Identifying Structural Problems in Your Historic Buildings Foundation | Wisconsin Historical Society

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Identifying Structural Problems with Your Historic Building

Identifying Structural Problems in Your Historic Buildings Foundation | Wisconsin Historical Society

Your historic house or building may show two common signs of settling: cracks in the plaster walls and sagging floors. In most cases, these effects of settling over time do not create any serious structural problems. The key is to evaluate the severity of the cracks and sagging. If you find large plaster cracks or severe sagging in your floors, contact a structural engineer to investigate and recommend the right repairs.

Identify the Severity of a Plaster Crack

EnlargeA diagonal plaster crack coming off the corner of an interior door, a sign the floor has sagged.

A diagonal plaster crack coming off the corner of an interior door is a sign the floor has sagged. Source: Bob Yapp

In order to evaluate the severity of cracks in your plaster, you must understand the difference between hairline cracks and wide or diagonal cracks. 

  • Hairline cracks. Small hairline cracks rarely signal a structural problem. Hairline cracks happen as a house contracts and expands. Whether you have a masonry building or a wood framed house, your building materials will take in and let out moisture. As this process happens, your house or building will creak and move. Hairline cracks can appear regularly in areas where the plaster was laid on the walls a bit thinner or where there's a structural beam. You can conceal a hairline plaster crack by digging debris out of the crack, taping it and re-plastering the area.
  • Wide and diagonal cracks. Plaster cracks that are 1/16 inch or wider and diagonal plaster cracks signal that your structure is moving more than normal. Diagonal plaster and even drywall cracks most often appear above doors and windows, in corners and around fireplaces and chimneys. These locations tend to sag or fail first because they are intended to support heavy loads from above. Windows and doors have a beam above them called a header. Headers are designed to support the weight of the wall above. Historic houses often have headers that were either undersized or installed in ways that were not as strong as they could be. Many historic header beams were two 2 x 4 inch or 2 x 6 inch boards nailed together and laid over the window or door on their face. Today, headers are still ganged together with 2-inch x material, but advances in construction have demonstrated that they are stronger when laid above the door or window on their edge.

Evaluate the Cracks in Your Plaster

You should watch cracks in your plaster over a period of time. Use this three-step procedure to evaluate the cracks in your plaster walls:

  1. Measure the length and width of the cracks and write them down.
  2. Take a close-up photo of yourself or someone else holding a tape measure that shows how wide the crack is.
  3. Put some blue painter's tape across the crack and watch it over a three-month period of time. If the tape breaks apart and the crack is bigger, contact a professional architect, structural mason or structural engineer for further investigation.


Identify the Severity of a Sagging Floor

Some minor sagging of floors is natural in your historic house or building. Masonry fireplaces and utility that protrude through your ceilings, floors and roof framing create holes in the framing structures. These holes must have doubled up 2-inch x framing around them to support the ceiling and floor joist beams as well as the roof rafters. Sometimes nails loosen, or wood is destroyed by termites or carpenter ants, or wood rots from a water leak. As these wood structural elements sag or fail, the floors around them sag as well.

Most fireplaces have an even heavier load on the structural framing than a utility chimney. Fireplace hearths, fireboxes and fronts were traditionally made of a mortar bed with a brick and tile surface. These heavy materials create a massive load on the often undersized structural framing.

Evaluate Your Sagging Floors

EnlargeImage of a 4-foot level being used to determine how much the floor is sagging.

You can use a 4-foot level to determine how much the floor is sagging. Source: Bob Yapp

Any floor that sags more than 1/2 inch from one end of the room to the other has excessive sagging. You can evaluate the severity of sagging in your floor with a 4-foot level. Place the level on the sagging floor and follow these guidelines:

  • If your floor drops 1/8 inch in 4 feet, you can assume your room is sagging 3/8 inch in 12 feet. This is within the 1/2-inch tolerance range, and you should probably learn to live with the "character" the sagging floor brings to your room.
  • If your floor is sagging more than 1/2 inch in 12 feet, you should call a professional structural engineer to investigate.