Poor design and/or construction of a flat roof may result in frequent or recurring problems.
Old roof has structural issues requiring significant repair or renovation. If corrective measures are extensive, the cost of fixing a flat roof may be close to the cost of converting to a pitched roof—or close enough.
Roofing may last longer. In theory, roofing on flat roofs can last as long as standard materials on sloped roofs, but in reality, roofing on houses with pitched roofs has greater longevity, on average. Again, it’s important to compare the durability of pitched-roof materials (like shingles) with high-quality flat-roof materials (not just tar-and-gravel or torch-down roofing).
Easier to insulate and ventilate. Flat roofs often aren’t (and usually shouldn’t be) ventilated, and their space constraints can make it challenging to achieve high R-values with insulation. Pitched roofs offer more space for insulation and can be ventilated with fewer problems.
More space. A pitched roof can create an attic space that can be used for storage or possibly additional living space, or both. How much space depends on the size of the house and the slope and structure of the new roof. Alternatively, a pitched roof can add visual space if the old roof is removed and the new roof structure is designed for a cathedral ceiling.
Aesthetics. This is a case-by-case matter. A pitched gable roof is the quintessential roof style in most of the Western world. That’s why it’s easy to claim that pitched roofs look better than flat roofs. But of course, it’s not so simple. Flat roofs have been a characteristic architectural feature of Modern home design for many decades — and for centuries before that on various styles of homes. It all depends on the house and the owner’s taste. A run-down midcentury modern house might be beautifully made over by a new pitched roof. At the same time, the midcentury look is so popular these days that new custom houses are being built with flat roofs. In any case, some homeowners make the conversion for nothing other than aesthetics.