A roof-over (in code language, a roof recover) puts a new covering on top of the old one. It is quicker, and it is often not allowed. In Philadelphia the rule is in the EZ re-roofing standard that every roof covering permit follows.
The City's rule
All existing layers of roof coverings shall first be removed prior to installation of new roof coverings under the following conditions: Existing roof or roof covering is water-damaged or deteriorated Existing roof or roof covering is wood shake, slate, clay, cement, or asbestos-cement tile Existing roof consists of two (2) or more roof coverings
Philadelphia L&I, EZ Permit Standard: Re-Roofing, rev. 7/26.
The code behind it
The International Residential Code adopted in Pennsylvania says the same in R908.3.1.1, "Roof Recover Not Allowed": a recover is not permitted where the existing roof "is water soaked or has deteriorated to the point that the existing roof or roof covering is not adequate as a base for additional roofing," where it is slate, clay, cement or asbestos-cement tile, or "where the existing roof has two or more applications of any type of roof covering." The code's definition of a roof replacement (R908.3) is "the removal of existing layers of roof coverings down to the roof deck."
What it means for an old rowhouse
A house built before 1940, and 39.9% of Philadelphia's homes were (ACS 2020–2024), has usually been re-roofed several times. If the last job went over an older roof, there are already two coverings, and the next roof starts with a tear-off to the deck. The same is true if the roof holds water or is soft underfoot.
A tear-off has advantages the standard builds on: the deck can be seen, rotted boards replaced, flashing at the parapet and party walls rebuilt (rusted or damaged flashing may not be reused), and insulation added if wanted, which the standard then requires to reach R-49.
When a roof-over is allowed
Over a single sound covering that is not slate or tile, a recover can be permitted. Over wood shingles, the standard requires the whole surface to be covered first with gypsum board, mineral fiber, glass fiber or another approved material. Even where it is allowed, a contractor may advise against it once the roof has been seen; ask why, in writing.
On the paperwork
If damaged sheathing is found during the tear-off, the standard says the inspector must be notified, and more than 10% of the sheathing takes the job outside the EZ permit. The contractor's TP_019_F certificate at the end records whether less than 10% was replaced. Read more: what a replacement includes and roof permits.