Dry CMU Walls


Designing and Building Dry Single Wythe Block Walls

By Jason Yana

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    NCMA Techical Notes

pocketb02After receiving a question from a local home inspector this morning, I decided to post my answer to the question here in a quick article. The question essentially addressed what can make or break a single wythe wall design.

It has been a recent trend among architects / developers / builders to utilize single wythe cmu for low rise residential structures.  The Masonry Industry has always promoted using a wall system that is designed with inherent redundancy against water penetration.  Cavity walls with a clear drainage / airspace are always suggested for this reason.  In the real world, however, this redundancy in design usually takes a back seat to economy.  Since the margin for error is tighter when using these single wythe walls,  there are several measures that should be taken during the design phase, during construction and throughout the life of the building.


jhangerb02The major objective in designing dry concrete masonry walls is to keep water from entering or penetrating the wall. In addition to precipitation, moisture can find its way into masonry walls from a number of different sources. Dry concrete masonry walls are obtained when the design and construction addresses the movement of water into, through, and out of the wall. This includes detailing and protecting roofs, windows, joints, and other features to ensure water does not penetrate the wall.

The following moisture sources need to be considered in the design for dry concrete masonry walls.

Driving Rain

Moisture in liquid form can pass through concrete masonry units and mortar when driven by a significant force. However, these materials generally are too dense for water to pass through quickly. If water enters the wall, it often can be traced to the masonry unit-mortar interface due to improperly filled joints or lack of bond between the unit and the mortar - emphasizing the importance of proper mortar selection, handling and installation. Cracks caused by building movements, or gaps between adjoining building segments (roofs, floors, windows, doors, etc.) and masonry walls are other common points of water entry.  For this reason as an inspector, you should be looking for control joints in the block walls spaced no more than 30 feet apart.

Capillary Suction

Untreated masonry materials typically take on water through capillary forces. The amount of water depends on the capillary suction characteristics of the masonry and mortar. Integral water repellents greatly reduce the absorption characteristics of the units and mortar, but may not be able to prevent all moisture migration if there is a significant head pressure – 2 in. (51 mm) or more. Post-applied surface treatments reduce the capillary suction of masonry at the treated surface as well but have little effect on the interior of the units.

Water Vapor

swcoping02Water as vapor diffuses toward a lower vapor pressure. This means it will move from the higher toward the lower relative humidity regions assuming no pressure or temperature differential. Vapor in air of the same humidity and pressure, but of different temperatures, will move from the higher temperature to the lower. As air is cooled, it becomes more saturated and when it reaches a temperature called the dew point, the water vapor will condense into liquid form.

Integral water repellents make masonry materials hydrophobic, thereby significantly decreasing their water absorption and wicking characteristics. While these admixtures can limit the amount of water that can pass through units and mortar, they have little impact on moisture entering through relatively large cracks and voids in the wall. Therefore, even with the incorporation of integral water repellents, proper detailing of control joints and quality workmanship to preclude beeholes and unfilled or inadequate mortar joints is still essential. Another advantage of integral water repellents is that they not only help to keep water out but also inhibit the migration of water to the interior face of the wall by capillary suction. 

This integral water repellent, (Dry-Block for example)  only adds a few cents to the cost of the block and our biggest manufacturer in Chicago, does not sell split face units without dry-block in it - a pro-active measure that has reduced the amount of callbacks for them and for myself.

For colored architectural masonry it is recommended that a clear surface treatment be post-applied whether or not integral water repellent admixtures are used. Most post-applied coatings and surface treatments are compatible with integral water repellents although this should be verified with the product manufacturers before applying. When using standard units for single-wythe walls, an application of portland cement plaster (stucco), paint, or opaque elastomeric coatings works well. Coatings containing elastomerics have the advantage of being able to bridge small gaps and cracks.

Walls incorporating integral water repellents should not be cleaned with a high-pressure wash as it drives water into the masonry. Acidic washes should not be used since they may reduce the water repelling properties of treated masonry. Keeping the masonry wall clean, as the construction progresses, using a brush and water minimizes cleaning efforts after the mortar has hardened. Consult the integral water repellent manufacturer for detailed cleaning recommendations.

If I were building one of these buildings, I would do the following and this could be a nice checklist to use when inspecting such structures.

1. Use a block from a reputable manufacturer that contains integral water repellent.

2. Use real Portland / cement / lime mortar (not a pre-mixed masonry cement which contains air -entrainment additives)  Type N -  It is important to note that when using integral water repellents, there is a mortar additive that goes with it that needs to be added to EVERY batch of mortar used on the job- this keeps the mortar from absorbing moisture.

3. Detail walls with all flashing details suggested by the Masonry Advisory Council and NCMA.  Use a quality flashing material, not the cheapest stuff available, such as IPCO self adhered rubberized asphalt flashing (

4. Specify hot dipped galvanized ladder type joint reinforcement in the bed joints of the block spaced at 16" o.c. vertically (every other course).

5. Specify and indicate control joints on project drawings in accordance with industry standards (no more than 30 feet apart)

6. Supervise the construction process insuring that all mortar joints are full and properly tooled (concave or vee joints)

7. Taking a belt and suspenders approach - apply a quality water repellent coating that is made specifically for concrete block.

8. Monitor the building closely looking for any cracks - caulk them.

9. Re-apply the water repellent coating periodically - 5-10 yrs. or as recommended by the manufacturer.

10. When mortar joints begin to show erosion, cracking or excessive wear - a proper repointing should be performed.

We take our cars for oil changes every few months, and have them serviced regularly - remember that a building requires maintenance to perform properly just like our cars do.


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