As Jud stated in his, this post is based on the limited information that you have provided.
Typically, there is a date of loss (DOL) associated with a claim assignment. You are there to determine the damages resulting from an occurrence on the DOL. If certain damages pre-date the DOL, they should be observed and reported, but you should ask the carrier if they wish an estimate of those damages. Some carriers are quite benevolent and will ask an adjuster include them in their estimates, while others will not. Benevolence should not be confused with obligation in these cases. That said, there could be code upgrade allowances and a host of other factors that could benefit the insured in the situation that you described. If the roof is not properly installed and was damaged, it could go either way, but you need to make sure that whichever way you lean is supported by policy language. If it's a standard ISO or AAIS policy, the vast majority of those policies have specific exclusions for "mechanical defect", "inherent vice", "improper installation" and several other terms that may apply to your loss (with limited info, there is only so much we can help with here). If it is a manuscript policy, you never know what might be in there and you have to read it. If you do find something that tells you to lean one way or the other, keep reading; rest assured that you will find something that contradicts it :).
If there are "pre-existing" damages, as you put it, you should make mention of them in your report and have the insured file a claim with a proper date of loss for those damages, as they are unrelated to the loss that was reported and most policies have a "per occurrence" deductible. If the insured files a claim for damages that pre-date policy inception with a corresponding DOL, well, the answer there is self explanatory. The only policies that I have ever seen that are retroactive are PL policies, and that would not apply here.
This really all comes down to reading the policy and it's endorsements. As I tell our field guys,our examiners and myself on a very regular basis: "If you are going to say no, or you think you should say no, you had better be able to justify it with policy language.". Every single "no" that we have ever presented a carrier has been backed up with policy language, so that when the insured (or their contractor or their counsel or their brother in law's cousin's best friend who knew someone who worked at a Baskin Robbins next door to an insurance company a few years ago) inevitably asks "Why did you say no?", they can be given an answer supported by the policy. If there is a gray area that is very open to interpretation, you don't say "no".
In this specific case, without getting into the various aforementioned potential exclusions, the DOL is the biggest factor. Right at the top of every policy is an inception date (this is an important part of the policy). If the loss pre-dates the inception date, and the policy is "new business", meaning that the policy is new and not simply a renewed policy, the loss becomes the problem of whatever carrier (if any) was insuring the risk at the DOL.