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You probably know that roofing slope is the amount of incline in a roof surface. It is usually expressed as a ratio of "rise over run". For example, a 5:12 slope rises 5 vertical inches for every 12 horizontal inches. It is easily determined with a level and measuring tape.

What you may not know is that roofing slope is the most important (yet frequently ignored) factor in making a roof safe, functional and affordable. Here are ten factors to consider...

1. Safety. Neither you, nor the workers you hire should have to risk life and limb to work on your roof. The steeper the roof, the more hazardous it is.

2. Degree of Difficulty. Your roof should be relatively simple to work on. I'm not saying it should be easy, but it shouldn't require scaffolding, mechanical lifts or mountain-climbing equipment.

3. Drainage. Your roof should not pond water as dead-flat roofs do. Ponding causes leaks and premature roof failure. Building codes, warranties and good roofing practice all require positive drainage. Tapered systems can correct slope deficiency, but they are labor-intensive and pricey.

4. Do-it-Yourself Friendly. If you desire (or finances dictate) that you do the roofing work yourself, your roof should accommodate you. Roofing slope that is too low or too steep are both troublesome for the handyman.

5. Curb-appeal. Roofs that are too steep tend to make a home look top-heavy. (Except for A-frames, which look great!) Roofs that are too low tend to make a home look boxy or bottom-heavy. Many manufactured homes have this problem.

6. Quantity of Material Required. The steeper the roof, the more framing, sheathing and roofing needed. For example a roof with a 12:12 slope uses 30% more material than a 5:12 slope.

7. Quantity of Labor Required. The steeper the roof, the more labor required. Not only is there more material to be installed, but it is much slower working on a steep roofing slope. A 12:12 can take twice as long per square to shingle as a 5:12. A near-vertical mansard can take four times as long.

8. Compatibility. Roofing systems are designed to function properly within a certain range of slope. Violating these requirements invites roof failure, and can void your warranty.

9. Special Requirements. Some systems require special considerations for marginal applications. Asphalt shingles for example, may require a two-ply underlayment on roofs less than 4:12.

10. Contractor Interest. The best roofers (which are the only kind you want) often have all the work they can handle. They can afford to pass on dangerous jobs. They don't want to risk injuries on steep roofs if they don't have to. If they bid a steep roof at all, you can expect high prices, because they probably don't really want the job.