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Parapets: where roofs meet walls.

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Historically, so many problems have occurred with parapets that we have a name for it: "parapetitus." They have a long history--which of course is not always clear--that allows me to embellish without threat of peer review reversal. * Their major function today, aside from confusing architects, is to protect the edge of roof assemblies from wind uplift forces. Not so in the old days where they were useful in fire protection.

When wind blows against a building it produces vortices at the roof edges (Figure 1) that create huge pressure differences (Figure 2) at roof perimeters that can suck roofs off buildings. Parapets dramatically reduce these pressure differences at roof edges (Figure 3). Neat eh? All this from a University of Toronto guy, go Varsity Blues (Leutheusser, H.J., 1964 (2)).

The easiest thing to get right about parapet construction is to keep rainwater from getting into the top of them. The principles are easy. Slope the top of them inward so they don't stain the building facade. Make sure that there is a waterproof membrane under the coping. Always. Metal and stone copings leak at joints. And always have drip edges--front and back--so that they don't stain the building facade. Did I mention the staining of the building facade? Check out Figure 4 and Photo 1 to see it done right. If you want to get depressed, look at Photo 2.

Are we done yet? Nope, not by a long shot. Now it gets weird, not the physics, but why so many buildings get the physics wrong. For the physics we go to another one of those legendary old guys who got it right and made it simple for the rest of us to understand--Max Baker. Check out Figure 5, adapted from his book "Roofs." Connect the water control element/layer of the roof to the wall, the air control element/layer of the roof to the wall, the vapor control element/ layer of the roof to the wall and finally the thermal control element/layer of the roof to the wall. Sound familiar? Coming from me it should by now. ([dagger]) I call them the "Baker Principles."

This is what we typically get in the "real world" today (Figure 6). What a mess. No continuity of the four principle control layers:

* Water control layer: no membrane under the parapet flashing;

* Air control layer: no air control in either the roof assembly or the wall assembly;

* Vapor control layer: same goes for the vapor control layer; and

* Thermal control layer: thermal bridging everywhere.

And to make matters worse, structurally we also tend to have some issues. Ah, but not in the way you think. Think about the thermal stress a roof membrane goes through (Figures 7 and 8). The key is to transfer these stresses to the roof deck. In the old days it was easy; just fully adhere the roof membrane with a lot of goop directly to the structural deck so that each square foot of roof membrane stress was directly transferred to the square foot of structural deck directly under the membrane. No problem. Until, wait for it, some lunatic person introduced thermal insulation. Now we had to transfer the stress of the membrane through sometimes multiple layers of insulation before it got to the structural deck (Figure 9). If you didn't get it right, you concentrated the stresses at roof edges, and you could suck in a parapet (Figure 10) or tear or rip a membrane at the parapet (Photos 3 and 4).

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Transferring loads in multilayer compact roofs is quite controversial. There are lots of opinions, and I want to point out right from the start that only I am right. Let's start out in the field of the roof. This is how a compact roof would be constructed if I was in charge (Figure 11). There should be a continuous fully adhered air control layer supported by gypsum sheathing on the top of a metal deck. ([double dagger]) The gypsum sheathing is screwed to the metal deck. There should be a whole bunch of rigid thermal insulation on the top of this air control layer--in two layers at least with the joints off-set horizontally and vertically. ([section]) This insulation should be screwed down to the metal deck. Then on top of the rigid thermal insulation there should be a coverboard. This coverboard is also screwed down to the metal deck. Finally, a roof membrane is fully adhered to the coverboard.

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The function of the coverboard is twofold. First, it is a hygric buffer that reduces roof membrane blistering. A discussion of this has to wait for some other time. Second, and most important to our story, is that its function is to transfer the stresses of the primary roof membrane to the metal deck. Stresses from the roof membrane are transferred to the coverboard, and the coverboard does the heavy lifting and handles these stresses finally getting them down to the metal deck.

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Next we have to deal with the potential for concentrated roof stresses at parapets. Figure 12a shows how the "old timers" did it--wood blocking and a cant anchored to the structural deck. Figure 12b shows how the "new pups" do it--a large backer rod supporting a bunch of extra membrane that lets things move when they have to move. As much as it pains this "old timer" to say this, with the newer more dimensionally stable membranes the "new pups" have it more right.

Now on to the continuity stuff. All we have to do is apply the Baker Principles (Figure 5) to typical roofs and walls. To that end, with the help of my colleagues at the Skunk Works at Building Science Corporation, I have drawn up a few of the more common parapet constructions following the "Baker Principles": the already discussed Steel Stud Parapet (Figure 12b), the Masonry Parapet (Figure 13), the Balloon Framed Steel Stud Parapet (Figure 14) and, finally, the Cantilevered Mini Parapet (Figure 15).

All of the "good" series of parapet details presented follow the "Baker Principles" and a little bit of other stuff (Figure 12a, Figure 12b, Figure 13, Figure 14, and Figure 15):

* Water control layer continuity: membranes continuous under the parapet flashing;

* Air control layer continuity: an air control layer in the roof assembly is connected to the air control layer in the wall assembly;

* Vapor control layer continuity: a vapor control layer in the roof assembly is connected to the vapor control layer in the wall assembly;

* Thermal control layer continuity: the thermal control layer of the roof assembly is connected to an effective thermal control layer in the wall assembly. The thermal control layer in the wall assembly is exterior to the structure--just as in the roof assembly.

* The roof membrane is fully adhered to a coverboard that is mechanically attached to the structural deck in the field of the roof and an allowance for membrane movement is provided at the perimeter of the roof assembly.

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The perimeter of the roof assembly insulation is wrapped to prevent interstitial airflow from the parapet into the multilayered rigid insulation of the field of the roof. **

The cure for "parapetitus" is continuity of the control layers and letting things move when they have to move. Max Baker and Stonewall Jackson would be proud.

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References

(1.) Baker, M.C. 1980. Roofs. Montreal: Polyscience Publications.

(2.) Leutheusser, H.J. 1964. "The Effects of Wall Parapets on the Roof Pressure-Coefficients of Block-Type and Cylindrical Structures." University of Toronto, Department of Mechanical Engineering.

By Joseph W. Lstiburek, Ph.D., P.Eng., Fellow ASHRAE

* The Italians have claim to the word "parapetto," which comes from "parare," which means "to defend," and "petto," which means "breast." The military calls "parapet fortifications"--defensive stonewalls--"breastwork." The dictionary meaning of "breasted" means "to confront boldly." So, low stonewalls historically are called parapets and are military in origin. "Stonewall Jackson" was also called "Parapet Jackson." OK, so that's not true, but with the way textbooks seem to be written today I bet I could get away with it if I decided to write one. So how did they come to be located on the edge of roofs? Ah, we can thank the English for that. In the old days, London tended to keep burning down and that tended to irritate the folks who lived in London. So, projecting wooden eaves were banned in the Building Act of 1707 as a fire risk. Instead an 18 in. brick or stone parapet was required, with the roof set behind, as fire protection (http:// en.wikipedia.org/wiki/Parapet).

([dagger]) Check out "The Perfect Wall," ASHRAE Journal, May 2007. Or go to: http://tinyurl.com/2upg3fv. It was my first column for ASHRAE and it was inspired by Max Baker's marvelous book Roofs. (1) The book, now out of print, was sponsored by the National Research Council of Canada and it brought together information into one document from Canadian Building Science Digests, Research Papers and Building Science Seminars and Workshops from the Division of Building Research. Much of this information is online (http://tinyurl.com/2eezyth). Awesome.

([double dagger]) For a more interesting discussion about the need for air barriers in compact roof assemblies, check out ASHRAE Journal, March 2008, "How Not to Build Roofs" or "Uplifting Moments-Roof Failures" at http://tinyurl.com/2vlgfll.

([section]) To more fully appreciate the need to offset the rigid insulation joints horizontally and vertically and to wrap the perimeter of the roof assembly insulation check out ASHRAE Journal, August 2009, "Complex 3-D Airflow Networks" or http://tinyurl.com/34wcqwz.

** See the previous footnote (Page 56).

Joseph W. Lstiburek, Ph.D., P.Eng., is a principal of Building Science Corporation in Somerville, Mass. Visit www.buildingscience.com
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Title Annotation:BUILDING SCIENCES
Author:Lstiburek, Joseph W.
Publication:ASHRAE Journal
Article Type:Reprint
Geographic Code:1USA
Date:Feb 1, 2011
Words:1669
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