Ever wondered about how houses hold themselves up?

You don’t have to be an architect or an engineer to appreciate structures. Whether you are in a construction or design program or you’re just curious about what makes structures stand, here is a simple examination of the very building you may be reading this article in right now.

Though this article details typical North American houses that are wood-framed (their skeletons are made up of primarily wood!), the principles of physics explained in the article apply whenever there is gravity.

Developing a bare-bones understanding of a house’s skeleton can help a homeowner grasp why the idea of rearranging load-bearing walls that already exist is far more than meets the eye. 

Your contractor will be impressed that you already know what’s up, as you discuss ideas about what your possibilities are. 

You can even impress your friends when you go with them to volunteer on a Habitat for Humanity build side, as you match theory with practical. Take care while you’re having fun to keep safety as number one though. After all, you need to take home all your digits on the hands you used to enthusiastically hammer together the wood frame.

All right, here we go… To the window….! To the walls! I will stop singing… promise. You know me, I had to get one out.

The Basics

You can have scores of four-credit structural engineering courses in your undergrad teaching you how to crunch numbers and make sketches worthy to lift the building off the ground. But often seeing how these courses connect to form the bigger picture should be a course in and of itself!

Ah!  But what you might learn here might impress even them. We will break things down - to build them up! Let’s examine the complexity of the structure through simple principles of physics.

This main principle of physics we will examine is called load transfer.

How about we brush through some definitions before we get into the meat of it? Worry not! We are going one step at a time.

Definitions and concepts

Load transfer: the transfer or distribution of load or loads across a structure so it can remain stable or standing.

Load path: the transfer of loads from one member to another.

Load: any force acting on an object, including its self-weight.

Self-weight: the mass of an object and the influence gravity has on it. (It can also be called the weight of an object. But since structures also have other loads applied, weight can be the total weight of the structure with those loads applied. Whereas self-weight is the weight of an object without those loads applied.)

Dead load: a load that is permanent, such as self-weight, and equipment loads.

Live loads: a load that is more temporary or dynamic/movable, such as people, furniture, wind, and snow.

Member: a singular object as part of a system or structure. This may either be load-bearing (like a beam, joist, walls, lintels, or foundation walls) or non-load-bearing (like a window, door, partition wall, or architectural post). Load-bearing members do the work of holding the structure up, while non-load-bearing members have other functions.

Fasteners: any object that fastens/connects members together, such as nails, bolts, screws, and welded or bolted joints.

System: an assembly of members that are unified in their purpose of supporting a load, such systems are walls, floors, roofs, and foundations.

Structure: a constructed form or an arrangement of members built to support its self-weight (like a statue or obelisk) or to support its self-weight and applied loads (like a bridge, dam, skyscraper, multi-storey house, or bungalow).

Substructure: the part of the structure that is below the ground level, such as the basement and the foundation. Typically houses have concrete foundations, and if they have basements, they are usually made of concrete as well.

Superstructure: the part of the structure you see above the ground level, such as the walls, floors, and roof. Typically houses have wood-framed superstructures. (I know, I thought superstructure meant that a structure was bigger than average, but that term is megastructure, like the San Francisco Bridge, Hoover Dam, or the Notre Dame Cathedral).

A simple structure is an arrangement of members (like your camping tent or bookshelf). A more complex structure is an arrangement of systems that are made up of members (like a house or high-rise building).

Load Transfer Through the Structure

Load transfer from top to bottom, system to system

Typically for structures, load transfer happens from top to bottom, from system to system.

Let's examine how load transfer works within a structure. 

Load transfer

Considering gravity, there is a load called “self-weight” which is the mass of all members that make up that structure as gravity influences it.

Many structures are built for the primary purpose of supporting loads applied to them, so they must support their self-weight as well as these loads. 

A bridge is built for the passage of traffic over a gap. 

A building is built to support and protect its occupants. 

A dam is built to hold back water in a reservoir. 

Therefore, both the self-weight and applied loads are considered in the design and construction of structures.

Even with structures that don’t have applied loads (such as statues, monuments, and sculptures), an applied load, a live load called maintenance load, is often considered for them. 

Am I saying that this is why one of the Sphinx statues in Egypt is missing her nose? (Lookin’ at you Disney’s Aladdin).

In this article, we will examine the load transfer of a typical house by first examining the types of loads.

Here is a list of typical load types applied to a house (as well as many other buildings):

1. Dead loads (including self-weight and permanent fixtures).
2. Live loads due to loads applied externally, such as snow loads, wind loads, and seismic loads.
3. Live loads due to loads applied internally, such as floor loads (including people and furniture).

Seismic loads are loads due to earth movements, such as tremors or earthquakes, and apply to buildings when they are built in seismic zones.

Snow loads are applied on the roof, wind loads on the exterior walls, and seismic loads on the structure.

Now that we have examined the types of loads, how does the structure withstand them?

But how are they transferred?  Well, they must be connected. 

Connectivity

Connectivity is a no less important principle of physics. Systems or members need to be connected for there to be load transfer.

If the floors are not connected well to the walls, the roof trusses are not connected well to the posts, or the stairs are not connected well to beams, disaster strikes! Members within a system also need to be connected properly. If a joist is not connected to its beam, the floor system is in trouble.

Here’s another analogy. If you’re not familiar with the folk song Dry Bones[i] by the Delta Rhythm Boys, you will be now. 

Dry Bones examines a simplified examination of the skeletal structure[ii] from bottom to top… saying that your toe bone is connected to your foot bone, your foot bone is connected to your heel bone, your heel bone is connected to your ankle bone, your ankle bone is connected to your leg bone, your leg bone is connected to your knee bone, your knee bone is connected to your thigh bone, your thigh bone is connected to your hip bone, your hip bone is connected to your back bone, your back bone is connected to your shoulder bone, your shoulder bone is connected to your neck bone, your neck bone is connected to your head bone. 

The song also thrills disaster when you “disconnect dem bones”. Principles of physics applied to your own body!

Now that we’ve understood connectivity, here is a system-to-system examination of load transfer.

Top to bottom, system to system

Each system has dead loads (including its self-weight) and live loads applied to it and transfers them to the system supporting it.

Let’s have a closer look.

The roof takes off the dead loads and live loads (including snow loads) applied to it and transfers them to the walls.

Each suspended floor (upper floor) takes off the dead loads and live loads applied to it and transfers them to the walls.

The ground floor, if it is a suspended floor (such as a house with a crawl space between the ground floor and the foundation footings, or a house with a basement), takes off the dead loads and live loads applied to it and transfers them to the walls.

The ground floor, if it sits on the ground, (such as a house with no crawl space between the ground floor and the foundation footings, when the ground floor consists of a slab on the earth called a slab-on-grade), takes off the dead loads and live loads applied to it and transfers them to the earth.

Each wall takes off the dead loads and live loads (including wind loads) applied to it and transfers them to the walls below, and eventually to the foundation walls.

Each foundation wall takes off the loads applied to it and transfers them to the foundation footing.

Each foundation footing takes off the loads applied to it and transfers them to the earth on which it sits.

And about seismic loads, they are taken off by the systems as well and are transferred back into the earth. 

Structures that are built in seismic-prone regions are specially designed and constructed to withstand the repetitive lateral (side-to-side) loads of seismic energy without suffering significant structural damage.

Structures specifically designed as such may still be vulnerable to damage when seismic energies become too great (anything six and up on the Richter Scale). A structure therefore can never be designed to be earthquake-proof but can be designed to be seismic-resistant[iii].

Now that we examined load transfer through the systems, let’s take a closer look at the systems themselves.

Systems of the Structure

We will examine the systems of a typical house (or typical building) one by one:

1. Roof
2. Walls
3. Floors
4. Foundation

These systems are made up of members, some of which will be mentioned as you read along.

We will examine how the structure is built from bottom to top, system by system. But first, let’s have a look at the design of a house to understand the bigger picture before we delve further into detail.

The floor plan

When a house is designed, floor plans are an integral component. Floor plans are the common name for drawings of a building.

Any detailed design of anything, from your camping tent assembly to the boxer engine of a Subaru SUV, has drawings. Drawings are sometimes called blueprints. (They are called blueprints because designs used to be printed in blue ink on large specialized plotters. Now we can have multi-coloured prints on plotters that are a bit more sophisticated.)

The drawings of a building contain plans (looking top-down) of each floor, elevations (these are exterior elevations of the building facing North, East, South and West), and sections (which are slices cut through the building and at certain complex assemblies).

What you often see in showrooms when shopping around for houses are the floor plans (of the suspended floors and ground floors) and elevations of the building. The sections, foundation plan, and roof plan often remain with the designer. When a homeowner buys a home, they don't get all of these drawings. 

However, when a homeowner hires a designer or architect to design their house, they can request a copy of all of these drawings as part of their contract. Otherwise, designers keep their drawings are part of their intellectual property so that no one can copy their hard work.

Alternatively, a homeowner can have contractors/builders design and construct their house. The contractor usually subcontracts the design to a design consultant, depending on how complex the structure is.

Houses are designed and built according to certain standards, otherwise, things could get very confusing all the time!  These standards are called the local building code.  The builder applies for a local permit from the government to build, and then designs the house while complying with the standard.  The build site may be subject to inspection by the government. 

The design of the house involves drawings. 

A homeowner can grasp a general layout (locations and dimensions of rooms and spaces) by looking at the drawings, but may not necessarily understand how the structure stands. However, we can deepen our understanding by having a closer look.

The roof outline will match the wall outline, which will in turn match the foundation outline. The roof outline, however, since roofs usually overhang, will show a larger area than the walls. And the foundation outline, since foundation footings are wider than the foundation walls, will also show a larger area.

These outlines may change slightly from level to level when different features are involved, such as differently shaped roofs, floors that have balconies, ground floors that have patios, ground floors that have garages, foundations that have basements, etc.

The wall outline is of the exterior walls (aka, the perimeter walls), and these transfer the structure’s load unto the foundation. Since houses can cover big spans, however, more walls need to be added in the middle, and foundation walls to support them. These walls in the middle are often called interior walls. The floor plan should also show them. These interior walls are not to be confused with partition walls that are non-load-bearing. Interior walls are often the target of homeowners to knock down when they want to rearrange spaces. 

And remember what happens when we interrupt systems?  Just like a circuit, it will disconnect flow, by disconnecting load transfer, causing structural problems.  But what if there's still a connection but members are only shifted or narrowed or now have openings?  This still affects load transfer.  Because if a load-bearing member is designed for a certain load and suddenly gets diminished, whole systems are affected. 

So the outlines of the floor plans, which are usually outlines of walls (load-bearing walls) give us an idea of the load path.  And sections show us a deeper look. 

A builder's drawings usually contain a full section of the house that can help us understand in more detail how the structure stands. This full section is explained closer to the end of this article after we grasp more of an understanding of building the systems.

So let’s return to building the systems for now.

Building a structure from bottom to top, system by system

This is how you typically build a house from bottom to top. I mean you can start from top to bottom, but that would involve more strategy, time, and expense.

From bottom to top, we go from the foundation and the walls to the floors and the roof.

The foundation

Aka the substructure. Ain’t nothing can happen without this.

The foundation of a house is made up of foundation walls supported by foundation footings. If there is a basement, there are basement walls and a basement floor, the floor being a slab-on-grade. The basement walls sit on the foundation walls, which are then supported by foundation footings.

The foundation footings are usually called strip footings in typical North American construction. This is because the footings spread out under each strip of a foundation wall, taking all the load transferred from the structure and distributing it over a large area of the earth.

These foundation footings sit under each foundation wall. And foundation walls support the walls of the superstructure.

Foundations can be a little different from house to house. 

Bungalows are one storey and therefore may not require strip footings so they often sit on their ground floor slab alone – and that is their foundation.

However, in many places in North America, there is something called the freeze-thaw effect which happens above the frost line.

This effect happens in areas that experience four seasons, and the temperature differences that come with it. Winters freeze the water present in the earth, while spring and summer thaw it back out. When water freezes, it expands in volume. As you go deeper than the frost line, the freeze-thaw effect is not as prevalent because deeper earth maintains more of a constant temperature being further away from surface temperatures.

The frost line is the level under the ground (around four feet deep in Ontario Canada) where the water in the ground expands and contracts in volume as it is affected by freezing and thawing, season by season.

This freeze-thaw effect is powerful enough to crack and even fail foundations of various structures, from residential privacy fences to multi-storey buildings, if their footings lie within this depth.

For this reason, many bungalows, though they do not structurally need footings, often have them poured anyway more than four feet underground.

There is another reason why bungalows may have footings. Homeowners may want to expand the structure by building upper floors later on. Therefore, builders will construct the bungalow to have footings and sturdier walls.

And speaking of sturdy, the foundation is the strongest system in the house, because it needs to support everything else.   Therefore, concrete is often the structural material of choice for foundations. 

For either bungalows or multi-storey houses, foundations are usually made up of poured concrete and often have rebar (reinforced steel rods).  And that's the substructure. 

But how does a superstructure fit onto its substructure, if they are made of two completely different materials? The superstructure (wood-framed house) is made of wood, whereas the substructure (foundation) is made of concrete.

That’s where some integral fasteners come in. And what are these fasteners called?.... Anchor bolts! And they do just that; anchor the house unto its foundation.

Anchor bolts are affixed into the foundation while the concrete is poured, so they protrude upwards for the superstructure to be bolted unto it.

An equally integral piece in anchoring the house is the sill plate or plate.

This treated wood member lies on top of the foundation wall and acts as the base for the superstructure. It is treated because there is always moisture content in concrete and moisture can cause wood rot in untreated wood.

We’ve got our foundation. We’re movin’ on up!

Platform framing

Before we continue to examine the building from bottom to top, let’s examine the arrangement of the structure. The arrangement of wood frame houses in North America is called platform framing.

Houses were usually built with posts, where posts would act as columns from top to bottom – as long continuous members. Fires loved this way too much and a house would be gutted far faster when posts were eaten in one fell swoop. Platform framing not only distributes loads in a more interlocked way but provides more diversity of members for fires to work through.

Though this offers more buffer, it does not mean that you take your own sweet time exiting in an emergency. Most people don’t succumb from burns, but from smoke inhalation, taking in all of the toxic chemicals released when building materials burn, like drywall, insulation and asphalt shingles.

Post framing limited houses to be built only as tall as posts would go, whereas platform framing does not rely on tall posts.  The longest vertical member in a wall is a typical stud anyway. 

Platform framing is also why you look at the skeleton of the house, the wood frame, the superstructure, and scratch your head as to where the columns and posts are.

With wood-framed houses, though there are makeshift posts and columns, they are small and are made up of clusters of studs within wall units or outside of wall units.  These clusters support point loads (like roof trusses) or stairs. 

The load transfer however is still primarily to the walls. This does not mean that you should knock out load-bearing posts or columns, or even deign to knock out walls, without the direction of a structural specialist.

The platform interrupts continuous columns or continuous posts. The platform here is the floor system. Because instead of the floor system connecting to continuous columns, it extends where the beams on either end of the floor system are sandwiched between the walls on the upper level and the walls on the lower level.  This sandwich system provides the interlock. 

This sandwiching will be further explained when we examine the walls and the floors.

Now that we have an idea of what platform framing is, we can get back to building from bottom to top.

Walls

Walls systems are made up of sills (horizontal members) that sandwich studs (vertical members).

Since walls in a house are constructed in this arrangement, they are often called wall units or wall assemblies.

Sills of walls are often double-plated between floors for a sturdier design, and often sandwich metal flashing to help with noise dampening between levels.

Within a wall unit, some studs are fastened together into a makeshift post or column to transfer point loads better.

A point load (or concentrated load) happens when the load on a system or member is concentrated rather than distributed along a line, strip, or large area.

What can cause point loads in a house? Roof trusses for example can take off larger loads as they can be placed further apart than rafters. Like rafters, roof trusses sit on the wall. However, the wall unit will contain a cluster of studs to support each truss. This cluster is called a built-up post or column.

But how come you don’t see that cluster protruding out like you would see a larger column? That is because the clusters are made up of typical studs stitched together by nails.

Walls not only carry vertical loads, but walls also make up the vertical load path of a house. However, just because the majority of the loads applied to them are vertical, they can still tip over due to other loads, taking the entire superstructure with it.

This effect of loads applied to a frame causing it to sway is called side sway. What can cause side sway? There is wind, for one. And even during construction, the entire wood frame can sway and fall flat due to movements if the frame is not properly protected. A frame can also have uneven loading during construction that can cause it to side sway. If the levels are being built up one side at a time, instead of level by level, this can create uneven loadings.

How is side sway prevented?  There are several ways. 

During construction, builders use blocking in the wall and construct shoring.  The blocking is permanent, but the shoring is temporary until the wall sheathing is installed. Blocking is made up of cut studs placed horizontally and sporadically between studs in wall units. Shoring is bracing or frames that are built up to support vertical systems.

When the wall sheathing goes on, this helps immensely in preventing side sway.  Sheathing is the wall's plywood surface on which wall finishes (exterior and exterior) are attached.  Sheathing sandwiches the wall units on the outside and inside. It makes it more difficult for the wall to fall to any side, as well as distributes wind loads more evenly during construction.

So there are walls everywhere in a house!  Are all of them load-bearing?  No.  Not all wall units that are built are intended to support applied loads. 

It would be difficult to tell whether a wall is load-bearing or not just by looking at it, especially when wall finishes go on and everything looks the same.  It would be best to have a structural specialist do an assessment before you attempt to modify any wall.

In constructing a house, how does the house go up straight when there are no columns or posts to speak of? The walls need to be installed straight up. Builders check their straightness by using tools like spirit levels, and they check that each house level is plumb, plumb meaning vertical and even. And they check this often. 

We're almost done with walls.  

For the structure to serve its other building functions, walls have openings for windows, doors, vents, piping and ducts. The wall openings for doors and windows are topped by lintels (a short beam to hold up the wall from falling into the opening).

As for their look, walls can be complex and come in different shapes, such as arcs, and bay windows.

Now... if the superstructure of a house is wood-framed, why do houses look like they're made of brick? Because some of them are, and they have masonry walls instead of being wood-framed.  However, this is often not the case with houses in North America.  So... what is typical then?

This means that the exterior walls of a house can be wood-framed but have a brick surface.

This exterior surface of a house is called cladding or facing.

Wall cladding can be brick veneer, shingles (clay, tile or asphalt), or siding (aluminum or vinyl).

Brick veneers are heavy and are not only strapped to the walls, but just like the walls are supported by the foundation walls, they are too.

Floors

Floor systems are made up of beams or girders that support floor joists on which the floor sheathing sits. The beams then transfer the loads to the walls.

The joists are often affixed to the sides of the beams so the floor system doesn’t take up too much vertical space. Otherwise, joists are affixed on top of beams.

Either way, the top of the floor system must be flush, flush meaning horizontal and even so that the floor sheathing and then the floor finishes can have a smooth surface.

Since floors are built in such an arrangement, they are often called floor units, floor assemblies, or subflooring (the skeleton members you see before the floor sheathing and then the floor finishes such as carpet or hardwood are installed).

Floors can support some concentrated loads, such as those caused by stairs. Stairs need extra support. This is why the members (beams) that border a stair opening are doubled up or are made of stronger members than a typical joist until they connect with a beam perpendicular to them.  Alternatively, stairs are supported by a wall, post or column. Either way, the top and bottom of the stairs are supported. 

Since houses have floor assemblies and wall assemblies, this is why this type of wood framing is called platform framing. 

The member of the floor system that is sandwiched between the walls is the beam.

Each floor system is supported by the wall system beneath it.

The ground floor can either be a suspended floor (just like the upper floors) or it can be a slab-on-grade. When the ground floor is slab-on-grade, the dead loads and live loads applied to the floor are not transferred to the walls, but to the earth instead. Likewise for a basement floor.

In recent decades, for suspended floors, the engineered I-joist (a special wood joist that has an I-section, or an I shape when you slice across it) has replaced many common wood joists in the superstructure. Fewer of these I-joists are required than regular joists as they are stronger and can therefore be spaced further apart. They can even cover longer spans. Sometimes even floor trusses are used instead, capable of covering even longer spans.

We're almost done with floors. 

For the structure to serve its other building functions, floors have openings for stairways, hatches, vents, piping and ducts.

Openings for stairways are bordered by joists or beams which transfer the loads to other beams perpendicular to them or transfer the loads to walls, posts or columns sitting under them.

Floors can also protrude out to balconies on the upper floors, and patios on the ground floor. When a house has a garage connected to the main structure, the garage floor is slab-on-grade, is built for vehicular loads, and is separate from the ground floor (regardless of if the ground floor is a suspended floor or slab-on-grade).

Floor finishes can be hard (engineered hardwood, wood laminate, tile, concrete for slab-on-grade) or softer (carpet).

Roofs

We're made it all the way up the the last system - roofs!

Roof systems are made up of the ceiling system and the arrangement on which the roof sheathing sits.

The roof ceiling is a makeshift floor system that is built to support the ceiling beneath it or to support the floor if the attic is walk-in.

Roof systems are some of the most complex systems of a house as roofs can take on many shapes, from simple lean-to and pitched designs, to complex hip and gable designs with multiple sections covering the floor plan.

This is often where a two-dimensional thought process morphs into a three-dimensional one, requiring more complex geometry.

Either way, the roof system is supported by the wall system beneath it.

In recent decades, prefabricated roof trusses have replaced a lot of common roof members, as their arrangement gives the advantage of being stronger as a unit, less stocky, using less materials, and can therefore be spaced further apart, and even cover longer spans.

In a traditional roof system, the ridge and rafters act as the backbone and ribs of the roof. The ridge is the highest and longest horizontal member, and it is situated in the middle, from which the rafters fan out. The rafters sit on top of the upper floor wall, or on top of the roof wall such as in an attic space.

Purlins are affixed on top of the rafters, providing laterals for roof sheathing to be attached on which the roofing materials (roof finishes) are installed.

In a roof system that employs roof trusses, the trusses replace the ridge and rafters, and purlins are affixed to them instead.

In the photo below, some of the roof trusses are installed, while others are lying across the tops of the walls ready to be erected. 

We're almost done with roofs. 

For the structure to serve its other building functions, the roof can have openings for chimneys, skylights, vents, piping and ducts.

Roof cladding (roof finishes) can be shingles (clay, tile or asphalt) or sheeting (aluminum or vinyl). Roofs can also exist as a flat poured concrete slab or a deck.

Now that we have examined building a house from bottom to top, let’s have a look at it all together.

How it All Comes Together

Now that you know about the systems, how do they all come together? The drawings can tell us a lot.

The full section

A full section is often called a building section and is a slice from top to bottom throughout the structure.

We know that load transfer from system to system goes from the roof and floors to the walls, the walls to the foundation, and the foundation to the earth.

With a full section, we can see more detail.

The building section also identifies the members in this load path.

The roof’s rafters or trusses sit on a sill and are supported by a wall unit. Wall units sandwich the floor beams on each level until you get to the ground floor, where the ground floor beams are sandwiched by the wall above them and the foundation wall below them, with a treated sill plate on top of the foundation wall. The foundation footings support the foundation walls and are built deeper than the frost line.

Notice here that the beams are being called floor beams since they belong to the floor system. However, many builders simply call them beams, because they are horizontal members that transfer loads to vertical members.

The Structure that is Your House

So there you have it. The structure as it stands. How it transfers its loads from top to bottom, while it is built from bottom to top.

Notes

This article contains no text pictures to ensure that every word can be read aloud by a text-to-speech application. And was tested using Google Chrome’s “Read Aloud” add-on.

Bio

Tiffany Persaud, E.I.T. is a civil-structural engineering intern and freelance writer that resides in Toronto Canada. Her experience designing residences is coupled with her portfolio of physically building homes across Greater Toronto with crews from Habitat for Humanity. She also designs and supervises the construction of apartment buildings in Guyana, South America.

Resources

Have a construction project you’d like a quick consult on? Book me for more information. Rates apply.

References

The Honest Carpenter: “All House Framing EXPLAINED… in Just 12 Minutes (House Construction/Framing Members)”. Feb 20, 2021. Accessed on Oct 20, 2022.