If your roof only leaks during winter months, I doubt anything wrong with it.
Having heat loss due to poor Insulation dept in the attic in most cases why the roof seems to leak. Before spending thousands on a new roof, have a professional evaluation to identify permanent solutions.
Also having poor ventilation in the attic is another no brainer. Stop spending thousands and address the source of all the problems. Bandaid can only last for a short period of time.
Flat roofs may or may not have ventilation. Commercial applications have no attic space and insulation is right under the roofing membrane. As long as 2.5 inch insulation with vapor barrier is valid, type of membrane can be modified Bitumen, epdm or tar/gravel. Residential flat roofs do have a small crawl space for the Insulation.
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