Jamieson, Fausset and Brown on Mark 2:4

4. And when they could not come nigh unto him for the press--or, as in Luke (Lu 5:19), "when they could not find by what way they might bring him in because of the multitude," they "went upon the housetop"--the flat or terrace-roof, universal in Eastern houses.

they uncovered the roof where he was: and when they had broken it up, they let down the bed--or portable couch

wherein the sick of the palsy lay--Luke (Lu 5:19) says, they "let him down through the tilling with his couch into the midst before Jesus." Their whole object was to bring the patient into the presence of Jesus; and this not being possible in the ordinary way, because of the multitude that surrounded Him, they took the very unusual method here described of accomplishing their object, and succeeded. Several explanations have been given of the way in which this was done; but unless we knew the precise plan of the house, and the part of it from which Jesus taught--which may have been a quadrangle or open court, within the buildings of which Peter's house was one, or a gallery covered by a veranda--it is impossible to determine precisely how the thing was done. One thing, however, is clear, that we have both the accounts from an eye-witness.