A sub-roof sheathing (4) is combined with two web members (5 and 6) manufactured from paper/cardboard. The entire unit may be folded into a compact, elongated package which may, if desired, be folded transversally. One web member (6) has a height of fold ridges which is lower than the height of the fold ridges of the other web member (5), and the same goes for the sub-roof sheating (4). When said package is stretched out of its folded position the sub-roof sheathing and said one web member (6) will be stretched to be substantially flat, whereas the other web member (5) will still be provided with stiffening folds. By the aid of one element a sub-roof with underlying air slots and a windscreen above the roof insulation will, thus, be formed.