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home page » passive solar design chapter » dry stack block detail page ... you are here
Dry stack concrete block (cinder block) technique uses surface bonding cement (SBC) instead of mortar for high thermal mass walls that are in fact stronger than conventionally laid block walls. Only the first course is bedded in grout to establish plumb and level for the wall. The rest of the dry stack block courses are laid without need for any mortar. This building method has been around for well over a century, so it is nothing new or even very unique. SBC was formulated to build million gallon municipal cisterns for water storage; the original recipe used horse hair for the structural fibers! There is absolutely no need for special "dry stack" blocks, but they are available in some areas. Typical concrete blocks are fine as long as the quality is good. And the quality is almost always very good these days, what with computerized forming and automated molding of concrete blocks. Plus, you will likely be buying a local product - block manufacturing yards are to be found in nearly every part of the country.
Blocks need not be of any special design; they don't interlock, and best of all you don’t have to know how to lay block! You simply stack the concrete blocks in a running bond pattern and then parge coat both sides with a single layer of fiber reinforced, surface bonding cement. Applied 1/8 inch thick (minimum) to both sides, surface bonding cements have strengths that are superior to conventionally mortared block walls and they look a lot better too (no grout lines)! Once your block walls have been surface bonded, hollow vertical cores (minimum every four foot and alongside every opening) are filled with ready-mix concrete and a #5 rebar for an exceptionally strong heat storage mass. Mortared block walls are just as effective, but dry stack is easier for the unskilled homeowner-builder. You don't need any experience laying block to excel at dry stack with SBC!
Please note that solid, poured in place, concrete walls are just as effective, but outside the skill level of most people. Hiring contractors to build the forms, pour the walls and then return to strip the forms, is always an option when you have the budget. Empty cores in your block walls (those not having structural rebar) should be filled with something for extra thermal mass. There is no advantage to having empty cores in the block walls and absolutely no advantage to filling them with insulation. The more thermal mass in a home, the better. Most clients simply order more concrete grout and fill the empty cores while they fill the structural cores, but some clients opt for using sand/dirt from the site. The only cavaets to using soil to fill the empty block cores is to compact well and "cap" the top of the core with SBC to prevent moisture from penetrating.
One key to thermal performance and indoor air quality is to have waterproof walls and surface bonding cement was originally used to line water tanks. You need to prevent water from traveling through the wall, sucking away energy and presenting possible indoor air quality problems. The same applies to poured-in-place concrete walls - the wall needs to be waterproofed on both sides to prevent water transfer. Several coats of low VOC non-porous latex paint is a good solution for waterproofing the interior of a concrete wall. The exterior footers and wall need a minimum of two heavy coats mop-on waterproofing treatment or better yet, a roll roofing underlayment - sheet bithumin, like Grace® Ice and Water Shield.
One of the most popular questions is: how do you wire a home made out of concrete or block? The answers are varied since there are probably as many different ways as there are electricians, but as with most design elements, look to commercial construction, not residential for the best and most economical solutions.
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In-floor electrical outlets like you see in commercial buildings are one way, knocking holes in the block and running conduit down through the cavities is another, and simply attaching Wire-Mold® strips along the wall is yet another solution. As with most commercial approaches, running conduit ends up being cheaper and sturdier than the residential alternatives used in stick frame construction (bare Romex® run through holes drilled in studs). One of the most visually pleasing and creative ways is shown here. Half-round logs are routered out along the back and holes are cut for the outlet and switch boxes. |  | 
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With any surface bonding cement (SBC), conventional mortared block or poured-in-place cement wall, it is key to note:
If you are not familiar with laying block (mortared or surface bonded), after the corners are stacked six to eight rows high, setting the string line is critical. The corners dictate the walls between - starting from corners, you simply stack block between. You can alternate last blocks, but it is generally the center block and it rarely needs shaved to fit. There are probably as many ways to knot the line block (shown below) as there are masons. "X"ing at the knuckle (full corner end of wooden line block) certainly helps stabilize or you can just wrap the string once or twice around block lengthwise, then anchor back to pin. You can pull in slack a lot easier this way (cinching to line pin), plus it ensures the string line will remain taut and true. While it is possible to simply set the wood line block on corner and tie off with tension alone, having an anchor post (pin as shown) is a little more elegant and saves the string from being subject to "jumping off".
As with most everything in life, there are at least two opinions on the correct usage of the string line (mason line).
Most prefer to keep string line a set distance from block (5/8 inch as pictured above with the Marshalltown® line block).
This allows you to use your fingers as a quick spacer - as soon as fingers touch line, you know you are about true for stacking (5/8 inch gap).
Classic masonry with grouted courses, forces you to hold the block at center pull / cavity opening (fingers inside block to steady and drop plumb).
You can't always be looking down on the block, though, and you really don't need to with dry stack.
Some masons like the string line right where the blocks are going.
You just have to maintain clearance (even if only 1/16 inch) to keep true (nothing should ever touch the string line).
It is possible to keep the wall plumb (vertical) and level (horizontal) without a string line reference, but the only way to keep it straight (true) is with a stringline.
Wander 1/16 inch off true every coarse and you will find the wall with a 1/2 inch curve before you know it.
The major labor/time saver with dry stack SBC versus mortar block is not having to make sure each and every block is plumb, level and true before setting the next block.
When you are rolling along with stacking the block, you never stop to check every single block ... you lay the course just shy of stringline true and then come back to adjust all the blocks over to the line.
A large rubber mallet does a quick and efficient job of trueing the wall until you get two or three courses atop the errant block.
This is where you notice the great strength provided by friction (six blocks touching the one you want to move) and gravity alone.
After the surface bonding cement is applied and structural cores filled, a SBC block wall is actually much stronger than a comparable mortared block wall.
A major time saver is to never cut any blocks for the layout - design around using all full and half blocks.
The block layout we normally work with is eight inch wide block (8" wide, 15.625" long, and 7.625" tall).
Maintaining a running bond stack (upper block overlaps half of lower block) is easy with eight inch wide since the corner turns on 1/2 of the width (15.625 inch long and eight inch wide).
When the design requires change to 12 inch wide block (berming into a hillside) you suddenly need to cut a block every other course.
The first course is traditionally all full blocks (12 inch by 15.625 inch long).
The second course (as shown below) uses a cut block - one 4 inch and the other what is left over (about 11-1/4" to 11-1/2" long).
Half blocks are only used at the end of a wall or doorway opening, never in a corner or on a straight wall run between two corners.
One key to SBC (and any other Portland® cement based stucco, concrete or mortar) is to keep the material moist for the first few days of curing. Any cement based product needs to cure slow and steady for optimal strength - up to a month for a concrete slab. If you want a quality product, get several hand pump sprayers for misting water during construction. Tent and heat the first course with mortar bed - and keep moist - for the first few days of set when temperatures are going to dip below 50 degrees. The same applies to any SBC work later - when dry stacking (or mortar block laying) in a cold climate, you should consider covering the entire structure with string reinforced poly sheeting. The tops of block walls can be spanned with a few beams and clear tarp material draped across all walls. Common roll size is 20' by 100', with 40' by 100' available in some areas (there is also a woven mat material). Commercial projects often leave the plastic "tent" in place for the entire construction, cutting away with razor knives after the ceiling has been oiled and interior is ready to work. Then the clear tarp just falls to the floor and becomes a drop cloth to keep traffic off concrete.
You can purchase surface bonding material at any building supply center or at wholesale (by the pallet) from your block yard.
We do not sell it and we are not the manufacturer.
Our only interest in proposing SBC as a building method is to help our clients save some money while having the ability to construct their own home with their own hands.
Here's a link to a three printed page Adobe® Acrobat® (.PDF) file with the technical data for:
Quikwall® brand surface bonding cement.
If you don't have Adobe® Acrobat® Reader on your system, take this link for a free download.
For a look at common dry stack cinder block construction techniques, please take this link: Quikrete® SBC Project Page.
Quikwall's manual is a very helpful "how-to-do" surface bonding cement outline.
Please note that other surface bonding cement manufacturers exist, so check with your local block yard.
We have always delighted when new professionals become familiar with dry stack cinder block techniques.
Cinder block has been a popular commercial building material for ages, but wood has always ruled for residential here in the USA.
That's what makes an HTM so economical and functionally sensible - this common commercial style of block or poured-in-place concrete construction.
HTM design just adds a coat of surface bonding cement (SBC structural stucco) to stop water transference and exterior wall and horizontal wing insulation for heating and cooling retention.
Plus, we throw in a little passive solar design, maybe some shade cloth, and voile, HTM.
Concrete and block are very low-tech LOCALLY available products without all the marketing fluff associated with SIPS, ICF pre-packaged insulated forms, special block shapes, and other trick methods.
Reinventing the wheel is not necessary ... any local commercial contractor should be more than capable of working on the project.
Here's hoping more websites, architects, engineers, and building departments get familiar with a worthy method.
For a technique that has been around for centuries, it gets entirely too little attention.
But like any product where scant little money is to be made by marketers, dry stack tends to get ignored.
We do not sell any tools, but darbies, pool floats, magnesium hawks, arch raspers and bucket scoops can be found locally at construction tool suppliers. The same tools used for stucco work are used for surface bonding (parge coating) dry stack cinder block. The pool float is a hockey rink shaped applicator preferred by many to work the stucco or SBC into place. A foam handle is nice, if you keep your tools clean. Otherwise, composite handles are much easier to maintain. Magnesium blades are preferred by many for their ability to "open" pores in the cement (versus seal smooth and tight, as with steel blades). Magnesium has a nice "feel", being rust proof and lightweight, yet thick and strong, making it preferred for hawks, floats and darbies. Arch rasps and bucket scoops are the other two images above - specialized tools for unique needs can cut labor time dramatically. The darby, for instance, is a two to four foot long two handed leveler that makes it possible to flat-level large surfaces quickly. Some darbies come with teeth for a sawing action as you level across the wall or floor - others are flat with rounded lips along edge for sawing (smoothing and leveling) more gently at the stucco, plaster or surface bonding cement.
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dry stack block shed plans - details common to any size building
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