Aquaseal No5 paint or trowel on. overlap joint 1.5 inches top and 2 inches at bottom.
I could be capillary creep which can be stopped by using small 1 or 2mm thick plastic to make wedges to place under the top sheet/s so at joint the top sheet sits 2mm above the bottom sheet so breaking capillary action. Try the wedges on one sheet use wood if you have no plastic. If it cures the leaks then don't bother with aquaseal just get plastic for wedges. If sheets are iffy under joints then yes go on the aquaseal route
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February 2011