What is the meaning of "v'Safun ba'Erez"?
Rashi: It was a roof over the Aliyah (upper story). Radak citing Targum Yonasan - it consisted of cedar planks.
Gra: There was a ceiling on the pillars. Additionally, there was a cedar roof on top of the Tzela'os.
What are the Tzela'os?
Rashi: They are walls on top of the Kerusos. [Each wall] was four walls, one on top of another, along the length of the house. They had window openings, for this was a summer house for air. The roof was on top of these walls.
Radak: They are the Kerusos mentioned (in verse 2). Also Yonason, who translates Mechitziya means like this, for this is a wall. 1
Gra: They are rooms that stood on the pillars after they were covered with cedar beams. Each room is 10 by 10 Amos, the Shi'ur of a Traklin (Bava Basra 6:4). Each of the three rows had 15 rooms, for it was 50 long (the width of the house) and 30 wide (the distance between rows of pillars. 2 Alternatively, each room is 30 Amos long (the distance between rows of pillars) by three Amos. There were 15 such rooms in each row of the roof, along the width of the house, which was 45 Amos, after deducting five Amos for the thickness of the walls between rooms. 3
It seems that Radak holds that the Kerusos were very thick, and comprised a wall. (PF)
The Vilna Gaon did not explain why in this Perush, the thickness of the pillars themselves counts in one direction (the house was 50 wide including the pillars) but not in the other (the distance between rows of pillars was 30 excluding the pillars. - PF)
Malbim: There were 15 such rooms in each row of the roof; we deduct five Amos for spaces between the rooms. Alternatively, each room was three and a fifth Amos wide.
How were the ceiling planks placed?
Rashi: They were in three rows of 15 each; each row was over a space between adjacent rows or pillars. The length of each plank was along the width of the house, over a gap between adjacent rows of pillars.
