The mortar used to lay the concrete masonry unit (CMU) blocks in a Broken Arrow masonry outdoor kitchen base is not a detail that most homeowners think about — but the mortar type directly affects the outdoor kitchen base’s structural performance, moisture resistance, and long-term durability in Oklahoma’s freeze-thaw climate. Using the wrong mortar type in a Broken Arrow outdoor kitchen CMU base is one of the most common quality deficiencies in lower-cost outdoor kitchen construction, and the consequences show up as mortar joint cracking, joint erosion, and eventual block instability within 5 to 10 years of construction in outdoor exposure conditions. VistaScapes & Design specifies Type S mortar for all CMU outdoor kitchen base construction in Broken Arrow and builds to this standard on every project.
Mortar Type Classification
Masonry mortar type classification for Broken Arrow outdoor kitchen construction: masonry mortars are classified by the ASTM C270 standard into four types — M, S, N, and O — based on their compressive strength, bond strength, and durability in different exposure conditions. Type M mortar (highest strength, 2,500 PSI minimum compressive strength): appropriate for masonry in contact with the ground or below-grade exposure; used for foundation walls, retaining walls, and below-grade CMU construction; provides high compressive strength but lower bond tensile strength than Type S; generally not used for above-grade outdoor kitchen base construction because the high stiffness reduces its flexibility to accommodate minor thermal movement. Type S mortar (1,800 PSI minimum compressive strength): the recommended mortar type for outdoor kitchen CMU base construction in Broken Arrow; Type S mortar provides the optimum combination of compressive strength, bond tensile strength, and durability for above-grade masonry exposed to Oklahoma’s outdoor conditions; Type S mortar contains Portland cement, masonry cement or lime, and masonry sand in proportions specified by ASTM C270; the bond tensile strength of Type S mortar is significantly higher than Type M, meaning the mortar joint resists lateral loads (wind loading on the outdoor kitchen veneer) and moisture-driven expansion-contraction cycling better than Type M; Type S mortar is specified for masonry exposed to severe weathering — the classification that applies to Broken Arrow’s freeze-thaw outdoor environment. Type N mortar (750 PSI minimum compressive strength): a general-purpose mortar appropriate for interior masonry and exterior masonry in above-grade, non-severe weathering exposures; Type N mortar is adequate for veneering a CMU substrate with natural stone but is not appropriate as the bed mortar for laying the structural CMU blocks of an outdoor kitchen base in Broken Arrow’s outdoor exposure conditions; Type N mortar’s lower compressive strength and lower bond strength make it susceptible to mortar joint erosion in freeze-thaw cycling. Type O mortar (350 PSI minimum compressive strength): extremely low strength; appropriate only for interior non-load-bearing masonry; never appropriate for outdoor kitchen construction in any climate. Pre-mixed mortar bags: most masonry mortar in the Broken Arrow residential market is available in 60-pound or 80-pound pre-mixed bags that require only the addition of water; the bag must be clearly labeled as Type S for outdoor kitchen base construction; generic masonry cement without a type classification should not be used for outdoor kitchen base mortar.
Mortar Joint Quality and Tooling
Mortar joint quality and tooling for Broken Arrow outdoor kitchen CMU construction: joint thickness — CMU blocks are laid with 3/8-inch mortar bed joints (the horizontal mortar between courses) and 3/8-inch head joints (the vertical mortar between adjacent blocks in the same course); maintaining consistent 3/8-inch joint thickness throughout the CMU base construction produces a structurally sound and visually uniform masonry structure; thicker joints (above 1/2 inch) reduce the mortar’s compressive performance and create visible irregularity in the block coursing; thinner joints (under 1/4 inch) reduce mortar coverage and bond area. Mortar consistency: Type S mortar for outdoor kitchen CMU construction should be mixed to a workable consistency that allows the block to be positioned and leveled without the mortar bleeding out from under the block’s weight — the correct consistency holds a peak when a trowel is pulled away and the peak curls slightly but does not slump; too-wet mortar shrinks excessively on drying and produces weak, porous joints; too-stiff mortar does not achieve adequate bond with the CMU block face. Joint tooling: after the mortar has reached initial set (the mortar is firm enough to resist finger pressure but not fully cured — typically 30 to 60 minutes after laying in summer conditions), the exposed mortar joints are tooled with a convex jointer to compact the mortar surface and produce a concave joint profile; a properly tooled mortar joint is harder and more weather-resistant than an unfinished flush joint because the tooling compresses the mortar surface and forces the mortar tighter against the block face; concave-tooled joints shed water more effectively than flat or raked joints in outdoor exposure. Full mortar bedding: all CMU blocks in the outdoor kitchen base must be laid with full mortar bedding on the bottom and both vertical head joint faces — no spot bedding or face-shell bedding of the structural CMU courses; full bedding produces maximum compressive and bond strength and prevents water from migrating through air voids in the mortar joint plane. VistaScapes & Design uses Type S pre-mixed mortar, maintains consistent 3/8-inch joint thickness, and tools all exposed mortar joints on every Broken Arrow masonry outdoor kitchen project.
Call VistaScapes & Design at (918) 779-1317 for a free outdoor kitchen consultation in Tulsa. We build every masonry outdoor kitchen base to the Type S mortar and full-bedding standard that produces the most durable structure in Oklahoma’s outdoor conditions.
