Should a roofing quotation explain the cause of the problem?

The reported symptom, what was inspected and how, what was found and where, the link between the finding and the symptom, the work proposed, and anything that remains unconfirmed with an explanation of how it will be handled.

None of that is onerous. Any contractor who has actually diagnosed the problem can write it in a few lines.

If the cause is stated and the work fails to resolve the symptom, the diagnosis is demonstrably wrong and the conversation about putting it right is straightforward. If no cause was stated, there is nothing to hold the work against.

It also makes second opinions meaningful, because two contractors can be compared on findings rather than only on price.

Sometimes the honest position is that the route is not yet established and investigation is needed first. That is a legitimate answer and should be stated as such, with the investigation quoted separately.

What is not legitimate is a confident specification built on an unstated assumption.

If a contractor is reluctant to put findings in writing, or responds to a request for the cause with reassurance rather than detail, that is useful information in itself. It does not necessarily mean the proposed work is wrong, but it does mean you cannot yet judge whether it is right, and a decision made without that judgement is a guess dressed up as a quotation.

In that situation, a short, independent inspection focused specifically on establishing the cause is usually a proportionate step before committing to any specification. It need not duplicate the whole quotation process; its purpose is simply to produce a stated finding that either supports the original proposal or shows why a different scope is needed. Where two contractors reach different conclusions after seeing the same roof, an independent finding is often the quickest way to move the decision forward without simply picking whichever opinion is more convenient.