1.   As the ice collects in a gutter, it lifts the shingles, creating an opening for seepage.

2.   First use a pry bar to gently lift the shingles on either side of the vent.

3.   Lift the shingles above and on either side of the pipe and slip the flashing underneath.

4.   The second way is to lift each shingle and dab roofing cement, liberally, under that shingle, then press in place.

5.   Try lifting the shingle above to allow the nail to be driven home under the shingle above.