Presuming that we are talking about the collecting and drinking of rainwater in an emergency, rather than under normal conditions, then I would not worry overmuch about lead in the water.

Lead is a long term chronic poison and unless present at most improbable levels is of little short term concern.

Presuming that the roof is not made of lead, then the only source of lead in the water is industrial air pollution.

Whilst times are normal, I would drink city water, or deep well water, or bottled drinking water.

In a short term but serious emergency I would drink roof water after boiling or chlorinating to kill germs, and not worry about lead.

In a long term TEOTWAKI situation then industrial activity and the pollution that it produces would cease within a few months.

When collecting roof water, it is desirable to discard the first flow of water from the roof as this will be contaminated with bird droppings, dust, and leaves etc.
Automatic devices to do this can be improvised.
If water be very scarce, then this first flush of water can be put on the garden or used to flush a toilet.