The chain of trust is described here: https://letsencrypt.org/certificates/
There is a link to both Root Certificates and Intermediates that LetsEncrypt uses.
If you are looking at broader CACert Bundle for example for wide range troubleshooting I suggest you review the Mozilla CA Bundle
https://www.mozilla.org/en-US/about/governance/policies/security-group/certs/
Note that for the chain of trust to work you should have both the roots and the intermediates. The Server should serve up the intermediates.
A list of all the included roots can be found here: https://mozillacaprogram.secure.force.com/CA/IncludedCACertificateReport
Andrei