When it comes to a new roof, most people are happy for any savings they can get (like our flash sale!). One such possible savings can happen when you are re-roofing one or two layers of composition asphalt shingles. One of the many advantages of re-roofing with metal is that its extreme light weight means it can often be installed directly over the top of existing comp. There are primarily two reasons not to do this, however, and instead remove the existing layers before installing the new roof. One of these reasons is regulatory and the other is structural.
In the case of regulations, these are regional and dependent on building codes adopted by the various jurisdictions. Depending on what codes are enforced at your location, you may have no choice but to remove what's there now and have an inspector view the structure beneath before you'd be allowed to install a new roof of any type. If this is an issue to you, check with your local building department to learn more about their specific requirements.
When a roof is constructed, the designer or builder allots a certain amount of weight to the roof covering. It varies by location, as homes built in snow country tend to be of much stouter construction than homes built in Arizona, for example. The weight allowance for the roof material itself is referred to, in engineering terms, as the "dead load" (when you walk on the roof or snow acccumulates on it, that extraenous weight is called the "live load"). Most dead load allowances are in the ten to twenty pounds per square foot range, but this is a very general range and will vary with location, complexity and pitch of the roof, as well as the type of roof the builder intended. If yours was of average contraction standards at the time of construction and you have two or three layers of roofing, you may still be well short of the dead load capability of your roof - in which the two pounds per square foot a new metal roof might add won't compromise your structure in any way.
On the other hand, if you can see the rafters showing signs of being at their limit, or you can sense an uncomfortable amount of flex in the roof deck when you walk there, then the roof may already be at its limit and going over top is not recommended.
My rule of thumb for recommending an over top installation or a tear off is this: if the roof looks nice and straight - no obvious bowing of the rafters - AND there is no apparent rot or cracking in the rafters or roof structure when viewed from beneath AND you can get a nail or screw through to something solid (a rafter, batten or plank) in a regular pattern, then going over top will save time, money and mess.
Since most metal roofs are the lightest and strongest roof you can buy - not to mention the longest lasting - they can be perfect for over top installations. If your roof is suitable for such an installation, metal can help pay for itself by saving you the cost of removing and disposing of your existing roof (sparing the landfills). It's something to consider when you can get a superior roof for the price of a temporary one, just by saving the cost of demolishing what's there now.