# Flashing under precast sill in a single wythe CMU wall?



## jslack (Mar 20, 2021)

Hi everyone. I'm remodeling my house on an owner/builder permit. The work has been straightforward so far but a window flashing requirement really has me stumped. Currently, the windows in my kitchen are lower than the countertop backsplash and I want to raise them up above the counter so that they're easy to open and don't collect dust and trash, etc. My house was built in 1968 and it's constructed of single wythe CMU walls with no stucco. I'm located in Florida and this construction is very common for older homes here. Most of the windows in my house are 5 blocks tall with a precast concrete sill. The kitchen is an addition made in the late 1980s. Its windows are six blocks tall and the existing sills are wood. My goal for this work is to make my kitchen windows more functional but also make them match the aesthetic of the windows of the rest of the house.

My plan for this work is to remove the existing wooden sills, lay an additional row of bond beam block at the bottom of the existing window openings, and for extra strength, tie them to the existing side and bottom blocks with rebar. Lastly, I'll fill the webs of the CMUs with grout and mortar on a new precast concrete sill. Lastly, I'll install new wood bucks in the head and jambs, and install the new window, flashing it according to the manufacturer's instructions.

The decision to do this window work was a recent one, so I've just tried to amend my permit to add it to the scope of work. However, my city is asking me to show specific compliance with with Florida Building Code (6th edition) and the American Concrete Institute (ACI) for this alteration. I've spent a lot of time reading the FBC so this isn't unexpected. ACI specs are totally new to me, so I'll need to figure that out, but my question here is about a provision in the FBC about flashing under the sill.

Section R703.4 "Flashing." Specifically, "approved corrosion resistant flashings shall be installed at the following locations... #3 Under and at the ends of masonry, wood or metal copings and sills."

Does this mean that I must install flashing under and to the sides of the precast concrete sill? If this is required, how do I attach the sill to the CMU below it? If I use a metal sill pan, for example, how should I mortar the sill to the CMU? Would this not be the ultimate bond breaker? With the use of flashing, what will hold the sill in place? The use of tapcons or something similar will pierce the flashing, so this is no good. As I mentioned, I will be flashing the window according to the window manufacturer's instructions, but these of course don't say anything about the window rough opening or sill construction.

My hope is that I'm misreading or misunderstanding this flashing requirement and that it doesn't actually apply to the work that I'm doing, or in the way that I think it does. Can anyone help clear this up for me?

I've included a drawing below which should hopefully convey the work that I'm doing. Any help at all is appreciated because this really has me stumped. Also, if anyone might know which American Concrete Institute specs I should research, feel free to reply with that, too. Thank you in advance!


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## e hilton (Mar 20, 2021)

Is that your detail, from architects plans, from a book ... ?


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## jslack (Mar 20, 2021)

I drew that. Does it make sense? The window has its own architectural/installation drawings, but they don't concern the rough opening.


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## fatboy (Mar 20, 2021)

Welcome!

Nice drawings! 

Not familiar with the FBC, nor that much with CMU construction, I am sure others will weigh in during the week. 

I can't think of ever seeing a flashing detail for CMU openings, quick scan of my electronic IBC was no help.

RDP's specification?

Check with the AHJ is where I would go next. 

Good luck!


GENERAL
2101.1 Scope. This chapter shall govern the materials, design, construction and quality of masonry.
2101.2 Design methods. Masonry shall comply with the provisions of TMS 402, TMS 403 or TMS 404 as well as applicable requirements of this chapter.
2101.2.1 Masonry veneer. Masonry veneer shall comply with the provisions of Chapter 14. 2101.3 Special inspection. The special inspection of masonry shall be as defined in Chapter 17, or an itemized testing and inspection program shall be provided that meets or exceeds the requirements of Chapter 17.


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## mark handler (Mar 22, 2021)

I believe the intent of R703.4 is wood and metal sills not precast Concrete sills. 
I would want the bond between the precast and the filler block.

You do need to properly flash between the window and sill, hot reflective in your drawing.


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## jslack (Mar 23, 2021)

@fatboy the "2101.1 Scope" etc. code you pasted is from the FBC. Are you familiar with ACI requirements?


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## jslack (Mar 23, 2021)

@fatboy interestingly, the 6th edition of FBC, "2101.2 Design methods" says


> Masonry shall comply with the provisions of TMS 402/ACI 530/ASCE 5 or TMS 403 as well as applicable requirements of this chapter.


which points me to ACI 530 which I think will get me started.


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## fatboy (Mar 23, 2021)

jslack said:


> @fatboy the "2101.1 Scope" etc. code you pasted is from the FBC. Are you familiar with ACI requirements?


No, the Sections I posted were from the IBC, which the FBC starts out from. 

And no, I am not familiar with them, hence my statement that I was not familiar with CMU construction. 



jslack said:


> @fatboy interestingly, the 6th edition of FBC, "2101.2 Design methods" says
> 
> which points me to ACI 530 which I think will get me started.



There you go!

I would like to think I walked you down the path!


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## jslack (Mar 23, 2021)

@fatboy absolutely you did, thank you!


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## Mark K (Mar 24, 2021)

ACI 530 is focused on structural issues.  I do not believe it addresses the design and installation of flashing.


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## jslack (Mar 27, 2021)

@mark handler 


> I believe the intent of R703.4 is wood and metal sills not precast Concrete sills.


 R703.4 specifies flashing requirements for "masonry" sills. As the term is used here, is "masonry" distinct from concrete?


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## e hilton (Mar 29, 2021)

jslack said:


> @mark handler
> 
> R703.4 specifies flashing requirements for "masonry" sills. As the term is used here, is "masonry" distinct from concrete?


2015 ibc chapter 2 Definitions.  Masonry. A built up construction or combination of building units or materials of clay, shale, concrete, glass, gypsum, stone or other approved units bonded together with or without grout orvother accepted methods of joining.   
{followed by several sub-definitions}


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