The best position for the panels is on a south-facing roof with a roof slope of between 25 and 70 degrees.
Although a south-facing roof gives the best performance, panels pointing within 70 degrees of south and angled between 10 and 90 degrees can still achieve 90% efficiency. Even a panel sited on an east or west facing roof with a roof slope of 20 to 40 degrees can reach an efficiency of 85%.
For south-facing panels a more steeply raked installation (towards 70 degrees) is best for year-round performance. Lower installation angles (towards 25 degrees) are better for sites where the panel does not point south. Angles of less than 20 degrees should be avoided as contamination with dirt and bird droppings can become a problem.
Viessmann's Vacuum tube collectors are more flexible than flat solar panels as the individual tubes can be twisted towards the sun. See a diagram of where different Viessmann solar products can be used
Click on Solar can advise on the best products from the Viessmann range for your requirements.