Docs can you explain, is root canal a must before a cap and does it help build up the surface to glue cap to?

No and Yes: Not all teeth that have been crowned need root canal treatments, but most root canal treated teeth have crowns. And yes, it definitely help to have the root canal done then build up the tooth cause the build up is now anchored into a more solid foundation. Then the crown can go on top. However, it is not always necessary to do the root canal just so the build up can stay. Talk to your dentist.

Sometimes: A root canal is needed when a cavity invades the nerve of the tooth and kills it or when there is minimal tooth structure between the tooth and the nerve of the tooth and a post needs to placed to help aid buildup of the tooth prior to a crown preparation. So yes, sometimes a root canal is needed to increase surface area prior to crown preparation.