There is no set answer to this question, as each circumstance will vary. That said, there are ways to determine whether or not you should request roof replacement or a credit towards such repairs when purchasing a home. It is quite common for the condition of the roof to be a sticking point in home negotiations. Replacement costs are high, and neither party will want to incur the cost. Consider the following before demanding roof replacement.
Home Inspection Contingency
If the offer you made for the home was contingent upon the outcome of the home inspection, it is your right to request roof replacement or a credit should the roof be found to be in poor condition. The one exception would be if you had already taken the condition of the roof into account when making your offer. Making requests for replacement or a credit after making an offer in which you accounted for the condition of the roof is often referred to as "double dipping." This is not illegal, but is considered to be unethical.
Requesting Roof Certification
Should your home inspector determine that the roof is in need of repair, a roofer could be called in to complete the roof certification process. This would include a complete roof inspection, including attachments such as gutters, downspouts, and flashing in order to determine how much longer the roof might last before replacement is needed. This process will allow you to know exactly what repairs need to be completed right away. If the answer is none, the inspection will simply tell you the estimated remaining lifespan.
Preparing To Negotiate Roof Replacement or Repairs
In order to successfully negotiate the cost of replacement or repairs, you will need to be prepared to prove that the roof is damaged and what the cost will be to restore it. A home inspection report and roof certification should both be presented to the agent of the seller. Regardless of how much paperwork you present, ultimately it will be the decision of the seller as to how to proceed. Should you not be able to prove imminent danger or need, the seller may not offer anything. There are no guarantees.
In the current market many sellers are open to negotiations for needed repairs. However, not all will have the resources necessary to fund repairs of replacement of elements such as the roof. As a buyer, it will ultimately be your decision whether or not to demand a new roof and hold to this demand. Your own finances and desire to purchase the home will both play role in the final decision. Consider also your own personal comfort level in regard to what you have personally seen, as well as the information provided to you by your home inspector.