http://revach.net/article.php?id=3690

Section: Halacha   Category: Tshuvos
Shevet HaKehosi - Siddur With Tehilim, What Can You Place On Top Of It?

The Shulchan Aruch writes (YD 282:19) that you may place a Chumash on Nevi'im-Kesuvim but not vice versa.  Someone asked the Shevet HaKehosi (2:299) that he read in a sefer that this problem doesn't exist in today's Chumashim since most of the words on the page are Peirushim on the chumash and the words of the chumash are Batel B'rov?

He answered that although the words of the peirushim are in fact more in number than the chumash, since all the peirushim are coming to explain the chumash there is a rule from the Lekach Tov that the Mi'ut HaEichus, quality minority, takes precedence over the Rov HaKamus, majority in number.

Based on this same logic he says that a Siddur that has Tehilim in the back does not have the kedusha of Kesuvim with regard to this halacha, since it is the minority and the sefer is considered a siddur and not a tehilim.  Therefore you may place seforim on top of a siddur.

 Important Note: We try to convey the Tshuva to the best of our ability. We admit that our understanding may not be accurate. One should learn the tshuva to verify the accuracy of our interpretation.  Please also understand that this Tshuva may not be the final word on this topic. One should consult a Rav before drawing any conclusions.