The pop tops are usually hollow box section, panels around vents and fittings like aerial with a couple of cross members of light timber ......... When I did a reseal of mine many years back I had in mind if the water had got in and rotted stuff I was going to pull off the pop top and replace it with some 25mm fridge panel and use the old trim around the edges, probably be a better roof, think they came in 2400 x 1200mm panels ... may have needed a few mods but that was the start of a "Just in case idea".