of 1.5.1) can spawn on glass. Spiders require a 3x3 area to spawn in. If you place halfslabs so that you have 2x2 cubes of non-opaque blocks and halfslabs around them, they will not spawn. But if you place glass on the same y axis and 2x2 cubes of non-opaque in them, for some reason, they spawn. Don't believe me? Try it. I did it 5 minutes ago.

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They still don't spawn on glass... they spawn on the 2x2 solid blocks, just as other mobs can. –  Arperum Apr 30 '13 at 18:43
    
Glass is a weird block. What's happening is that the spiders are spawning on the opaque blocks, and the glass doesn't count as "solid" blocks that interfere with their bounding box placement, the way slabs do. You see this as well with glass over chests—you can still open them—or glass above rails—you and mobs won't suffocate when your head goes inside the glass. Mobs still won't spawn on glass though. –  SevenSidedDie Apr 30 '13 at 21:31
    
@Arperum The thing is, they don't. I've built my mob trap high up in the sky and my spawn platform is made in the pattern above. There are absolutely NO spiders spawning if I place half slabs there. I waited 10 minutes and there were over 60 mobs in my trap. I went to the chamber above and: NO spiders. I can record it and put it on YouTube if you want... –  SeiKun Apr 30 '13 at 21:57
    
@SeiKun Nothing can spawn on halfslabs that are on the bottom half. –  Arperum May 2 '13 at 10:21
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