First off, do check to see if it's wind-driven rainwater that's running along the underside of the sheets.
If not, then a common thing is for the sheets to crack with age.
It's rarely a good idea to apply coatings on the inside. It's rather like dealing with the problem "after the event". Even if you can get a sealant to stick well, further water will eventually force it off.
If you do need a quick inside repair, just go for general purpose silicone sealant squirted from a frame-gun.
For outside, any of the "Ruberoid" mastic products smeared on with a simple wallpaper type scraper. One of these on this page (some are ok for external as well)...
http://www.screwfix.com/p/isoflex-liquid-rubber-2-1l/36010
If you want to further, you could even use the messy old "Turnerising" system the used to be applied to roofs to stop slates from slipping.
A coat of this first..............
http://www.screwfix.com/p/cementone-aquaprufe-5ltr/36405?_requestid=31077
Then lay glassfibre netting over the whole (possibly a garden centre if it's still available ......)
Or this maybe ..............
http://www.screwfix.com/p/scaffold-netting/95543;jsessionId=BMFnRFvPkfFnByn2TtnDS7lvMT11ZB1yg1v3wz9dHnPJFKLVl7KV!-905979492!1359294383619#
Then a couple more coats of Aquaprufe.
The net is just for reinforcing. You may well think it unnecessary in your case.