All Brickwork needs a good strong foundation to take load bearing , only lay on brick or block, never on shingles or tiles, remove shingles and wall plate back to Brickwork and build upwards, with lead flashing to roofworks.
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Yes firstly a shingle roof would not support the weight of bricks or blocks and it would need to be on brick or block for the bond of the bed of cement
Yes, the shingle will provide no real foundation to have a brick laid ontop of it, I would strip the shingles back and lay brickwork off the original brickwork/blockwork/foundation