A low pitch roof can be done with rolled roofing, but this method can not be used on a totally flat roof. The roof must have at least a slight pitch to it. The roof will also need to have a solid wood deck beneath it. Due to the fact it will still need to be nailed. Having planking spaces between the boards is not acceptable. Measuring the roof is always done first to determine how much of the rolled roofing material will be needed. To properly measure, you would measure the nearest foot, width and length of the area. Times these two numbers and that is the area of the roof. Times by 20 percent, add the two numbers, and you will get the amount of roll roofing you need. When doing rolled roofing, black tar paper will also be needed. Plastic roof tar, galvanized nails are also some supplies that will be need. The best size nail for this job is 1 1/4 to 1 3/4, and that is for laying it over a existing roof. The basic is if there is only one layer you can go over it, more than that must be removed. Depending on the size of the job, it could take a few days.