February, 2018
Inspection News and Views from the American Society of Home Inspectors

Inspecting Old Houses: Wood Structures, Part 2


If you have any questions about inspecting old homes, please contact me at ABLE Building Inspection, ableinsp@centurylink.net or 540-636-6200.  

For this article of the “Inspecting Old Houses” series, I’d like to start by explaining load support. The object of a structure is to carry dead and live loads down to the ground. Dead load is the weight of the structure itself, including fixtures and finishes. Live load is the weight of the building’s occupants and contents.

Walls and columns carry loads down to the foundation and, ultimately, to the ground. Joists, rafters and beams carry loads horizontally until the load can be carried vertically down to the ground. Bearing walls should be located over beams or multiple joists. Columns below headers and beams should have support below them. Roof structure should include triangles or support at all intersections. That is the basis of residential structural analysis.

Structural Spans
The lumber used in the wood frame construction of old homes is different than from the lumber used today. This lumber—from large, old trees—is stronger than “modern” lumber and it can bridge long spans successfully. However, there were most likely no tables or building codes to specify the size, type and grade of lumber needed for a particular span. Much of the sizing was done by the builders or carpenters, who based the sizing on their knowledge and experience.

The amount of deflection in a floor or sag in a roof that may seem excessive today was acceptable in the past or may have been the result of undersized framing members. Failure of the framing is indicated by joists or rafters literally cracking or breaking, a relatively rare situation. Usually, the wood perseveres under duress and the framing has multiple members to share the loads. Wood also generally becomes harder and stronger with age. Trying to drive a new nail into an old board with a hammer isn’t easy.

Like today, the budget can determine what goes into a home. Nice, old homes have framing that barely deflects or moves. Homes built on a more limited budget can have framing that is grossly “over span” and shows it. People built Sears or other types of “kit homes” economically at the time, but the roof framing, in particular, which was part of the kit, might have been grossly over span.

Sagging is different from failure. If the framing is serving the purpose for which it was intended and has been doing so for many decades, it is working. Floors that drop in a particular direction or floors with high spots may be indicating a problem other than over-span joists. Porches were considered less important than the interior of the home. The spans of the porch framing in old houses can be truly impressive.

Excessive floor spans or bounce can be corrected by adding beams and posts below or possibly sistering them with additional framing members. However, once wood has taken a shape and has sagged, it is difficult if not impossible to straighten the deformed wood.

Structural Connections
Before the early 1800s, blacksmiths made nails individually, so people used nails sparingly. Cutting timber to form joints between the members was common. However, many of these joints, particularly mortise and tenon joints, don’t work well structurally. A large piece of wood is cut to produce a tenon that is inserted in a beam with multiple holes cut into it. These are inherently weak structural connections that should be examined closely for cracks and failure. 

People began producing cut nails in the early 1800s. Once the wire nail manufacturing process was developed in the 1860s, people began commonly using wire nails for structural connections. Nails are quick and inexpensive fasteners, but they are marginal in their ability to hold structural connections together.

Joist connections to beams or sills are areas of potential movement. Notching the end of a joist to sit on a beam, sill or ledger can weaken the joist at a critical area. Cracks at the notches are common. Adding joist hangers or 90-degree framing clips can reinforce failing connections. Attachment of the ledger boards also can be weak. Check the tops of the ledger boards and the ends of the joists for gaps that indicate that a ledger is rolling away from the beam. 

Ideally, any splice between structural members should be lapped or supported directly under the splice. Rafters and ceiling joists should lap at their intersections. Joists and beams should lap at their splices or the splices should be over columns. Nails alone will not make a very strong connection. That is why you see metal framing connectors at important structural connections in new construction.

Wood, Masonry and Soil Contact
Today, wood that is in contact with masonry comes from decay-resistant species, or it has been chemically treated against decay and insect damage. In older homes, this may or not be the case. Older wood has much more resistance to damage than modern lumber, however. Large, old trees had generations or centuries to develop their resistance to damage from wood-destroying organisms (WDO). However, particularly when adverse conditions persist, “old wood” may eventually succumb to adverse conditions and decay. 

Clearance between the wood and the soil, inside and outside, is critical. Wood that is close to or below grade is subject to moisture and damage. A foundation should extend 6 to 8 inches above grade to provide clearance between the wood and soil. Beams should be 12 inches and joists should be a minimum of 18 inches above the soil in a crawl space. If these conditions are not met, problems with WDO may occur. Two inches of clearance is recommended between wood framing and masonry chimneys. Chimneys should not support wood structures. 

Older masonry buildings, especially brick and block buildings, commonly would have had the floor joists built into the masonry walls when the building was being constructed. Now, the wood is inside a moist, limited air circulation environment that is conducive to every type of concern regarding WDO and it is difficult to inspect for this problem. Repairs are difficult because the joists are longer than the span and the ends are encased in the masonry.
A sister repair that extends to the masonry, but does not bear on the masonry, is simply adding weight to a damaged structure.

Beams should bear 1½ inches on wood structure and 3 inches on masonry. That may seem counterintuitive, but that’s the standard today. Splices in beams should be over posts or columns. Beams should have no notches or holes. Spans can be referenced from the building code. Beams should go from bearing point to bearing point, and cantilevers should be no more than one-third of the span. A 6-foot, 4-inch x 4-inch beam on a single 4x4 post that is sitting on the brick cellar floor doesn’t do much for the joists above. 

Columns should bear on beams, multiple joists, foundation walls or footings. Footings should spread the weight of the column onto a larger area of soil and should not be made of wood. Columns should have appreciable width for their height so they don’t bend. Columns and posts added after the initial construction may be reasonably and properly done, but more likely, they’re not.

Joists and Rafters
Wood structure is designed to deflect under load. No wood floor is level. Ceilings can be flat because plaster can straighten the deflection of the joists, but by the time we see the ceiling, it is most likely no longer flat. New structure and finishes can be installed over the old and conceal the original deflections. Look carefully because new finishes may conceal damage or deflection. Don’t be surprised by what may be wildly undersized members. The key question to ask is: Is it working or not? If your client wants to straighten the floors and make the ceilings perfect, you may want to suggest that they look for a newer house in which the defects are less obvious (but even newer houses have defects).

Damage and Repairs
Plumbers, electricians and heating contractors always have had hammers, saws and drills, but they may have little respect for or knowledge of structure. Restricted access into a foundation area can make repairs difficult and expensive. Repairs can be performed by people who are knowledgeable, experienced, competent and conscientious, or people who are not. The wood used in later repairs may not have the same structural properties and resistance to WDO as the original material had. Placing fasteners into damaged wood adds no structural value.

Termites are lazy and many times will leave the old wood alone unless the wood is softened by moisture. Borers don’t care and can chew a large log or timber to dust. Mold is far less frequent on old structures than on new and conditions have to be persistent for years before damage will occur. Look carefully and poke, poke, poke.

Homeowners may significantly remodel or add to a house every 20 or 50 years, and they do repairs continually. A house that is 100 years old probably has been remodeled two to five times and a 200-year-old house, four to 10 times. That represents a lot of opportunities for human ingenuity to improve or destroy a home.

Inspection and Reporting Techniques
If there is inadequate clearance in a foundation or attic space, it may be physically difficult or impossible to inspect the space. You should clearly note this in the inspection report because it represents a significant unknown and risk to a potential buyer. It is just as important to document why you can’t see or assess an area as it is to report on those areas you can see. Get in there and look. My rule of thumb is, “If I can fit, I git.” Where you didn’t go is where you should have been. 


Current building codes can be used as a general reference for spans of beams, joists and rafters. You may not be able to identify the species or grade of wood, but you can see that the joists 16 inches on center under a dining room shouldn’t be 2x6s for a 14-foot span.

It is very important to probe suspected or representative areas for possible deterioration. Technology is making robots that can access and visually show what is in a crawl space, but until a robot can poke an awl or screwdriver into wood with significant force, it cannot replicate what a determined human inspector can do. Wood that does not appear damaged may have significant structural deterioration.

Most of this information is applicable to old and new structures. When inspecting older structures, you will see many things that don’t seem possible. I used to say to my clients, “You can’t do that,” but I’ve changed my comment to be, “You shouldn’t do that,” because, many times, someone has managed to do something that I thought couldn’t be done. I also don’t spend a lot of time speculating why someone did a particularly hair-brained thing. Usually, I don’t have enough information to know and the genius who thought of the idea most likely isn’t there to ask.

In the next article in this “Inspecting Old Houses” series, I’ll discuss exterior sidings and roofing materials. If you’d like to ask any questions about inspecting old homes or invite me to present a topic to your chapter, please contact me at ABLE Building Inspection, ableinsp@centurylink.net or 540-636-6200.