Are there any unknown costs when my roof is being installed?

There are two items that are unknown while we are estimating the roof.  Any roof decking that is rotten or broken will have to be replaced before installing new shingles and there is no way of knowing this prior to replacing the roof so we charge a per foot amount that will be added to the invoice.  The other item would be any unknown structural damage you might have that can not be seen.  Most of the time while estimating the roof we will suspect this and will warn you prior to doing the roof.