Vertical wood framing is usually not a critical item in masonry row houses, but what really matters is the horizontal framing, specifically the floors. In most row houses in the 16-feet-wide range, the joists span from party wall to party wall, in some cases with a major beam for intermediate support. In wider houses, the framing gets more complex.

How do you know you may have a structural problem? One sign may be noticeable deflection or bounce in a floor. Such floors may not be hazardous per se, but they make installation of floor tile, such as in a bathroom, more difficult and the deflection causes unsightly cracking in plaster and drywall. Oftentimes, structural issues will need to be resolved when it comes time to renovate, and then you’ll need professional assistance.

There are five common causes of problems in structural floor timbers and most of them cannot be detected until finishes are removed. Termites and water are two big enemies of wood. Termites, being hungry little critters, eat wood and weaken it. Water causes rot, and there are three big sources of water: leaky roofs, leaky plumbing fixtures and moisture-laden masonry. In the case of the first two, a failing wax ring on a toilet, or failed flashing can keep wood in the immediate area perpetually moist.

Moisture in the masonry is more insidious. The joist ends rot out in contact with masonry and over time they settle into the joist pockets. Moisture wicks through masonry walls, even in the party walls between houses. To avoid this, keep your roof, metal copings and roof flashing in good condition. Check exterior walls for cracks and to make sure the mortar is sound. If your masonry needs re-pointing (sometimes called “tuck-pointing”), bring in a masonry contractor with experience in historic brick and the appropriate low strength mortars. It’s best to do an entire wall surface. Not only are spot repairs unsightly, they are generally ineffective in keeping the moisture out. These days pressure-treated lumber is specified for any wood in contact with masonry, in order to avoid this problem.

Plumbers sometimes cut away at timbers willy-nilly to install their drain pipes. These days there are strict codes governing how and where holes are placed in timbers. In the “good old days,” plumbers were not as circumspect, and it’s a common problem in existing bathroom areas.

Joists can also simply be under-designed from the beginning or weakened through inappropriate removal of interior bearing walls. There’s really no good way for the untrained eye to tell if a wall is load-bearing or not. Consult with a professional before attempting to remove an interior wall, particularly for a wall that runs perpendicular to the direction of your floor joists.

Timbers sag with age. The technical term for this is “creep.” In one Capitol Hill house I’ve worked on, the timbers sagged about an eighth of an inch per foot. Meaning in a 16-foot-wide house, the center was an inch lower than the walls. The floors sloped visibly. The floor joists were still perfectly sound, but the sag can cause doors to stick and makes installation of new partitioning more of a challenge.

What can be done to fix these problems? In the case of sag in the absence of deflection, my advice is live with it as one of the quirks of living with an old house, unless you want to gut it to the bare brick walls and basically start from scratch. For the rest there are techniques that vary in cost and complexity. Sistering is making a sandwich of new timbers alongside the old. That’s good for small repairs, such as a rotten bathroom floor. In larger areas, selected areas of old material can be removed and completely reframed with modern materials. Still larger areas of framing can be reinforced with additional columns, beams or bearing walls.

Then there’s wholesale replacement of the floor structure, including changing the direction of the framing, if damage and deterioration are pervasive, or if the floor plan changes radically as part of a renovation.

In the case of wholesale changes, the structural design should be prepared with layout of other systems in mind. A sensitive structural design will make layout of plumbing and ductwork much easier, minimizing the use of bulkheads and keeping ceilings as high as possible. The key is good coordination between your architect, your structural engineer and your contractor.

A few notes on materials. When most of our houses were built, rough-sawn dimensional lumber was the only game in town. Today we have modern alternatives, engineered lumber that’s made in a factory and able to span greater distances and bear larger loads. Engineered joists, often called “TJIs,” are wood “I” beams. TJIs are much easier to penetrate with ductwork and drain lines than dimensional lumber. LVLs, short for “laminated veneer lumber” and variously called microlams or “glu-lams,” are thin wood strips glued together – basically plywood on steroids –and they are often bolted together in double and triple assemblies to form very strong wood beams.

Paul A. Wilson, AIA, PhD, is principal of Paul Wilson Architect, PLLC, www.paulwilsonarchitect.com, located on Capitol Hill. The author acknowledges the assistance of Robert Wixson, PE, of APAC Engineering, www.apacengineering.com, in the preparation of this article.