Abarim Publications' online Biblical Hebrew Dictionary
There are two separate roots רצף (rasap), which don't seem to have anything to do with each other. Both roots appear in cognate languages and are clearly distinct.
The root-verb רצף (rasap I) probably means to fit together or to fit out. It's used only once, in Song of Solomon 3:10, where the choir besings the travelling couch of king Solomon, with its interior lovingly "fitted out". This verb yields two derivatives:
- The feminine noun רצפה (rispa), meaning pavement (Ezekiel 40:17, 2 Chronicles 7:3).
- The feminine noun מרצפת (marsepet), also meaning pavement (2 Kings 16:17 only).
The root רצף (rasap II) doesn't occur as verb in the Bible, but, judging from equivalents in cognate languages, it probably meant to glow or something in that direction. This root's sole derivative is the feminine noun רצפה (rispa), denoting a cooking stone or glowing coals (1 Kings 19:6 and Isaiah 6:6 only). Note that this word is identical to the first derivation of the previous root.