By laying metal roofing over the shingles of an existing roof you are protecting it from the weather and also from the effects of time.
A Serious Project
Overlaying a shingle roof with metal is a difficult project, one that really falls into the semi-professional category. If not done well, not only do you risk damaging the existing roofing materials, you may damage the entire roof. Care should be taken when attempting this sort of large-scale project to get the best materials and if possible enlist some help from others who have done this type of job before successfully.
Find the best way possible to get to the roof with the tar paper.
Starting at the bottom or at one end of the roof lay the felt across the entire length, and then work your way to the top or center with over laying sheets and as you overlay use the nails to pin this down. If you are working on a windy day, you will definitely need two people for this job.
When the felt is secured you can start laying the metal shingles. Again start at the bottom and nail the shingles into place. Then work your way up to the top or center of the roof. The shingles need to be offset in every space, so there are no valleys or blank spaces where a leak will start.
Make a chalk outline of the horizontal and vertical measurements so that you can lay the next set of shingles into place quickly.
If the roof is peaked, there will be a ridge. You will need to nail the ridge roll into place.
As you near the completion of the job look over the roof to make sure that rain or moisture will roll to the bottom easily and not pool or collect anywhere on the roof.
After completion there will be a need for maintenance of the metal roofing shingles.
Maintaining Your Metal Shingles
Mix a gallon of hot water, a cup of laundry detergent, and a cup of bleach in the bucket. Climb safety onto the roof with all your supplies including the bucket. Make several trips if necessary. Sweep any debris away using the broom.
With the spray hose on a high setting, rinse away as much grime and dirt as possible to reduce the amount of brush scrubbing later. Then scrub the remaining dirt away with using the brush and the mixture from the bucket. This process should not be too laborious as metal shingles are an easy cleanup. Work your way around the entire roof, and then use the hose again to wash away the residue.