When shopping for a home we often look at the candy coating and by-pass the structural components. I thought I would put together a short post about the basement foundations you would commonly find in Minnesota.
The foundation is what supports the entire house, so it can be said the home is only as good as its foundation.
How a basement foundation goes in today is not all that different from it was 100 years ago. Instead of digging the basement by shovel, we are using backhoes. Instead of Stone or 17″block we are using 8″ block or poured concrete walls. The big advancement is basements is not as much in the structural aspect but rather the waterproofing and radon ventilation, those I will address in another post.
To get a better understanding of how the foundation system works, let’s look at how goes together.
When building a new home the surveyor stakes the lot and house with wood stakes that come from the architect/designers drawing. The excavator comes in and digs the hole for the basement. Once the hole is dug to the proper depth and soil conditions to support the home, the foundation is reading to go in.
The block layers come in, measuring from the stakes the surveyor put in, they measure and begin to lay the footing framework. Often this will be with 2×12 boards, but that is specified by the engineers to be thick enough and wide enough to support the entire weight of the home including snow loads. The engineer will also specify the thickness of rebar reinforcement steel in the footing.
In Minnesota the footings are required to be at least 42″ deep to get below the frost level, so often times it is not that much more expense to dig deeper to get in a full basement rather than a crawl space.
Once the forms are set in place, the local building inspector will sign off that everything is in place properly and then the cement trucks come in and the forms are filled with cement.
Now that the concrete footings have cured, the block layers begin to set their block. Whether they are using a block wall, poured concrete wall, or wood wall – the weight of the home will all be transferred down to the footings.
The most common foundation walls we see in Minnesota are block and poured walls. The poured walls are becoming popular as the price has come down on them with the form technology reducing labor time. Some will argue that poured concrete is much stronger than block, whether this is true or not – it is hard to argue with block walls that were put in the 1910’s-1920’s are still holding strong. So whether or not it is worth the extra money is up to the homeowner.
Block Walls: A block wall is installed one block at a time, and held together with mortar and core-fills. Core-fills are taking the hollow space in the block, filling that with a mortar or concrete mix with steel rebar for added strength.
Poured Walls: A poured foundation wall will be installed by the forms being set in place with the rebar installed and literally pouring concrete into these forms. The strength advantage of the uniform bonding is what people seek, however if you have a bad batch of concrete you have a major failure. Today it is less likely to come into a bad batch of concrete with the advancement in technology of the reddy-mix plants.
Wood Walls: Occasionally wood foundations are used. I have never personally worked with them, but I do know of several homes that were built about 30 years ago with wood foundations and appear to be holding up just fine.
Whatever foundation wall system you are using, the process will be the same.
When the foundation wall is up the next steps will be waterproofing the basement. These are not structural components so I won’t go into details on them other than; the exterior of the foundation wall gets waterproofed and drain tile goes in the inside of the footing and sometimes the outside. Both are designed to keep your basement dry, which is the big advancement in basement technology.
The walls are then braced with temporary bracing to support it as the excavators then push dirt back against the exterior of the foundation wall; back-filling.
There will be footings somewhere in the center of the basement without a wall on it at this point. These are the load bearing points for staircases and load bearing columns or walls.
The foundation is now complete and the framers begin framing the house. The basement floor (the slab) usually gets poured after the roof has gone on the home. Often times the interior drain tiling goes in at this time as well.
Below are some images of different foundation wall materials used by era. All of these foundation wall materials are adequate to carry the loads of homes if installed properly. The reason for switching materials has been largely due to labor cost savings and not that they were not good
foundation wall materials.
When shopping for a home, whether it is a newer home or older home – there are key signs to look for. I look for horizontal cracks, and am less concerned about vertical cracks. Vertical cracks although not ideal, will still transfer the weight coming from above to the footing below. Horizontal cracks will have a tendency to move inward from the exterior force of the dirt pushing on it. Once the foundation begins failing it can become costly to fix, and sometimes can’t be repaired.
Poured concrete wall failures can come from horizontal cracking or a bad concrete batch that starts to fail over time. You can see evidence in this by spalling and if you poke at it with a knife, it will be soft and powdery almost like a dry rot. Like block foundation walls, poured walls are only as good as the installation – in any foundation, if it was installed properly it will last.
A good home inspector will be able to spot potential foundation problems that are visible.
I suspect we will likely see a more cost-effective solution to foundations in the future, likely coming from an engineered material. Time will tell…
- Building Material for Homes: Foundations (UWEC.edu)
- Poured Concrete Foundation Walls (jkath.com)
- Integra Block: From the ground up (Part one) (darringraycorp.com)