Block vs Poured Foundation: What’s the Difference?

A solid foundation is essential to the structural stability of a building. Two common options are poured concrete or concrete block. If you’re wondering what’s the differences and which is better, concrete block vs poured foundation, we’re here to help.

A poured concrete foundation has no mortar joints and is easier to waterproof than a concrete block foundation. Blocks are more DIY-friendly, but reinforced solid concrete has a higher density and greater strength. Additionally, poured foundations are faster to erect than block foundations so framing can begin sooner, but they require more workers.

In this guide, we’ll explain the key differences between a block and poured foundations. We’ll discuss what block and poured foundations are and look at the pros and cons of each. Plus, we’ll identify which is better, concrete block or poured foundation. Our aim is to provide you with the information to make the best choice for your foundation.

Block vs Poured Foundation: Key Points

Choosing between a block or poured foundation may be strictly logistical, budgetary, and skill. Concrete blocks may be more accessible and transportable, but from a budget perspective, they may prove more expensive. Laying a block foundation or cribbing and pouring a concrete one depends on your personal skill set or the skill of those hired for the job.

The best way to choose which is better for your foundation is to compare key points shared by both. The Table below compares block and poured foundations based on key points.

What Is a Block Foundation?

A block foundation typically rests on a poured concrete footing. The walls are made of offset rows or courses of stacked blocks, so vertical seams do not run straight, but are stepped. Between each block and row, mortar is used to level and bond the blocks together. A block foundation is usually 10” thick or wide, but can range from 4” to 12” depending on the structural requirements.

To improve the lateral strength of the blocks, many masons embed expanded steel mesh in the horizontal seams every third or fourth course. They also add rebar vertically every four to six feet in the hollow cores, which are then filled with mortar or concrete. The expanded mesh and rebar help improve the lateral strength of the wall.

The two most common kinds of blocks used for foundations are concrete blocks and cinder blocks. Concrete blocks or CMUs (concrete masonry units) differ from cinder blocks, which are also referred to as CMUs.

Concrete blocks are made from a mixture of Portland cement, sand, and small gravel or other aggregates. Cinder blocks are also made of Portland cement, but instead of sand and gravel, they contain granulated coal cinders (fly ash) or volcanic cinders.

Both mixtures are poured into various mold sizes, shapes, and finishes, and are compressed as they cure. Some blocks are solid, others have open cores, and some are shaped for specific purposes. Concrete blocks are typically 10 pounds heavier and have greater strength.

Cinder blocks are lighter, and have lower density and little tensile strength, so are weaker and less durable. Plus, many building codes restrict or prohibit their use in foundations.

Concrete and cinder blocks come in different lengths, widths, and even heights. The most common CMU sizes used for foundations range from 4” to 12” widths. Blocks have a nominal measurement that includes half the typical mortar gap, and an actual measurement that identifies the true size of the block. The Table below identifies the most common foundation block sizes.

Common Foundation Block Sizes
Concrete Masonry Unit (CMU)	Actual Dimensions W x H x L in Inches	Nominal Dimensions W x H x L in Inches
4” Full Block	3-5/8 x 7-5/8 x 15-5/8	4 x 8 x 16
4” Half Block	3-5/8 x 7-5/8 x 7-5/8	4 x 8 x 8
6” Full Bock	5-5/8 x 7-5/8 x 15-5/8	6 x 8 x 16
6” Half Block	5-5/8 x 7-5/8 x 7-5/8	6 x 8 x 8
8” Full Block	7-5/8 x 7-5/8 x 15-5/8	8 x 8 x 16
8” Half Block	7-5/8 x 7-5/8 x 7-5/8	8 x 8 x 8
10” Full Block	9-5/8 x 7-5/8 x 15-5/8	10 x 8 x 16
10” Half Block	9-5/8 x 7-5/8 x 7-5/8	10 x 8 x 8
12” Full Block	11-5/8 x 7-5/8 x 15-5/8	12 x 8 x 16
12” Half Block	11-5/8 x 7-5/8 x 7-5/8	12 x 8 x 8

Block Foundation Pros and Cons

Concrete block foundation walls, like everything, have pros and cons. The level of skill of the installer can impact the structural integrity and aesthetic finish of the build, so ask for references and check them out.

Pros

Cons

What Is a Poured Concrete Foundation

A poured foundation can be a full foundation, a partial foundation, or a slab-on-grade. A double set of wall forms or cribs are typically erected on poured footings to form the 6” to 12” thick perimeter wall of a full or partial foundation. Overlapping rebar is placed at set elevations to strengthen the concrete. A slab-on-grade commonly has a single perimeter form with a rebar grid and framework reinforcing the combined footing and slab.

Once the forms are erected, leveled, braced, and rebar placed, they are filled with wet concrete – a mixture of Portland cement, sand, gravel, and water. The concrete may be trucked in or mixed onsite, poured, pumped, or placed into the formed foundation, and left to cure.

The wet mixture may need to be vibrated or tapped so that no air pockets exist. The surface of the slab-on-grade also needs to be smoothed and finished too. As the concrete sets up, special anchor bolts are often embedded into the concrete to help fasten and secure sill plates or bottom plates.

The size and complexity of the foundation determine the length of time to build. Once the excavation and footings are done, a typical residential foundation can be formed and poured in a day by a professional crew of 4.

The next morning the forms can be stripped off, sill plates installed and the rest of the build begun. Forming and pouring typically require more equipment and workers than a block foundation, so it is less DIY friendly.

The forms used for foundation walls that will be below grade are usually smooth, to leave a smooth finish once the forms are removed. The exterior surface is typically dampproofed or waterproofed below grade with a tar-like sealant, waterproofing membrane, an exterior concrete sealer, or a combination of materials. Forms for concrete walls with above-grade exposure may leave a surface pattern of stone, brick, plank, or other images when removed for a finished appearance.

Pros and Cons of Poured Foundation

A poured concrete foundation is the most common type of foundation today. However, whether premixed or mixed on site and poured, there are pros and cons to consider.

Pros

Cons

What is the Difference Between Block and Poured Foundation?

Building a foundation requires a solid level footing upon which to erect the foundation walls. Whether above or below grade, the foundation provides the support structure for the rest of the building.

Both poured concrete and block foundations need to be horizontally and vertically level, and corners need to be square – unless they’re not supposed to be. The greater the skill of those doing the work, the quicker and more accurate the build.

The type of foundation may be dictated by the availability of material, location, and accessibility, as well as other differences between a block and poured foundation. An awareness of the differences below may make the decision of choice easier.

Material

Both concrete blocks and poured concrete contain Portland cement, sand, and gravel. However, the size of the gravel aggregate is smaller in the blocks than in the poured concrete. Blocks are formed at cement plants and commonly have hollow cores, while concrete can be ready-mixed and trucked to the building site or mixed on-site.

Pro Note: Although the terms concrete blocks and cinder blocks are often used interchangeably, they are NOT the same thing! A cinder block is manufactured from Portland cement and granulated coal cinders (fly ash) or volcanic cinders. They are lighter, weaker, less durable, and more porous than concrete blocks, which is why many building codes prohibit or restrict their use in foundation walls.

Durability

A reinforced poured concrete foundation has great lateral and tensile strength and durability and will withstand water pressure and the expansion and contraction of soils. Concrete block foundations have very good load-bearing strength but limited lateral strength unless horizontal and vertical steel reinforcement is used and properly placed. If properly reinforce and maintained, a block foundation’s durability is almost equal to a poured concrete foundation.

Life Expectancy

Both a poured concrete foundation and a concrete block foundation will last 100-plus years. With proper care and maintenance, they can last significantly longer. Concrete has been around since approximately 6500 BCE and used by different ancient civilizations, with some examples still in existence.

The Romans also used concrete extensively in many existing structures too. The use of modern concrete containing Portland cement dates back to its invention in 1824. Some of those almost 200-year-old structures are still standing, so life expectancy is a work in progress.

Complexity

The complexity of a block or poured foundation really depends on the structural requirements and the ability or skill of those doing the work. Both require a solid, preferably level, footing upon which to start. A skilled DIYer with a strong back can manage both types of foundation construction, but their speed and neatness will seldom equal that of a skilled mason or cribber.

A poured foundation is usually quicker to erect but requires more tools and materials and can’t easily be done by one person, whereas a block foundation can be. A poured foundation requires forms into which the rebar and concrete are placed, a block foundation doesn’t. The forms for a poured foundation need to be leveled and braced. A block foundation is leveled as it is built and shouldn’t need bracing.

A solid, level, square-cornered foundation is essential for the rest of the structure. So, unless you have the ability, time, skills, and tools, even the simplest task can be complex. Skilled trades typically bring their own tools, skills, knowledge, and all the tricks of the trade with them, which is often well worth the expense.

Water Resistance

A poured concrete foundation is solid, whereas a concrete block foundation has mortar between each hollow core block, so it is less resistant to moisture. It is common to waterproof both poured and block foundations from the base to 6 inches above grade prior to backfilling.

The waterproofing protects the concrete and improves water resistance. The smoother surface of the poured concrete makes it easier to apply some types of waterproofing too.

Fire Resistance

Concrete is classed as non-combustible with a Class 0 fire spread rating, provided it contains no combustible materials. Concrete blocks have the same rating provided the mortar between the CMUs also contains no combustible materials.

The duration of the fire and the weight the concrete material must support, affect the burn time rating. So, the longer and hotter the fire burns, the greater risk of structural failure.

Maintenance

Most poured or block foundations are treated with a black tar-like sealant prior to the foundation trench or excavation being backfilled. The material helps to protect it from moisture. Plus, there is commonly weeping tile or Big ‘O’ laid alongside the footing and covered with gravel.

Depending on the structure’s location, the Big ‘O’ will move water collecting at the base of the foundation to the municipal sewer, or to a sump pump which will move it further away. Some just have an extension that moves the water further away from the footings. In most situations, this is enough protection for the first 25 to 50 years.

However, foundation maintenance is recommended for all poured or block foundations, and prevention is the best medicine. To maintain soil stability, ensure the ground around the foundation slopes away from the structure so roof and ground runoff is moved away.

The foundation perimeter is usually loose fill and will settle, so bringing in more ‘earth’ and compacting it until it slopes enough to keep moisture away from the foundation is important. Over time it can settle, so it may require attention over time.

Sidewalks, patios, and decks should also slope away from the structure, so they too are part of the maintenance to protect the foundation. If they settle over time, they can redirect runoff toward foundations, which isn’t good.

Gardens next to the foundation can also trap moisture, so they should be sloped to move water away from the foundation. Gutters and downspouts should be inspected and maintained in spring and fall to ensure they move water away from the structure too.

Trees and shrubs near the house can be problematic too. The root systems cause soil shrinkage by removing moisture, which can create subsurface settling under footings, leading to structural cracks. Additionally, the pressure from the roots themselves can damage foundations.

The recommendation is that trees not be planted closer to the structure than their mature height, so a sapling that will become a 20’ tall tree, should be no closer than 20 feet to the structure. Since this isn’t realistic in most urban subdivisions, consider pruning branches as the tree grows, it’s a good way to limit root mass growth.

Repair and Maintenance

Identifying if a foundation requires repair can be difficult if the interior walls are finished, or it could be as noticeable as a blinding light. Settling, water pressure, and tree roots are just some issues that can cause cracks in foundation walls, which can allow moisture to seep in and mold and mildew to grow.

Poured foundations are less susceptible to cracks but they do occur and are more difficult to repair. Cracks in concrete block foundations commonly occur in the mortar seams making repairs ‘easier’.

Cracks in the foundation are often structural issues. Repairing a foundation wall of any type, although possible for a DIYer should be left to qualified professionals. Foundation walls are typically built on footings. The foundation walls transfer the structural loads to the footings which in turn, transfer them to the ground. So, just patching the crack usually isn’t enough. The cause needs to be addressed first.

Foundation walls are commonly moisture protected with a tar-like sealant and the footings with weeping tile or Big ‘O’. Over time, the waterproof sealant on the exterior of the foundation can weaken or be damaged and the weeping tile become plugged, so they may need to be repaired. Repair of either requires the exterior perimeter of the foundation to be excavated for proper inspection and repair or maintenance.

A camera scope can be used to ensure the weeping tile is still functional, or since the excavation makes it accessible, consider replacing it. If the cracks don’t involve the footing, they can be repaired with new mortar or a concrete epoxy compound. Once the repair is done, waterproof the foundation with an elastomeric sealant, exterior primer, self-adhering waterproof membrane, rubber membrane, or a combination of materials.

Subsoil settling or erosion under the footing can cause it to crack or break, requiring a significantly more complex repair that also addresses the cause. Once repaired, the rest of the foundation can be repaired as required. The excavated foundation should also be waterproofed to protect it from moisture damage, and the backfill compacted to reduce settling.

Best for

Poured concrete is excellent for half or full foundations, those with an open side for a walkout basement, slab-on-grade foundations, retaining walls, and other foundation types. They can be used in dry or damp soil conditions or on bedrock. However, trucks delivering ready mix are heavy and often require supportive access to build sites and may require a pumper truck too.

Concrete blocks are great for half or full foundations, partitions, firewalls, pile caps, piers, retaining walls, and many other applications in remote and urban areas. They are easier for a DIYer to build and don’t require a lot of extra materials or tools. Since they are more permeable, though, they require more maintenance and aren’t ideal for wet soil conditions.

Cost

A foundation often requires excavation to below local frost levels and poured footings. The deeper the excavation, the greater the expense, however, a full basement provides extra living and storage space, and higher resale value. The footing supports the foundation, which in turn supports the building. The foundation alone can range between $6,000 and $45,000 or more.

The cost of a concrete block or poured concrete foundation depends on many factors. The physical size and complexity of the build, height and thickness of the walls, ZIP code, soil conditions, build site location and access, steel reinforcing costs, and who is doing the work all affect the foundation cost.

Additional costs include the building permit, forms if making your own, waterproofing and sealing, backfill, and grading. So, costs for any foundation will vary from site to site.

A nominally 8”x8”x16” cinder block ranges from $2 to $3 each and a concrete block of the same size is between $3 and $4. Ready mixed concrete is sold by the cubic yard or meter and often can include delivery fees.

Prices average between $112 to $150 in the U.S. depending on the strength of the concrete selected and the ZIP code. A cubic yard of concrete is equivalent to about 46 solid core 8”x8”x16” blocks, so its comparable price point would be $2.44 to $3.26.

An 8-foot-high foundation, including rebar and labor, typically averages between $12 and $23 a square foot. So, a 1,000 sqft basement foundation will average $12,000 to $23,000, not counting permits, excavation and footings.

A concrete block foundation will take longer to erect and requires steel reinforcing plus mortar as well as labor. The same foundation with concrete blocks would cost between $16 and $27 a square foot or $16,000 to $27,000. So, a poured foundation is about 20% less than a block foundation.

Which Is Better Concrete Block or Poured Foundation

A concrete block foundation is more DIY-friendly since it can be done by one person, but it takes longer. A poured concrete foundation typically requires a crew of 4 to erect and pour but can be done within two days and be ready for framing much faster. Plus, a poured foundation has greater density and strength and is easier to waterproof.

The cost savings can amount to 20% too. However, both concrete block and poured foundations are excellent options.

Hopefully, we’ve provided you with the information to make the best choice for your foundation.