Block Wall (CMU) Thickness

Started by Squirl, July 26, 2012, 01:26:07 PM

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I have been meaning to make a post about this for a while.  I did one on girders a few years ago, and I wanted to do another on how to read the code charts for concrete masonry units (cmu or concrete block) foundations.  This is based upon the code charts and minimums and is not to advocate against added reinforcement or larger sizing.  This is for the overall ICC guidelines for 2009 and may vary slightly by local state guidelines.
Here is the code section:
Here is the main chart with footnotes:

I will focus on the first column first.  Wall height.

This is the height of the wall from the top off the footing to the bottom of the sill plate.  If going with less than a five foot tall wall, use the 5ft for the chart, if going with a greater than 9 ft tall wall, use a different chart.  Most people with crawlspaces will be in the 5-6 ft area and most people with basements in the 7-9 ft area of the chart.

The next column is the height of unbalanced back fill.  I can't state it better than the code so here is the code definition. Unbalanced backfill height is the difference in height between the exterior finish ground level and the lower of the top of the concrete footing that supports the foundation wall or the interior finish ground level. Where an interior concrete slab-on-grade is provided and is in contact with the interior surface of the foundation wall, measurement of the unbalanced backfill height from the exterior finish ground level to the top of the interior concrete slab is permitted.


This one of the most important factors determining sizing design.  As you can see in the chart most, but not all, of the block sizes are the same based upon how much unbalance fill there is rather than how tall the wall is.  This is because this is the amount of weight the wall has to stand up to.  If you evenly backfill both sides of the wall or your wall is not that deep and you have less than 4 ft unbalanced, just use the 4 ft minimum.

The last column is Soil Classes.  This is broken down into three different sections.  This is to determine the weight of the soil pressing against the wall and how thick the wall has to be to withstand that.  This is based upon the Unified Soil Classification System that can be found in table 405.1  It sounds complicated, but it isn't. Clay is heavier than sand and loam and soils. You can usually find many maps locally or online as to what type of soil you have.  You can even sometimes have it tested locally free or cheaply.  Or at worst you can just build for the heaviest soils and know you will be safe.

With this category there are some foot notes to translate.
A. Mortar shall be Type M or S and masonry shall be laid in running bond. Ungrouted hollow masonry units are permitted except where otherwise indicated.

There are many mortar mixes. Type M and S are the strongest.  M is stronger than S.  Both are fine.  Not type N, O, K, or others.  For the novice, they are sold in premixed bags by most hardware stores. This is a running bond pattern.  Granted it is impossible to do at openings like doors.

The last about ungrouted hollow units coincides with footnote D Solid grouted hollow units or solid masonry units.  In the masonry sense grout is a very thin mortar used for filling blocks.  It is to be thin so that it can easily fill the insides.  Normally this is ordered from a concrete company, but it can be mixed by hand. These two footnotes combine to state that unless specified where there is a D in the chart you can use the hollow concrete blocks you find at most hardware stores.

Let's say you have a 4 ft deep crawlspace with 2 ft above grade in a heavy clay soil. 

You would have the option of going with a 6 inch wide solid concrete block,  6" wide hollow concrete blocks filled with grout, or an 8" wide hollow concrete block (the most common kind at big box retail stores, pictured above.)

   You may notice something absent from this chart.  Rebar requirements.  That is because as long as you are not exceeding the sizing requirements in this chart and you are not in Seismic Category Zone D or worse, it is not required by the building codes.  It is not to say rebar is a bad idea.  It is cheap and adds a lot of strength.  It is just not required by the ICC.

Footnote E, Walls Taller than 9ft, and Seismic Category D or worse.
Now you have to use charts Table R404.1.1(2), Table R404.1.1(3), Table R404.1.1(4),
They are very similar to the previous table except the charts are now broken down by block width (8", 10", or 12", no more 6") and they require rebar.
I used TABLE R404.1.1(4) (12 inch) because it converted correctly the rest of the charts are similar and can be found at the link in the beginning of the post.

As you can see from the previous example if you wanted the same wall in a seismic category zone D with 12" thick block walls you would be required to place #4 (1/2 inch) rebar spaced every 72".
The major differences between the charts are in the footnotes. Footnote A is the same.
Footnote B. Alternative reinforcing bar sizes and spacings having an equivalent cross-sectional area of reinforcement per lineal foot of wall shall be permitted provided the spacing of the reinforcement does not exceed 72 inches
This is footnote lets you use alternative spacing/sizing of rebar.  This is found in TABLE R404.1.2(9) .
Let's say the chart required you to use #6 rebar every 48".  This may be difficult for the average DIY person without access to contractor supply places because most retail stores only carry up to #4.  So with this table the person can find out that they can use #4 rebar spaced every 22" instead.

Footnote C is the only other footnote that is different.  It specifies that grade 60 rebar must be used and how far away from the soil bearing side the rebar must be.  Rebar is stronger the more concrete, mortar, or grout is between it and the load (soil). The distance, d, from the face of the soil side of the wall to the center of vertical reinforcement shall be at least X inches.

If I missed anything, please let me know, especially if I made any mistakes, so I can edit the post.

alex trent

This is very useful info...especially down here in the land of "block".

Down here we see in walls (almost all)  horizontal columns, typically 8"x8" with rebar every six feet of height. Also all corners are vertical columns...also with rebar.  The horizontal columns are tied into  the vertical columns.

Blocks here are very variable. some are downright crap. you can avoid them, but the ones that are "close'" but not up to spec are difficult to sort out. This column cross hatch would seem to compensate for this since the blocks essentially fill in the column space.

Kind of like and "unwritten code...very, very few walls go up otherwise.

Is there any written rationale for this, or you care  to comment?  Looks like a sound system to me, but who knows?  You think it is better than all block? Asuming the blocks are good.


It sounds like some of the FEMA concrete column foundations combined with block walls.  I've never seen in myself.

I do know that the western side of Central America is one of the highest seismic activity areas on the continent.  Maybe it is some special way they deal with it.


Tin roof land

Most of latino land is build as Alex says- more than likely due to the quality of the bricks and the quality of the laborer

They lay a slab with some mesh - integrate rebars for the "posts" and pour the floor

The posts are planked out and poured, then the second floor - if they have one

the walls are then built in place- the quality is rarely good- the whole lot then has a scratch coat of concrete- and your done

It probably comes from original construction methods

The wooden frame would be built with a latice structure horizontaly and the void filled with hair-mud and poop

Ie a frame with a non strutural fill

pics not taken for engineering but you can see some details

This house is over 100years old the last inhabitant the last of his family

In this one you can see some later concrete repairs over the original structure