Learn the ins and outs of using concrete masonry units, or CMUs, in construction, including how to calculate the amount needed for a project. This article dives deep into the architectural details and provides a step-by-step guide to constructing CMU retaining walls, including waterproofing considerations.
Key Insights
- In construction, CMU stands for concrete masonry unit, often referred to as concrete blocks or cinder blocks. They are used to support structural framing, especially in the foundation and crawlspace areas of a building.
- Calculating the amount of CMU needed for a construction task involves figuring out the heights and square footage of CMU. This involves referring back to the architectural and structural drawings and making certain assumptions, such as average height, to move forward with the project.
- Waterproofing is an essential part of constructing CMU retaining walls, especially where they are below grade. This involves applying a waterproof membrane to the exterior of the CMU walls and calculating the square footage necessary for ordering.
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So what we are going to take off now is going to be some of the CMU that goes from the foundation up to some of the framing that holds up the kind of underside of the first floor and frames out the crawlspace area. In construction, CMU stands for concrete masonry unit.
These are precast concrete blocks, often referred to as concrete blocks or cinder blocks. So let's look at those details back on the architecturals. If we go into Sheet 8, sorry, Sheet 9, and we look at Detail 1 on A4.2, we can see that on top of some of the framing underneath the guest room, there is going to be some CMU that supports the structural framing as well.
If we go to this note number 2, we can see that it is concrete block with typical stucco exterior finish. And if we look at this detail here, 10 on D7.3. We're on D7.3, and we go to look at 10. We can see that we've got our CMU retaining walls.
We can see that we have a waterproof membrane on the front of it. And we can see that we have just kind of like a standard French drain and filler media that helps carry water away from our foundation. So, okay, let's now try to figure out the heights of this CMU so that we can do an accurate takeoff of it.
And in looking back at our section cuts here, it appears as though there is going to be CMU around kind of sitting on top of our entire foundation of our concrete footing. Seeing as this is just a section, it does look like there is going to be CMU in a kind of square formation underneath this guest bedroom. Let's just see if we can see any other maybe details that would help us out here.
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So number 7 and number 5. 5 is just saying an insulated crawl space. Okay, that's great. We'll get that insulation later on.
Number 7, we have the face and finish of wall above foundation stem wall system. Okay, so this doesn't really change our understanding. Let's go down to the structural drawings then.
And let's basically just ring this entire concrete footing with another takeoff. And that takeoff is going to be our CMU. And I guess for the height of this, let's go back to our number 9 sheet here.
And there isn't really a great elevation to follow. You know, there's not necessarily a top of concrete elevation that we can see. The grading totally changes here.
So, but we can definitely see that it's going to be like 10 feet from underside of finish floor to the ground. We know that this footing is going to be a couple feet down. Let's just assume something like maybe we have like 2 feet over on this side of CMU, maybe like 4 feet.
Let's just assume like a 3-foot average height just for the purposes of moving forward here. So to do this takeoff, we're going to go, like we said, back to the structural drawings here. We're going to go to S2, and we're going to do a wall area.
So let's go ahead and do a poly length. And then with that poly length, we can go ahead and add a height to it. So that way we can get kind of like a square footage of CMU.
So let's get the poly length here. Let's go ahead into our properties. Let's call this CMU wall at foundation.
All right, that's great. We'll go ahead and put this at an O for concrete masonry units specification section for organizational purposes. All right, great.
And let's just go ahead and start taking this off. We're going to go outside to outside to outside to outside. So that is going to be the length of our CMU wall around.
And then let's click on that takeoff, and let's go up to our units dropdown up in this properties bar here. And let's go ahead and give this a depth. And we're going to give this a depth of three feet.
Like we said, we're just going to do a three-foot average here. And then once we go over into our wall area in our totals kind of dropdown, we can see that that is going to give us that square footage number that we're looking for. So having that amount of square footage is going to allow us to figure out how many actual CMU blocks we needed.
You would typically just take that square footage number and divide it by the square footage of one block and the associated mortar joints, and that's how you'd come up with the actual number. But that right there is going to be our CMU. And now that we've got our CMU takeoff completed, we do know that there was also some waterproofing that was called out to be on the face of this CMU as well.
So let's go ahead and run that waterproofing takeoff right now. So I guess let's take a look at our detail again before we move forward. Detail is 17, D7.3. We see this below-grade waterproofing here.
And we can see that we have waterproof membrane applied to the exterior of CMU retaining walls where they are below grade. We didn't have a clear, totally clear understanding of where these CMU walls would be below grade or not. But let's go back to sheet 9 and take a look here.
Again, I know we kind of called out like a 3-foot average height for the CMU wall. So maybe let's just say that like half of that is expected to be below grade. So let's just give it like a 1.5-foot height all the way around for our waterproofing.
That'll just be kind of like an assumption that we will make here. So let's go back to our structural drawing where we've done the CMU takeoff already. And probably the easiest way to do this would just be to choose that CMU takeoff and just copy it and then paste it.
And when we go to paste it, that pasted section, we will have to change the information about. We'll go into properties. And we will just call this, let's say, foundation waterproofing.
Well, CMU, not foundation. I guess we'll call it CMU waterproofing. Oh, and we'll clarify here that it is going to be where below grade.
Where below grade. All right. And let's go ahead and just change that color.
Let's just make that blue for, you know, waterproofing purposes. We can change the specification section, not 08, jump down to 07. We'll just throw it under membrane roofing for the time being.
I don't know if we're going to break out a separate one or if it's going to be a similar kind of like waterproofing type product. We'll just throw it into that one for now. We can always change that up later on.
Go ahead and just put this right in the same place as our CMU takeoff. And then we are also going to want to make sure that we change the height of that waterproofing as well. So the depth is not going to be 3 foot.
It's going to be 1 foot and 6 inches to account for our assumption of how much is actually below grade. And if we again go into our totals page and we hit this drop down and we look at the wall area, that gives us the square footage of this waterproofing membrane. That's how we're going to know how much of it that we need to order.
And let's just do a quick reminder to save our work here so we don't lose anything that we have done.
