I have a metal gutter connecting the house roof tiles to a large veranda and indoor pool area. Rain falls into a zinc gutter in the valley between the two structures. Unfortunately, once in the gutter, the water does not exit properly into the downpipes. There a down pipe at each side of the roof. but the ends of the gutter are higher than the middle so I constantly have 2-3 cm of water in the centre of the gutter but the ends (downpipes) are dry. Is there a self levelling compound that I can pour into the middle of the gutter that will force the rainwater to exit in either direction?