Creating a Foundation Plan

First a bit of background knowledge:

A foundation is the structure that holds up your home or building. Like anything creating a good solid

foundation before building on top of it is essential. This is true for buildings, schooling, sports etc. In

school you need to create a solid base of learning before you can move onto the more complicated

tasks. In sports you need to master simple skills before you can learn the more advanced ones. When we

talk about buildings we need to create a solid foundation that the home or building can sit upon so that

it won’t fall down or partially sink into the ground.

A buildings foundation is built of concrete on what is called “undisturbed soil”. The footings of the

foundation (the solid base) are placed a specific minimum distance below the surface of the ground, or

below the frost line. A frost line is the distance below ground that the earth will freeze and then thaw

out therefore changing from a hard (frozen) structure to a soft (thawed) structure. Below the frost line

the soil stays a consistent density and this is the soil that we want to build our foundation on. In

Chilliwack the minimum distance below the surface in which we can build the footings of our foundation

is 18”. This means that the Footings of the foundation will be below the frost line and built on dense

undisturbed soil. Note that if you dig to deep in any area and have to fill in areas to create a flat surface,

the areas that were filled must be compacted properly to imitate the density of the solid undisturbed

soil.

Starting your Foundation Plan

You will either be creating a crawlspace or a basement.

If you are creating a crawlspace you will be creating plans for the structural elements that hold up your

building.

If you are creating a basement you will be creating another floor plan as you did for the Main and

Second floors and then adding extra information to this plan that include all the information for the

structural elements as with the crawlspace.

1. Copy your main floor plan within your drawing to a new location off to the side

2. On this new copy delete any electrical, appliances, fixtures, doors, and window as these items

will need to be placed appropriately in the Foundation plan if necessary later.

3. Currently your exterior walls are 6” thick. Your exterior concrete foundation walls need to be 8”

thick. Change the current 6” exterior walls to be 8 inches in the properties (AutoCAD

Architecture) or by offsetting the inside lines towards the middle of the building by 2 inches

(Regular AutoCAD)

a. It is important that the outside dimensions of your building do not change and that the

2 inch difference happens on the inside of the building.

b. Think of the garage as it’s own building (all 8” thick concrete walls with the extra 2”

being added towards the center of the garage.

Before moving onto #4 you will need to know a bit about floor joist size and span capabilities.

(see next page)

Canadian wood-frame house construction (based on 2005 national building code)

Floor Joists support the floor in a building. They span from one wall to another allowing plywood to be

placed over them in which the next floor can be built. Our task is to figure out what size (blue box),

spacing (green box) and support (yellow box) are necessary for our design. NOTE: The Orange boxes

separate the supports into their 3 sections.

An example of a Floor Joist choice (purple oval): 2x12 Floor Joists @16” O.C. with Bridging (This spans 16’-2”)

24” O.C.

O.C. = On Center

4. Creating adequate structural supports (walls that the floor joists sit on)

a. Basement: The walls of the rooms that make up your basement floorplan will be the

supporting structures for the floor joists.

b. Crawlspace: You will need to create structural 4” or 6” walls to be used for support of

the floor joists. Often these walls are placed under the walls of the main floor plan as

this supplies a more solid support up through the building, however these new

structural walls can be place anywhere necessary to support the floor joists adequately.

Note: Large open rectangular spaces are OK as long as the floor joists can span the short side length

Also Note: The garage area does not need any floor joists

Before moving onto step 5 make sure you have the following complete:

a. Exterior walls (green above) are 8” thick and the extra 2” is on the interior keeping the

exterior dimensions of your building the same (see step 3 for more detail)

b. Structural walls (red above) have been added and no span is more than 17’-10” as this is the

maximum span we can accommodate with the chart we are provided with.

5. Floor Joists (blue arrows with text in above photo)

Note: Most homes will use either 2x10 or 2x12 floor joists, 2x8’s are rarely used for main structural

flooring

a. Draw in your floor joist arrows and the arrows that show where that specific set of

floor joists starts and stops. Notice how the half arrows on the ends are on opposite

sides of the line allowing them to overlap with other floor joists arrows if necessary.

(blue arrows in above picture)

b. Ask yourself, “what is the longest span that I have to cover?” Ex. 16’-5”

Floor Joist arrow Arrow that shows where a

specific set of floor joists starts

and stops

Small circle that shows the

connection between the two

arrows

c. Using the chart on the previous page, find the appropriate floor joist combination that

fulfills your largest span and write in the appropriate information along the Floor Joist

arrow as seen in the sample picture on the previous page.

Ex. 2x12 floor joists @ 12” O.C. with bridging

This example floor joist spans 17’-2”. Note that the span distance is not listed on the plan.

Note: if you use 2x12 floor joists for your largest span you must use 2x12 floor joists everywhere

as using 2x10 or 2x8 floor joists in a smaller section will cause the flooring to be uneven.

d. Complete all of your floor joist information using the chart and choosing the appropriate

floor joist combinations for each. Remember that we want to try and use the cheapest

option… let wood means less money. Therefore, as an example, 24” O.C spacing is going

to be cheaper than 16” O.C. and 2x10 floor joists are going to be cheaper than 2x12

floor joists. Use the cheaper options whenever possible.

6. Creating openings in your walls for access to all areas of your crawlspace / basement

a. Basement: you will likely already have access as you have doors and rooms designated

for use.

b. Crawlspace: you will need to create openings in your walls (minimum 3’ wide) that allow

access to all areas of the crawlspace. This allows for extra storage and more importantly

access to check for and repair any damages in the exterior foundation walls.

7. Create beams over openings to support your floor joists

There are a few charts similar to the Floor Joist chart that allow us to chose an adequate beam

for a specific span and how much it is supporting.

I have included 3 tables for use in this project. Which one you use will depending on your specific requirements.

--- Table 1 of 3 --- Built up floor beams for not more than 1 floor

--- Table 2 of 3 --- Built up floor beams for not more than 2 floors

--- Table 3 of 3 --- Glue laminated floor beams for large spans

To select the proper beam we need to clarify some Jargon.

Span: the distance that a beam or floor joist is unsupported

Supported length: the distance or distances that are being held up by the beam divided by 2.

This can be a little confusing so I have included two examples below.

In example 1 I have created a 3 ply 2x10 lintel (Lintel = built up floor beam that is used to support the

top of a doorway or window) that will support the 2x10 floor joists over the opening at the end of the

wall. The span of the lintel is 3’ (the distance of the opening below it) the supported length is 13’-8” + 9’-

4” all divided by 2. The reason that we divide this value by 2 is because the outside walls are supporting

the other half of the floor joists.

13’-8” + 9’-4” = 23’-0” divided by 2 = 11’-6”

Our supported length is therefore 11’-6”

If you look at Table 1 of 3 you will notice that we will have to use the 12’ support length row and that all

of the spans are adequate as we are only spanning 3’. Because we are using 2x10 Floor Joists we should

use a 2x10 beam to keep things even.

Example 1:

In Example 2 there are 2 beams, a 4 ply 2x10 Built up floor beam and a 3 ply 2x10 built up floor beam.

Both of these beams are only supporting the weight of the floor joists that are running from the bottom

of the picture to the beam. The floor joists that are shown above the beam in the picture are running

parallel to the beam and therefore do not have any effect on what beam we need to use.

The span for the beam on the left is 12’-8” and it has a supported length of half of 15’-1”. Looking at the

beam table (page 5, table 2 of 3) you will find that we can use a supported length of 8’ as ours is only 7’-

6 ½” and that using a 4 ply is adequate. We use 2x10’s because the rest of the floor joist structure is

being built with 2x10’s and we want to keep all of this the same.

The span for the beam on the right is 8’-4”. Looking at the same table (page 5, table 2 of 3) we can see

that a 3 ply 2x10 is adequate as our span is less than the 12’-8” that we need to use a 4 ply for.

Example 2:

8. Creating the Footings

Footings are the structure underneath the foundation wall that supports the building from sinking into

the ground (kind of like your foot supports you). Think of walking through snow on stilts… you are going

to sink right to the base of the snow as the stilts have very little surface area pressing against the snow,

if you walk over the snow with your normal shoes you have much more support and will likely sink less

than if you were on stilts. Now think about wearing snowshoes and walking over the same snow. The

purpose of snowshoes is to spread your weight over a larger surface area allowing you to stay on the

surface and not sink into the snow. The footing on a building is doing just this same thing, giving a large

surface area for the building to sit on so that it doesn’t sink into the ground.

In the Chilliwack area footings are usually built between 18 and 24 inches wide and are usually 6 inches

thick. They are constructed on what we call “undisturbed soil” which basically means that the soil under

the footing is very compact and will not move with the weight of the building. If we were to build a

building on “disturbed soil” the soil would settle and the building would sink until the soil under it was

compacted enough to stop sinking.

This is a picture of a cross section (which we will be talking about later in the course). This image shows:

1. The footing sitting on undisturbed soil.

2. The foundation wall

3. The floor joists that hold up the main floor which we just created on our plans.

You will be creating a 24” footing for your building. To create the footing you will create lines that are

offset from your foundation wall by 8” (hidden lines shown below) to represent the 24” width of the

footing. You will need to create a footing under all walls in your foundation plan.

If you have a deck, it will likely need concrete supports to hold it that will also require footings. The

spans and beam support structures for a deck are found the same way they are found inside a house.

The picture below shows an example of what a decks foundation / footings would look like.

1

2

3

9. Dimensioning

Dimensioning a foundation plan is done the same way you would dimension a floor plan with the

same 4 layers of dimensioning we have talked about many times.

Layer 1 (closest to the building): Layer 2:

- outside corners - outside corners

- major breaks - major breaks

- middle of interior walls - middle of interior walls

- center of doors

- center of windows

Layer 3: Layer 4:

- outside corners - overall length

- major breaks

On a Foundation plan where you simply have a crawlspace you will not have any doors or windows so

these parts can be ignored meaning you will likely only have 3 layers. If you have a full basement you will

treat the plan as a floor plan as it is a floor plan for your basement and a foundation plan all in one.

10. Labelling parts of your plan (see next page for pictures)

The last step is to label the parts of your plan so that whoever is reading the plan can find

information easily.

You will be creating a chart similar to the one shown below and placing markers like the ones shown

below to label the following parts of you plan:

a. Foundation walls (A)

b. Bearing Walls (B)

c. Deck supports (C)

d. Garage Flooring (D)

e. House (basement or crawlspace) flooring (E)

Note that “Cont.” stands for Continuous and means that the foundation wall or footing is continuous

around the whole building.

Last but not least you will need to label your plan with room names (only if you have a basement) like

you have done on your main and upper floor plans with the size of rooms included. You must also state

how deep the crawlspace or basement is with a notation.

Well Done!