How far should the waterproofing shingle underlayment be installed up the roofline?

Waterproofing shingle underlayment (WSU) should extend at least 4 feet up from the interior wall. On low slope roofs (down to 2/12) it is recommended that the WSU be used on the entire roof surface. In situations where a WSU is applied over the entire roof it is even more important to use an adequate amount of attic ventilation because WSU is a vapor barrier and will trap moisture below it causing potential condensation problems. Check the shingle installation instructions also.