Under current IRC, a roof is depreciated over a period of thirty-five years. Not being able to deduct legitimate business expenses would be akin to a retailer, being taxed on the price of goods sold, but not being able to deduct what they paid for it, nor their rent for store front, telephone, utilities, employee wages, etc.

So, you buy a new suit for one thousand dollars. The retailer pays seven hundred fifty, for the suit. Their employee costs account for fifty dollars, on that sale; rent and expenses account for another hundred dollars... but the entire one thousand would, under your theory, have to be fully taxed as "income".

If you sat down at your sewing machine, after purchasing the fabric, and made a suit, you would not receive a deduction.

It's simple business. Goods or services, less expense, equals a taxable profit.