Philadelphia's row house stock is one of the great urban engineering achievements of the nineteenth century — 70,000+ brick structures packed side by side, sharing walls that have stood for 100 to 150 years. Those party walls are what make rowhome renovation different from any other building type, and they are what catch unprepared homeowners and their GCs off guard.
A party wall is, by definition, shared. It sits exactly on the property line — half on your lot, half on your neighbor's. Structurally, it is often carrying load for both buildings simultaneously: your floor joists frame into it from one side, theirs from the other. The mortar joints have been reworking slowly for a century, and in many rowhomes the wall has shifted, bowed, or cracked in ways that were stable until something in the renovation changed the load path.
What you need to understand before demo
First: if you are removing any portion of a party wall — even a small section to accommodate a beam pocket or a new opening — you need a party-wall agreement with your adjacent neighbor, in writing, before work begins. This is Philadelphia law, and L&I will ask for it if the scope triggers a structural permit. Getting this agreement after the fact is far harder than getting it before. We always initiate the conversation on behalf of our clients, early in the preconstruction phase.
Second: the joists in most pre-1940 rowhomes are balloon-framed and bear on the party wall — not on a ledger, not on a beam, directly bearing on the brick. When you demo interior walls and floors, you are temporarily removing the lateral bracing those joists depend on. A proper shoring plan — temporary walls and posts — is not optional. It is the difference between a controlled renovation and a neighbor's bedroom dropping six inches.
The joist situation
Original Philadelphia rowhome joists are typically 2×8 or 2×10 rough-cut lumber spanning 12 to 16 feet — a span that would be marginal even to modern code, and they have been carrying floor loads for a century. On most gut renovations, we sister at least a portion of them. Sistering means attaching a new piece of dimensional lumber alongside each original, glued and bolted, to restore the full design load. In severe cases we replace them entirely. Either way it adds cost — typically $8,000 to $18,000 for a full floor — and it is the most common "surprise" we find mid-demo. We try to probe through small access holes before signing a contract to give you a realistic picture upfront.
When the joist framing into the party wall is deteriorated at the bearing point — rotten or crushed brick at the pocket — that is a structural repair that requires our engineer. It is also one of the most important repairs in the building, because a failed joist bearing can migrate from a sag to a collapse over time.
Before you hire anyone
Ask your GC: have you sistered joists in a rowhome before? How do you handle party-wall agreements? What does your shoring plan look like for a full-floor demo? The answers will tell you everything about whether they have actually done this work or just said yes to get the deposit.