VIDEOClick to see full answer. Similarly, which side of the valley do you Shingle first?Each end shingle strip should have the upper corner trimmed and set in a bead of roof cement. NRCA recommends shingles first be installed on the valley side with the lowest slope or shortest distance to the ridge. If all conditions are equal, either side may be installed first.Likewise, how do you attach shingles to an existing roof? The easiest way to tie in the new shingles with the old is with a cut valley (see Four Ways to Shingle a Valley). After the addition roof is framed and sheathed, snap a line on the existing roof shingles 2 in. up from the valley center, and cut through the shingles and underlayment. Furthermore, can you weave architectural shingles? Woven valleys are suitable for three-tab shingles as their single-layer construction allows them to be woven together and overlapped more easily. When considering how to shingle a valley with architectural shingles, you must take into account their thickness and reduced flexibility.Do roof valleys need flashing?Closed valleys aren’t required to have metal flashing, but they should have additional underlayment lining the valley, preferably a couple layers of roll roofing. You can usually see whether this has been done by looking carefully at the bottom edge of the valley.