A weed barrier needs to be opaque. Roof felting meets this requirement. If you happen to have some on hand, it will make an effective weed barrier.
Also known as tar paper, roofing felt originated in Finland in the 1870's as a waterproofing underlayment for roofs. First, paper was impregnated with coal tar. Later, asphalt was used in the manufacture of roofing felt. Today glass fiber or polyester fabric may be used instead of paper.
The Asphalt Institute maintains that asphalt does not leach into soil. Even so, roof felting is not an organic mulch.
Asphalt is widely used to line the drinking water supply in California, and it is used to line the pools used in salmon farming without apparent harm to either fish or humans. Many wooden planting boxes are lined with roof felting to protect the wood from rot.
Alternative Weed Barriers
Commonly used weed barriers include newspapers, cardboard and black plastic.
If roof felting is found at hand, it will accomplish the task. However, newspaper and cardboard are effective, very easily available, economical and generally regarded as harmless.