Submitted by BillB on 10/9/2007
Wood Veneered Block Wall with Cat
This photo, featuring our fat cat, shows the new retaining wall, made of cinder blocks and veneered with pieces of redwood of varying dimensions but generally long and narrow, keeping with a horizontal feel. I considered ledgestone as a veneer but that would have introduced a new material into our landscaping which I thought might look out of place; I've read that using too many different materials can result in a chaotic look. I liked the look of the ledgestone however and realized I could get the same horizontal feel using wood, a material that would blend in with the rest of our yard.
The original retaining wall, made of redwood, had mostly rotted but I salvaged what was left for the veneer, supplemented with redwood from my own collection. There's gold in old redwood garden structures because over the years the interior of the pieces of lumber darken beautifully and often it's from old growth trees with tight growth rings and straight grain. I save my best stuff for cabinets around the house. To think this stuff could end up in a landfill.
What follows is a bunch of information about building a block retaining wall. If you're just checking out my wood covered wall, scroll down past the technical stuff to the next photo of the wall.
A quick disclaimer: Retaining walls over a certain height as dictated by your AHJ, Authority Having Jurisdiction, will need to be inspected by your AHJ and may require a plan drawn by an engineer. I'm not an engineer and despite a big effort to provide accurate information in this document, errors may remain.
This is the basic wall shape you'll see repeated in many documents from the web or in books. The footer cross section can be a simple rectangle but fancier designs like this one include what's called a key on the bottom, a ridge, providing additional friction against the soil helping to prevent the wall from moving forward or rotating against the lateral stress pushing from behind. There's also a groove in the top surface of the footer, centered where the wall will be built, which helps to bind the wall to the footer. To form the groove, rip the edge off of a 2x4 at a 45° angle and plop it into the concrete while it's wet, centered where the wall will go, along the vertical rebar pieces. Pull it out after the footer has cured a bit but before it hardens completely. The slant on the back edge of this groove allows for easier release of the 2x4 and I don't think there's any engineering purpose for it, though I'm guessing.
The rebar shown on the diagram is typical too. There are two pieces oriented in the plane of the page: one piece runs along the upper surface of the footer and turns down 90° into the key and the second runs near the bottom surface of the footer and extends out the top of the footer and ultimately to the top of the wall. A cell occurs every 8 inches along the wall but my engineer specified vertical #5 rebar spaced every 16 inches.
The six dots shown in the footer and the six in the wall are the ends of rebar running horizontally the length of the wall. The horizontal rebar is tied to the pieces in the plane of the page at every intersection with tie wire, forming the rebar cage. The rebar size number is how many eighths of an inch of the diameter of the rebar, so #5 is 5/8", #4 is 1/2" and #3 is 3/8". The rebar in the block wall, running horizontally, are called bond beam runs. Notice that my engineer wanted 2 parallel bond beam pieces run at the top of the wall, which is apparently under the most stress. (Makes sense when you see what a failing wall looks like.) The drawing is abstract when it comes to the bond beams as he shows 5 of them while specifying that the horozontal rebar only needs to be spaced 24" OC, (on center), which is every third row of blocks.
Notice the piece of rebar that runs in the plane of the page, near the footer bottom and bends vertically through the cells of the blocks to the top of the wall. In other drawings I've seen this piece placed so it runs near the top surface of the footer. Placement towards the footer bottom, as shown in the diagram here, makes more sense because that's where it seems the footer would be in tension and the top surface would be in compression. I think this way because I see the wall tending to rotate forward along an axis somewhere near the junction of the footer and the wall. Anyway, I'm no engineer so I could be way off. I suppose if I was building a wall without an engineer's design I'd put rebar all through the footer, along the top and bottom surfaces. But try to keep all rebar 3 inches from the surface for structural reasons and because concrete absorbs water and the steel will rust if it's embedded too near a surface.
Notice the note to "Grout all cells". That means you fill all the cells in the wall with grout, basically mortar-like material formulated to shrink less and flow well. Sometimes regular mortar and even regular concrete are used to fill CMU cells, but it's certainly best to use a product designed for the purpose. Talk to your city inspection department for recommendations. I talk more about grout and mortar below, as well as construction techniques.
Digging Deeper into the Design
If the ground behind the wall is level, the relationship between the height of the wall and the width of the footer seems to be straightforward and can be found in tables in books and the web. There are inconsistencies between the publications but they're close to one another. If you've got a hill above your wall or maybe a road running along the top you've got what's called a surcharge; anything that adds to the stress on your wall and you should probably call an engineer. Here's part of one table I've seen which assumes a footer depth of 1 foot. (In all the looking I've done, I don't remember seeing a wall plan with a footer other than 1 foot deep, not including the depth of the key.)
|Wall height||Footer width|
I'd include more information from the table but some of it didn't make sense and it got very complicated. It also included specifications for rebar size and spacing, which differed from my specs; sometimes they were beefier and sometimes wimpier. For a 5 foot high wall it specified either #3 rebar every 26" or #4 every 40" for the vertical pieces, where my plan specified #5 every 16". For the rebar in the footer, the table specified #3 every 14" or #4 every 40", where my plan says #3 every 16", so here my plan seems wimpier. (I used #4 rebar though.) (The table didn't make sense to me: how can #3 rebar every 14 inches = #4 every 40 inches. There must be some multiplier or other complicating factor that goes into these calculations. 3/14 = .214 and 4/40 = .1, so something else is going on. Also in the table, the spacing for the vertical rebar passing through the CMU cells wouldn't work for 16" blocks, which require spacing numbers that are divisible by 8, as that's how far apart the cells are.) If you're DIY'ing the the design, spend the little extra money and time and use thicker rebar and closer spacing. A wall like this is a bit of work and you don't want it leaning on you.
My engineer specified the following strengths for the various components of the wall:
- Foundation Soil Bearing Pressure of 1000 PSF - The engineer knows the soils around here and didn't need to bring in a soil engineer. Anyway, that seems like overkill for a residential wall and the inspector didn't need an additional stamp for the soil. Regardless of the soil type, the rule is to build your foundation on undisturbed soil. If you dig too deep, though I'm not sure why you would, don't fill back in with dirt to try to save a couple of bucks on cement.
- Reinforcing bars ASTM A615 Grade 40. This is your basic home center rebar. It's Plain Billet Steel and has a minimum yield strength of 40,000 psi. I think it's the weakest rebar made and the next stronger stuff is grade 60, min yield strength of 60,000 and it's rail steel, which I think means it's actually made of old railroad tracks or material that's as strong.
- CMU f'm - 1500 PSI - The strength of the whole wall assembly; the blocks, mortar and grout and speaks to the individual strength of each component and construction techniques. This is what's called the Masonry Assemblage Strength.
- Mortar minimum compressive strength to be 2500 PSI
- Grout minimum compressive strength to be 2500 PSI
Mortar types relevant for this discussion are M, S and N. I used a high compressive strength type M mortar. The Quikrete product lineup shows that their type M has the highest compressive strength at 2500 psi, followed by S at 1800 psi and N at 750 psi. A link to Fine Homebuilding about different mortars, http://www.taunton.com/finehomebuilding/how-to/articles/mortar-what-type-need.aspx, left me a bit confused as it says type M is usually recommended for retaining walls due to it's "durability", but then goes on to say type S, though weaker, it often used for applications under "soil pressure" due to it's "flexural strength". Soil pressure sounds to me like a retaining wall application. Another link, http://www.cement.org/masonry/cc_mortar_types.asp, which seems more authoritative, suggests the rule of thumb is to use type S for a retaining wall, per ASTM C 270, ASTM being the American Society for Testing and Materials.
As mentioned above, grout is specially formulated to flow down into the cells of a CMU wall but I've seen that some people use the term grout to refer to any concrete that's dumped down the hole. I'd talk to your AHJ or engineer about exactly what to use. If you're building a wall that doesn't require a permit, I'd guess using regular concrete would be fine. An 80 lb. bag of grout should fill about 3, 8x8x16 blocks. There are tables on the web for estimating how much material you'll need; Google "grout fill table" or something like that.
After filling a couple feet of a cell with grout, I'd pack it in by repeatedly shoving a stick to the bottom of the void, hopefully removing any air pockets. You can see why a thinner mix and smaller aggregate, which allow for better flow, work better for grouting. If your wall is very tall, you'd lay 5 rows or so of block and then fill with grout and tie on more rebar, 18" overlap, before adding more rows of block.
When you're laying the block, try to minimize the amount of mortar that drops down into the cell and make sure the mortar that has squeezed out between the blocks doesn't reduce the size of the block's cell, which would impede the flow of grout. Before moving on the a new row, go back and remove the excess without letting it drop into the wall. I've read that if you're grouting a wall greater than 4 feet high you need to drill holes in the bottom of each cell stack, to make sure you don't have air space or debris in the form of mortar droppings in the bottom. I didn't do this and neither the engineer nor the inspector mentioned it and after reading some online building forums I got the impression it's not even regular practice in light commercial construction. I filled all the cells in my wall, per the engineering specs, but you might fill only the cells that contain rebar, making what's called a "partially grouted wall". I've read that a fully grouted wall is only 10% stronger than a partially grouted wall.
Notice the engineer left out a value of block strength, so it needs to be inferred from the other values given. There are a couple of tables in the Uniform Building Code, tables 2-3 and 2-5, that can be used to figure out what type of mortar and block to use for a wall. Here are a few rows:
|I can get my wall with a strength of 1500 psi by using type N mortar and block of 2150 psi compressive strength or using type M or S mortar and 1900 psi blocks. So, you can compensate for a weaker block by using stronger mortar and visa vera, based on this table.|
Block of strength 1900 psi is what's called standard type block and is probably the stuff sold at your local home center. Another type block you'll see in various publications is High Stress, with a compressive strength of 3750 psi. The managers at the Home Depot where I bought my materials had no clue what strength block they sold, but I used it anyway and the city inspectors never brought it up, so I assume block strength is a concept beyond the realm of residential retaining wall work.
To ramble on a bit more about CMU, block strength is measured two ways:
- Gross Compressive Strength is the strength of a cross section of a block, a measurement of the block including the cells.
- Net Compressive Strength is a cross section without the cells; basically of measure of the concrete used in the block and not the whole block. A standard block at 1900 psi is a measure of net compressive strength and the same block's gross compressive strength would be 1000 psi.
One last bit of information I found that's interesting: There's a rule of thumb that says the strength of a wall is usually about 80% of the strength of the grout, mortar and block and speaks to construction methods. It's basically a real world average based on experience. So, the calculation for my wall was:
Finding which products available to you conform to engineered specs may take some work. Bags of mortar, for example, may just have an ASTM number on them, so you'll need to find out what that ASTM spec means. Go to the web sites of the products being sold in your local stores. (The people working there won't be able to tell you much.) Or, ask your engineer or inspector for specific recommendations of what to buy.
Cutting and Bending Rebar
I cut my rebar with a circular saw and metal cutting blades but it makes a lot of dust and sparks. I've since cut rebar with a reciprocating saw and a metal blade which was much easier. Put the bar in a vise if you can and if you're cutting shorter pieces, you can make a partial cut and break the rest by bending the bar at the cut.
I bent the rebar using a couple 3 foot lengths of 3/4" black pipe and a vice. Clamp one piece of pipe in the vice. Mark where your bend should be on the rebar and slide it into the clamped pipe so the mark is about 3/4" outside the pipe. Slide the other piece of pipe over the exposed end of the bar till it's end is about 3/4" from the mark. Now pull on the second pipe, using it as a handle/lever to bend the rebar.
To tie the cage, I used a roll of tie wire, which is annealed to make it softer and more pliable. My engineer didn't specify the tie wire size but I think I got 16 gauge; it comes in a variety of thicknesses. You can buy individual wire ties that come in various lengths, 3" to several feet, that have a loop on each end for use with a any of a variety of wire tying tools. A simple, manual hand-tying tool is just a handle with a metal hook extending from one end, called a pigtail tool. If you use wire off a roll like I did you'll probably use a pair of pliers to get a tight twist on the wire at a rebar intersection. You'll break some wire while twisting but it's nice to get it tight. The engineer didn't specify how many intersections to tie so I tied them all, called a 100% tie. My inspector said my rebar cage was "beautiful". I was so proud. I talked to a pro who told me he always uses a roll of wire and pliers; that the pigtail tools were a pain.
I checked whether you can use plastic zip ties to tie rebar and pretty much found out it was OK to do. The ties serve only to hold the cage in place during the concrete pour and provide little structural function in a backyard retaining wall. In big construction this may not be true as sometimes rebar is actually welded at the intersections, though this may be because the cage has to be solid enough to be moved without distorting. I'm guessing here. I tried plastic ties on another project and it worked pretty well, though I could get tighter connections with wire. Be careful to have your vertical rebar lined up with where the cells in your block wall will be, which was every 16 inches for my wall, such that every other void in the wall contained rebar. I'm tall enough to lift the blocks over my rebar verticals for this 5 foot wall but if your wall is too tall, you may need to tie more on as you go up. Continuing a rebar run requires you overlap the pieces by 18". Use wire to secure the splice.
The Rebar Cage
Use dobies, (small 3" square cement blocks with tie wire stuck in them), to hold your rebar cage off the ground. Once tied, your cage becomes surprisingly stiff but you may need additional support to keep it in place while you're dumping concrete into the footer form. You can use a few large chunks of old broken concrete inside the footer to stabilize the rebar that is higher than 3" off the ground. If your rebar cage is a little top-heavy, you could stabilize it by constructing a system of supports outside the area of the pour using wooden stakes and scrap wood attached to vertical rebar extending out of the footer, just like you would if you were setting and plumbing fence posts.
The horizontal bond beam rebar is laid and tied as you're laying block. You can buy what are called bond beam blocks, which are manufactured with cutouts to accommodate the rebar and used for just the bond beam rows. Or you can raise a huge cloud of cement dust and cut your own notches in regualr blocks with a circular saw and a masonry blade, just an inch or so deep and wide. Wear a mask and cover your porch furniture.
Waterproofing and DrainageI waterproofed the back of the wall with masonry coating, a cementitious powder you mix with water and apply with a brush. I laid lots of landscaping fabric in the trench behind the wall, placed two 4" diameter drain pipes in the bottom, which hook into a drain system emptying to the street and backfilled with drain rock to about 18" from the top of the wall. It's very important to provide good drainage behind a wall because the force exerted by water build up is huge.
Back to the Wood Veneered Wall
The footer of the block wall extends out a good foot and becomes part of the cement patio.
The ledgestone look I was going for, which has a craftsman-like, horizontal, Frank Lloyd Wright feel about it. My wood wall has a more pronounced horizontal feel, since the wood pieces are relatively long.
A piece of old redwood from my collection sitting on my trusty, 20 year old little table saw.
The cinder block wall with the facing panels ready to be installed. I glued the redwood pieces to Wonderboard, a 1/2" cement board made for the construction of shower walls and then screwed the resulting panels to the wall using cement anchors. This way, I could work on the panel flat on a table, much easier than working on a vertical surface so close to the ground. In addition to construction adhesive to attach the redwood to the Wonder board I used screws through the backside of the board but just into the larger wood pieces. Cutting and fitting the wood pieces was tedious but once I got into the zone it was fun. I laids the panels out on a long table I have outside, (you could use tall sawhorses), two at a time, ends butted together. As I glued wood strips to the area at the butted edges, I was careful to glue any piece that spanned the edge to just one piece of Wonderboard. Otherwise, I'd be gluing the two panels together and I wanted to be able to carry each piece separately. (The panels are heavy!) So, what I ended up with were two panels that had wood strips extending beyond the butting edges as well as areas that were still bare cementboard. When installed, the two panels edges fit together like intertwining fingers.
I trimmed the coutside corner of the wall with a 4x4 post with a 1 1/2" square chunk ripped out of one corner so it was shaped like a right angle, forming an inside right angle that would fit over the corner. You can see it in the photo at the top of the page, just over the chaise lounge and in this photo, already attached to the panel. But to make this work, when gluing the strips to the wonder board panels that would meet at the corner, I left bare 2" of each edge. This way, the corner trim post laid flat on the cement board and butted where the horozontal strips began. Actually, before gluing the strips, I clamped the trim post to one of the corner panels to push the strips against while gluing. For the other corner panel, I placed it against the wall, bare, (no strips attached) and the other, completed panel against it's wall and marked a line where the edge of the corner post trim overlaid the second panel. Then I clamped a board to the second panel, edge along the mark forming a nice straight fence line to push the strips against during glue up. That way, at panel install time, the corner came together nicely.
Another reason I used the Wonderboard was because I thought gluing the wood directly to the cement block would expose the wood to the moisture that might be in the block after a rain, though I suppose I'll have the same issue with the wonder board. I considered attaching something to the wonderboard before attaching the wood, which would serve to create airspace between the wood pieces and the wonderboard. Metal lath or chicken wire, used for reinforcing tile mortar beds might have worked for the purpose but in the end I skipped it. I still think it's a good idea though, I just got lazy. The construction glue would have an impact on the effectiveness of the airspace, but you could apply the glue in dollops so the water could move around it. I also put glue on the meeting wood surfaces, the edges of the strips, and you could rely more on those areas to hold the wood pieces in place, minimizing the glue between the wood and Wonderboard.
The footer is almost a foot deep, 2 feet wide. I also positioned rebar to extend vertically up through the cells in the cinder blocks, which I filled with cement and there are two horizontal runs of rebar in the wall, half way up in a bonding block run and along the top. In the photo you can see a piece of wood holding a vertical rebar piece to keep it plumb during the pour.
The footer excavation lacks a bottom key but does include some holes to further prevent movement of the wall. The holes were there already, used by the builder of the original wall for the redwood posts so I made use of them. Instead of manufactured dobies, I used some old concrete chunks in the bottom of the trench to hold the rebar off the bottom. I wrapped wire around the rebar and the chunks to hold the rebar in place during the concrete pour. I've mixed more bags of concrete around here than I care to think about.
Another view. The boards on the top of wall, forming a seat are made of wood from my in-laws old pergola that once protected their back deck. I jointed the edges, just using my little table saw, and glued two boards together edge to edge using biscuits to get the final 10 inch width.
The whole patio. By the way, one last thought. Notice the electrical outlet on the wall. (Scroll up to the photo of the bare block wall.) Whenever you're doing anything, inside or out, consider adding electrical outlets, lighting or anything else you can think of to add to the project while everything's already torn apart. Got some drywall torn down? Run some wire, even up the studs, strengthen the structure with Simpson plates, add insulation, add shear strength. If you've hired people, don't let your contractor get away without discussing the world of additional possibilities.