Should I use a poured concrete or masonry block foundation? It seems like a simple question, but the answer can get complex. In general though, a properly constructed foundation of either type is a good choice. However, there are many factors to consider that may make one of the two a better solution for your home.
First, both foundations are strong enough when constructed properly. However in general, a 8" thick, rebar reinforced poured concrete wall is approximately 2.2 times stronger than a 12" masonry block wall.
The type of concrete used in the poured wall can have a big effect on performance. At ProBuilt, we specify 4000 psi concrete in our poured wall foundations. The building code only requires 3000 psi concrete but upgrading to 4000 psi helps to minimize shrinkage cracks in the wall. Also, 4000 psi concrete is borderline waterproof, so it also helps to prevent damp basements. Water vapor will readily move through a block wall, but with nearly waterproof concrete in a poured wall, that vapor transmission is much more difficult. Thus, it has been my experience that poured wall foundations do not have that damp feeling you many times have with block foundations.
The masonry blocks in a block wall are very porous and will absorb water quickly. Properly parging a block wall is critical to it not leaking. Parging is the process where a cement-type material is applied to the blocks to close the pores. This prevents water from passing through the blocks. One of the main reasons I like poured walls over block is the fact that the 4000 psi concrete wall is borderline waterproof while a block wall has to be perfectly parged in order to avoid absorbing water.
Yes, poured wall concrete foundations do crack. Concrete always cracks. As it cures it shrinks slightly and this causes cracking. As a general rule you should expect 1/4" of shrinkage in a 40 foot long wall. In a poured wall foundation, a water-proofing membrane is applied to the outside of the foundation. This membrane is designed to span those cracks so water does not penetrate. If you do have a crack and it does leaks, they are easily fixed from the inside so you do not have to dig up the foundation to stop the leak. An epoxy is injected into the crack from the inside. The epoxy bonds to the concrete and permanently stops the leak. This crack repair is typically part of the waterproofing warranty on the basement and should not cost the homeowner anything.
Because poured walls only leak at a crack, the source of the leak is easily found and fixed. In a block wall, a leak is more difficult to diagnose. Just because you see water on the inside does not mean that the source of the leak is right where you see the water. The water could be entering the cores of the block 30-40 feet away and only showing up inside after it has migrated through many blocks. To fix a block wall that is leaking, you have to dig up the foundation from the outside until you find the cause of the leak. That cause could be a defect in the parging, a crack, or improperly applied waterproofing. You may have to dig up a good portion of the foundation before you find the source of the leak.
With both types of foundations you have to be careful not to place dirt, mulch, flower beds, patios or concrete pads above the waterproofing line on the foundation. If done, this can lead to water intrusion. However, it is much more critical with a block wall than a poured wall. This is because the 4000 psi concrete poured wall is close to waterproof so it is much less likely to leak.
Since block walls are assembled in small pieces the mason installer can make the wall more perfect than a poured wall. With a poured wall it is all poured at once and, due to the volume and weight of the concrete, the wall forms could move slightly. Thus, in a poured wall foundation you can expect around one inch of variation in trueness. This is not an issue to be concerned with though. The framer and veneer stone or brick installer can make adjustments to accommodate the variation.
I recently built my own personal home and chose a poured wall foundation. I have gone with poured walls in my previous two personal homes based on the pro's and con's listed above and prefer them for new home construction.
George Davis is the President and founder of ProBuilt Homes. He has over 23 year of construction experience. Contact ProBuilt today at (440) 255‐6535 or visit them online at www.probuilt‐homes.com to learn more about how ProBuilt is “Building Better”.