How To Design a Building Foundation (Ultimate Guide)

Giovanni Valle is a licensed architect and LEED-accredited professional and is certified by the National Council of Architectural Registration Boards (NCARB). He is the author and managing editor of various digital publications, including BuilderSpace, Your Own Architect, and Interiors Place.

Whether your project is large and complex or small and simple, you will need to create a foundation that fits the overall plan. The foundation is what you build everything else on. The most essential part of any building structure is planning to secure it into the ground building from above.

Here’s how to design a building foundation:

Decide what type of foundation you need.
Design your foundation layout.
Decide the location of columns & foundation walls.
Design drainage & waterproofing.
Decide depth of foundation & calculate foundation area.
Determine variation in vertical stresses.
Prepare your foundation area.
Excavating & compacting your foundation area.
Start building your foundation.

As a new homeowner or a professional builder, you often need to do some research and invest in tools that will help you while designing or building your home. In this article, I’ll give you some tips on designing a strong and solid foundation for any type of home or building. By the end of this article, you will have a good idea of how to design a building foundation.

1. Decide What Type of Foundation You Need

There are three main types of foundations: Slab-on-grade, Basement/Crawl Space, Full/B Wall.

A slab-on-grade is built on top of the ground, usually reinforced with concrete. The dirt below should be compacted or crushed before construction begins.
A Basement/Crawl Space foundation is typically one solid wall that goes straight down to the bedrock beneath the home. The wall may only be 1-2’ (0.30-0.60 m) thick because it does not need to go beyond 2’ (0.60 m) into the ground. But it will require waterproofing and insulation against moisture to protect your living space.
A Full/B Wall foundation uses steel beams to secure your home into place. This type of building requires a significant investment in time and money. Still, an experienced builder can create a beautiful piece of architecture while securing your home for decades to come.

2. Design Your Foundation Layout

The next step is to lay out the foundation so that it fits your needs.

If you are building on a sloping or uneven lot, be sure to have your builder create a plan for an engineered solution. Create a wooden frame, level off the ground with gravel and sand, then pour concrete around this structure.

This will reduce any settling of the slab or cracking within the future foundation.

If you are building above water, make sure that your builder plans how they will waterproof the foundation ahead of time. The most common method used today is called “waterproofing paint,” where they coat the whole area in several coats of sealant.

However, there are other options, such as French drain systems or Flexible Pavement Coating, which can dramatically increase the lifespan of your future foundation.

Designing your foundation layout is the most important decision you will make in your building project. Be sure to design the layout ahead of time because these decisions will affect how much space you can build on and where all your appliances and rooms fit into place.

Typically, waterproofing is applied first around the perimeter of your home.

Depending on local building codes, you may want to use another method since they are less expensive than mortaring but will not crack like concrete either. Mortar work ties everything together into a firm structure that can withstand water erosion over time.

If puddles and water lines appear at your property, you should hire a local building professional to help you determine the root cause. Having an experienced builder design and install your foundation will provide peace of mind that you are taking care of this problem once and for all.

3. Decide the Location of Columns & Foundation Walls

Another step in your foundation design is to decide where you want support pillars, walls, and plumbing vents.

Location of Columns – The best placement for columns generally falls between 6-12’ (1.82-3.65 m) apart. This distance was once determined by the length of wooden beams used for building foundations (generally about 12’ or 3.65 m), but now it is typically driven by electrical lines which run through homes, bringing electricity from one room to another.

If you must place a column either closer or further than normal, be sure that your builder understands how this change will impact the rest of the structure.

Location of Foundation Walls – The best location for foundation walls is usually on the outside of your home, away from any major traffic or heavy load-bearing rooms. For example, if you have a bedroom with no windows and only one door, you might want to place this room closest to the edge of your foundation where possible.

Also, consider whether your heater ducts will run along inside or outside of the foundation wall.

If this is part of your plan, you may need to arrange for different sized windows to accommodate a larger exterior wall which could reduce natural lighting in some areas of your home.

4. Design Drainage & Waterproofing

Designing drainage and waterproofing for your foundation is perhaps the most essential part of your foundation design.

Unless you are building on flat land, water will likely be flowing across your yard and under your home. A good builder should always consider this when designing your concrete foundation layout.

French Drainage Technique – There are several different drainage techniques available to builders today, but “French Draining” seems to be the best option for most homeowners.

French draining often involves running a perforated pipe or tubing through gravel below the entire foundation with weeping tiles around the edge. They empty into an existing sump pump with discharge pipes leading back to just outside the primary building wall.

Your Foundation Should Be Waterproofed – Your last step before construction begins is to make sure that your foundation is waterproofed and drained properly. This can save thousands in repairs later on if water is allowed to collect on your foundation.

5. Decide Depth of Foundation & Calculate Foundation Area

Remember that the deeper your foundation, the higher price it will cost to build. Plywood or other waterproofing material can be used on top of your concrete foundation if you plan to dig shallow trenches for your wire/cable.

An average foundation costs around $6,000-$15,000, depending on size and complexity. You will need to know the area of your foundation to budget for materials and manpower. Assume that for every $100 you spend on materials/labor, it will take roughly 4 hours of work to complete your foundation.

Time is money. Be sure to plan ahead and get more than one estimate so that you know how long the job will take and how much it will cost before construction begins.

6. Determine Variation in Vertical Stresses

After deciding how to waterproof and protect your future foundation, you’ll want to figure out what type of forces will press against the foundation, especially due to earthquakes or extreme weather conditions, such as floods or hurricanes.

This is most easily accomplished by hiring a local professional who can perform these tests.

The next step is to ensure that the area of your foundation does not exceed 75% of your soil’s ability to resist pressure.

Calculate Load Bearing Capacity Through Standard Penetration Test

A standard penetration test or Soil Penetration Test often called an SPT, is used to determine the LBC (Load Bearing Capacity) of an area by testing samples from different depths in your soil. The procedure involves inserting a metal rod into the ground at various angles and measuring the amount it penetrates into the ground.

Most soils have an average penetration resistance between 50-100 lbs per sq ft (0.34-0.69 psi), so be sure to choose a level lot when your SPT is below 75 lbs per sq ft (0.52 psi).

This process will take 4-10 days to complete and cost roughly $300-$500 depending on the size of the area tested. The average LBC for most areas is between 100-200 lbs per sq ft (0.69-1.38 psi), which means you need your soil tested before building.

If your soil falls above these levels, call in an engineer or construction company to create a plan for drainage and righting support structures if necessary.

7. Prepare Your Foundation Area

To prepare your foundation area:

Using a shovel or other carving tool, clear out the area where you will lay your foundation.
Be sure to remove any roots from trees and weed away any grass to not interfere with the construction process.
Next, mark off the ground where you are planning on making your foundation. Use wood stakes or markers if dealing with larger projects, or just use some string between two points if working alone.
What’s important is that both corners of the future foundation are marked off to get accurate measurements for construction.

8. Excavating & Compacting Your Foundation Area

To begin excavation, simply dig down at least 2’ (0.60 m) wide all around your marked area, leaving at least 1’ (0.30 m) above the ground as a safety net in case the ground below isn’t compacted enough.

After you have dug down at least 2’ (0.60 m), go back over your area with a compactor or roller to ensure all excess dirt is gone, and you are only left with what will be your foundation. Now fill in any holes or cracks with excess dirt if needed.

You want the surface of the earth to be as flat and even as possible before placing your foundation on top of it.

Build Forms for Your Foundation

As mentioned earlier, basements use one solid wall that goes straight down to bedrock. If this is your type of building, find where your footing will go and build a form from wood or other solid material.

The height of this wall should be equal to the thickness of your foundation plus 1’ (0.30 m). Then dig a trench around your marked area that is 2’ (0.60 m) deep and about 4” (10.16 cm) wider than the width of your walls on each side.

Fill the trench with gravel, then pour concrete into it until you have formed a solid footing for your foundation wall.

9. Start Building Your Foundation

Now that you have planned out your foundation design, it is time to break ground.

Make sure to plan with your builder by marking off where the following steps should be placed. For example, if you are building a basement/crawl space foundation, set up the timber frame before pouring concrete so that they fit together perfectly.

After all materials are on-site and ready to go, start digging. If you do not want any trenches showing later, say for additional plumbing or wiring, then make sure that these trenches are built into the wall itself before installation begins.

Move slowly but surely, keeping track of where each brick will be laid down because this process will be much harder to correct in the future if not placed correctly.

Choose Your Footings

After the foundation has been laid out, it is time to choose your footings.

Footings are essentially the base of your foundation, but they should always extend several inches beyond the outer border of your foundation. This will make leveling your home easier during construction and provide additional support for any weight that may come onto the main structure.

Building codes require that footings be at least 6″ (15.24 cm) thick unless you are building on expansive soil or an un-compacted base rock formation, such as granite. Make sure that you discuss what type of footing system your builder plans to use with them before starting construction.

Conclusion

Now that you have the basics of foundation design, it’s time to get started.

The first step is deciding what type of foundation will work best for your needs and property layout. Once this decision has been made, consider how many columns are needed to support your desired building height.

Next up is designing a waterproofing system that includes drainage systems and waterproof materials used during construction to prevent water from seeping into the basement area once the house has settled over time.

Follow these steps, and you’ll have a stable and strong foundation for your building project.