How to include roof price in mortgage?

[Therese V.]

There is a multifamily I'm looking at that appears to need a new roof. I believe the roof will be around $15-$20k depending on if it is a tear-off or not. If I were to make an offer on it, how can I have that extra $29k added to the mortgage so I'm not putting that out of pocket to get a new roof?

Let's say the building is listed at $160k and I would offer $140k because of the roof, can I offer $160k with a seller credit of $20k and get the $20k subtracted from the downpayment owed? So it'd be like $32k downpayment, but minus $20k credit (plus fees, but let's count those out for simple math) means that I'd pay $12k down to the mortgage company and have the extra $20k for the cost of the new roof. Otherwise, it'd be $28k plus the $20k for the roof, or $48k. With a seller credit, I'd be saving money. 

Is this allowed? What other ways can I accomplish that? The roofing company does not do payment plans for investment property.