vinyl tile on plywood question

I'm tearing out some floors in a building that was a daycare, and in the bathrooms there was the commercial grade vinyl tile installed on a plywood subfloor with what appears to be ordinary 15# roofing felt as underlayment. I couldn't tell what adhesives were used. Considering the abuse this floor no doubt went through, the plywood (and the vinyl flooring, for that matter) is in remarkably good condition. Anybody ever heard of glueing tile onto felt? bill
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