Overview of Wood Home Structures

Framing House Solid State Home Inspections

One of the cliches in home inspecting is when a client is told in a home inspection, 'the bones of the home are good'. There is some truth however in this cliche as every home needs 'bones' to hold walls vertical, floors level, and carry the loads of wind, snow, and people. These 'bones' of a home make up the structural support of the building and the performance of this system is evaluated by a home inspector during a home inspection.


Structural 'loads' occur on a building from 'dead' loads like the weight of the building materials and occupants belongings, to the 'live' loads from falling snow, moving people, and wind forces. A major role of the homes structural system is to resist the 'loads' on the building and to transfer associated weights from these loads to the earth beneath the home without moving. 


Footings, Foundations, and Floor Slabs

Footings are the first building materials placed on a typical building site. Footings are typically made out of concrete and carry the load of the building above onto the soil of the property below. In order to be a solid base, footings need to be deep enough to resist and sideways movement and any effects of frost heaves on surface soils. They also need to be strong enough to support the weight of building loads above.


Spread Footings and Foundation Walls

Spread footings are the most common type of footing in western building methods. They are designed to spread the weight of the building evenly onto the soil below. Spread footings are not commonly visible in a home inspection as they rest below the soil line outside and below the floor slab in basements and crawl spaces. Footing failure can be visible by movement in the foundation walls and structure above.


Foundation Solid State Inspections
A closeup of the cement foundation of a new housing development.

Spread footings are the lowest part of the foundation system on top of which concrete foundation walls are poured. Foundation walls are important to the structure a they typically define either a basement or crawlspace, carry the weight of the wood structure above, and hold back the soil pressure outside the basement area. The most common foundation wall failure is that they are not strong enough to hold back the soils and cave inwards. 


Cracks are common in concrete foundations. 'Normal' cracks occur as concrete shrinks when it dries however, a 'normal' crack may still be the early sign of a wall failure. As such, all cracks need to be treated seriously by home owners, buyers, and home inspectors. All cracks big or small in concrete foundation walls present an opportunity for water to penetrate the building which presents a second concern for foundation cracks.


The last structural element for footings and foundation walls is the concrete floor slab. The soil outside the basement walls is constantly trying to push the walls and footing into the building. Pouring a wall-to-wall slab of concrete that also ties into the top of the footings provides additional resistance and ridgitidy to the foundation system. To help keep footings and foundation walls from being pushed inward under the pressure of soil from outside the walls, a concrete slab is poured from 'wall-to-wall' in the basement or crawl space which will provide strength the base of the floor system. This slab also provides a sub-floor for interior finishes and objects to be placed on.


Slab-on-Grade Foundations

There are various forms of 'slab on grade' construction which integrate the main floor concrete slab with the footings for the structure. Slab on grade construction is found in areas where a high water table would impact structure and in climates with little concern for ground frost issues as the foundations are not very deep.


Slab footings do not incorporate a foundation wall system as they provide a direct surface and foundation bolts for exterior walls to be mounted on. To protect the wood structure above, the slab level should be 6-8" above the grade level of the property. Slab-on-grade foundations also provide the sub-floor for the first level of the home.


Pile or Pier Foundations

In soil conditions where the surface soils are unstable or large point loads of weight need to be managed, some foundation footing systems use 'piles' driven vertically into the ground to depths necessary to stability carry the loads of the building. Pile foundations are very common under structures like bridges and boat docks as the depth of the piles finds solid soil for the structure. In houses, pilings or pier foundations are more commonly found in boggy areas, reclaimed land, or uneven bedrock.


Pile foundations don't provide a continuous base for exterior walls. Structural beams spanning across individual piles creates the horizontal surfaces that loads above can be built on. Pilings need to be 6-8" higher than the solid level to protect the beams and wood structure above from ground water.


Walls, Floors, and Sill Plates

Ballon and Platform Framing

As building methods have evolved, so have the methods of carrying structural weight through homes. In the 1800's and early 1900's, 'ballon' style framing was common. In ballon framing, the outside walls are the main structural element. Outside walls were built continuous from foundations to the roof line with each floor level hung from the wall framing. Ballon framing came to an end as building and fire science identified that the open cavities traveling the height of the home in the outside walls would allow a fire to engulfing the entire home very quickly.


By the mid 1900's, ballon framing was replaced with 'platform' framing. With platform framing, the floor, or platform, of each level is the primary structural element and it creates a fire separation between each floor of the home. The platform floor also provides a solid structure to each floor while load-bearing walls provide vertical support for the floors.


Sill Plates

In the transition from concrete foundational structure to wood building structure, the transitional piece of wood material is called the sill plate. The sill plate is typically a 2x4 or 2x6 on its side and is separated from the concrete by a thin water proof membrane like tar paper. It is then bolted with washers and nuts to the concrete foundation using bolts set into the concrete foundation during pouring. The bolts ensure the wood structure is mechanically attached to the concrete foundations resisting movement forces such as those in high winds and minor earthquakes.


Sill plates provide a continuous nailing platform for the rest of the wood structure above and also transfer the weight of the wood structure to the foundation wall. It is critical that the soil levels around the home stay at least 6-8" below the level of the sill plate as water damage can allow the sills to rot and collapse causing structural problems above.


Platform Floors

Platform Floor Solid State Home Inspections

Close up of platform floor under construction with joists and beams (sub-floor not yet in place)

The next wooden structural element on a wood frame home is the first platform floor (except slab on grade homes which use the slab as the first floor). A platform floor is built by first attaching perimeter joists to the sill plates along the outside perimeter of the floor area and then hanging beams and floor joists to the perimeter joists as needed to support the design of the home.


The maximum 'span' of wood joists is the maximum distance a wood joist can travel unsupported and is based on 'load tables' created by engineers. Typically longer spans mean thicker joists but when spans are too long for standard joists, beams, engineered trusses, or load bearing partition walls can be used to support the floor structure. Builders also need to build and account for penetrations in the floor platform structure such as stair wells and chimneys.


Once all the floor elements are in place, the joists need to be connected together to prevent movement in the system (also called lateral bracing). Joists are connected at each end but if the spans are long enough, additional lateral support in the form of blocking or bracing may be required. Lastly, to provide a working surface for interior finishes, additional lateral bracing, and a fire stop, a sub floor of boards or plywood is laid on top of the joists finishing the floor system.


Load Bearing Walls

Walls are critical to holding up each floor of the home and the roof. Load bearing walls are those walls that support the weight of the structure above and always include the outside walls and depending on the design of the home may include some interior walls as well.


Wood Framed Wall Solid State Home Inspectiolns

Wood Frame Exterior wall with partial shear wall being installed

Load bearing walls are made up of either 2x4 or 2x6 vertical studs spaced 12, 16, or 24" apart depending on the loads that need to be supported above. The studs are connected by a single continuous sill on the bottom and a double sill above which transfers the loads above to the studs. Wall designers need to account for penetrations like windows and doors which impact the walls structural design.


Wood framed walls are very strong at carrying loads vertically but they need re-enforcement to prevent the walls from falling over in winds or minor earthquakes. This is accomplished by adding plywood , chipboard, or shiplap sheeting and shear walls to the outside wall of homes. This provides rigidity from movement in all planes of direction. Drywall finishes on the inside of the home also add secondary lateral support but are not considered to be primary structure.


Partition Walls

Partition walls are not actually part of the structure of the home as they do not carry a load from above but they are part of the framing for the home and increase the rigidity of the entire structural system. Identifying partition walls from structural walls is not straight forward as they are often built from the same materials and methods as the load bearing walls by the same framers. Always have a structural expert determine if a wall carries a load before making any adjustments to a home.


Roof

The roof itself is a major part of the homes structural system. It provides bracing to hold the walls vertical as well as carrying the loads of snow, rain water, and roofing materials from above. Roof design and structure is critical to roof performance. Most single family homes have a 'steep' roof system built from rafters or trusses. Flat roofs are less common.


Rafter and Joist Construction

Until the mid 60's, it was common for builders to make site-built rafter and joist systems as the structural 'bones' of the roof system. Rafters are the angled wood pieces that run from the outside walls to the peak of the roof. Joists are horizontal wood that connects the wall ends of opposing rafters to each other. The three points of connection (wall, opposing wall, roof peak) form a structural triangle which is one of the strongest shapes for carrying a load. Additional collar ties, purlins, and knee walls could be added by builders to increase load strength and horizontal spanning distances. 


Site-built rafters were typically very functional but they had a few down sides for home owners. First, they are only as good as the builders skill is at making them and structural failure or high snow loads in some climates would cause home owners to need to re-enforce weak rafters. Second, rafters limited house designs to simple roof shapes that could be built with the traditional skills and construction load tables of on-site labourers.


Engineered Truss Construction

Roof Truss Solid State home Inspections
Wooden roof trusses stacked together on a building site

Starting in the late 60's, it became common to order factory built engineered trusses for new construction. Engineered trusses are custom built for each home based on home drawings sent to the truss manufacturer with the goal to maximize strength and minimize material costs. A common way to recognize engineered trusses is the use of metal connector plates installed by the factory at the wood piece junctions. Site-built rafters would be nailed together.


Engineered trusses can be made much stronger than site built rafters using more complicated angles and additional 'web' supports. With engineered trusses, builders and architects could also begin to design more complicated home designs along with larger open spaces which radically changed the design of homes in the later half of the 1900's.


Roof Sheeting

Like exterior walls, the trusses or rafters of the roof system need to have lateral support to keep them in place. Plywood, chipboard, or wood slats are used, depending on the final roof covering system, to provide rigidity to the structural system and a surface for the roofing material to be attached to.


Putting It All Together

All the various elements of the structural system work together to create a single structural system designed to carry the loads (weights) of the building to the earth the building is set on. Houses need to be able to carry these loads on different potential planes of force:


Vertical Load (Gravity)

Ideally, the home only need to manage a perfect vertical weight load of building materials, objects, and people. However, our buildings need to be strong enough for our environments. 


Simplified Example 1 - A heavy snowfall on the roof needs a strong enough roof structure to carry the load. This snow weight transfers vertically from the roof to the load bearing walls. Then it carries down the walls through floor platforms to the sill plate. The sill plate transfers the load to the foundation wall, which transfers the load to the footing and ultimately the earth below the footing.


Simplified Example 2 - A grand piano is placed on the second floor. The joists in the floor carry the load horizontally to the load bearing walls, which transfer the load through as above to the earth below the footings.


Horizontal Racking Forces

Square faced wood frame houses are not very aerodynamic. Even modest winds can exert thousands of pounds of force all trying to push a home over. A homes structure needs to be able to resist this racking force.


Simplified Example - Wind blowing on the West side of a home will cause the west wall to want to lean into the house. The West wall's stud configuration and top and bottom sill connections keep the west wall ridged and transfer the load to the North and South walls. The North and South Walls are kept ridged by the plywood sheeting which transfers the load to the back wall which is held vertically by the roof and floor connections. In this way, the entire structural system creates a ridged cube to resist racking forces.


Lifting Forces

In our example of the West wind above, the homes structure resisted walls from racking but as the wind blows on the ridged structure, the wind pressure on the top of the west wall will create a lifting force at the bottom of the wall. This lifting force is resisted by the bolts and washers holding the sill plate to the foundation wall which is one reason it is critical to bolt foundations to sill plates and to correctly connect all the structure together.


Earthquakes

Earthquakes present a major challenge to our homes structures. Earthquakes can subject homes to vertical lifting and dropping forces, horizontal shaking forces, and homes will attempt to oscillate (wave) along with the frequency of the earthquake. Despite all the engineering that can be done to make homes more resistant to earthquakes, the earth has the ability to overcome our engineering with levels of power we cannot engineer against.


Earthquake retrofitting is a complex business with its own sets of experts. The basic principle is to ensure the homes structure can absorb the earthquake forces and the structural elements of the home are all well connected to each other. This area of engineering and advice is best left to the experts in this field.


Other Supports

This overview is meant to address the most common structural systems found in wood frame houses but only scratches the surface at the complexities and options involved. There are as many ways to build the structure of a home as there are engineers and architects that can design them.


FInal Thoughts

Home Inspections done to the standards of practice in the industry are performance based inspections and when it comes to the structure of most constructed homes, there is often little if any of the structural elements available to home inspectors for direct inspection. Home inspectors will be looking for signs of performance failure in the structural system such as cracks in finishes, out of level floors or walls, or various other clues about structural system failure. As always, if you are looking at buying a home, always call a professional home inspector who will help you know the home you are buying is safe and solid.


By James Bell - Owner/Operator of Solid State Inspections Inc.