Around the sides of the vinyl upper apron on most pop top caravan a series of elasticized 'bars' known as 'Pop-top tension straps'  are installed into sleeves or pockets around the insides of the vinyl...the purpose being to force the apron to bend towards the inside of the caravan and to reduce the possibility of the vinyl becoming captured in the gap as the roof is lowered......on each poptop caravan we have owned these devices have failed quite early on and have require replacing...it is a fiddly job and many owners will have them replaced by a caravan repair shop....sadly the replacements, if of original equipment manufacture, will of course fail again after not a long time. 

I have found a better method of dealing with this issue....our 2010 Jayco 17' Outback needed these things replaced and this is how I did it....read on...