A state of the art poly tunnel (10) is fitted with a particular light permeable cover (22) extremely thin, but durable. On a hot day the material (22) extends unfortunately forming depressions for the accumulation of rain water and offering a grip for heavy winds. The new design of the structure assembled of several vertically and inclined positioned metal pipes (24) is fitted with additional beams (24) underneath the original ones and with an extending device (26) inserted between both layers (24). The cover (22) is tightened if required by adjusting a screw (42) at the device (26).
