Assuming that you need the adjusted basis of your property in order to determine the amount of deductible casualty loss:
From IRS Publication 547: If your property is personal-use property or isn't completely destroyed, the amount of your casualty loss is the lesser of:
- The adjusted basis of your property, or
- The decrease in fair market value of your property as a result of the casualty
For property you buy, your basis is usually its cost to you. For property you acquire in some other way, such as inheriting it, receiving it as a gift, or getting it in a nontaxable exchange, you must figure your basis in another way, as explained in IRS Pub. 551.
While you own the property, various events may take place that change your basis. Some events, such as additions or permanent improvements to the property, increase basis. Others, such as earlier casualty losses and depreciation deductions, decrease basis. When you add the increases to the basis and subtract the decreases from the basis, the result is your adjusted basis.
If only your roof was damaged, you can get an estimate from your insurance company or real estate professional as to the value of the roof by itself. What you pay to bring the property back to its previous condition (not to "brand-new" or improved condition) may be one good standard to use for the decrease in the fair market value, together with estimates of the overall drop in market value of homes in the area.
See also this TurboTax tips article and this help article regarding casualty losses.
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