Unobstructed Construction: Open Grid Ceilings

MeyerFire University | FX108.52C

By Joe Meyer, PE

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RESOURCES

FX154 SERIES

RESOURCES

NOTES & SUMMARY

• One-Page Summary [PDF]
• Notes Page [PDF]

CODE & STANDARD REFERENCES

• NFPA 13 – 2022: A.3.3.41.1(5) Examples of Unobstructed Construction: Open-Grid Ceilings
• NFPA 13 – 2022: 9.2 Allowable Omission Areas for Sprinklers, including Open-Grid Ceilings
• NFPA 13 – 2022: 9.3.10 Required Clearances between Open-Grid Ceilings and Sprinklers Above

FX154 SERIES

1. Introduction to Unobstructed Construction
2. Difference between Obstructed & Unobstructed?
3. ​Unobstructed Construction: Bar Joists
4. Unobstructed Construction: Open-Grid Ceilings
5. Unobstructed Construction: Smooth Ceilings
6. Unobstructed Construction: Standard Mill
7. Unobstructed Construction: Trusses

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TRANSCRIPT

Examples of Unobstructed Construction: Open-Grid Ceilings 

DEFINITION

Today, we’re continuing our series on Unobstructed Construction. So we last spoke about bar joist construction, and here is our second of five examples of Unobstructed Construction that we’re given in the annex of NFPA 13. 

So, what is an open-grid ceiling? 

The definition we’re given for open-grid ceilings are when the openings are at least a quarter inch or 6 mm, the thickness of the ceiling material does not exceed the least dimension of the opening, and the openings constitute at least 70% of the ceiling area. (add highlight box) 

70% open? If you’ve been around the sprinkler industry for long, you know that’s come up before. Let’s start with a breakout on each portion of this definition and then we’ll circle back on that 70% open in here in just a bit. 

DEFINITION

The ceilings must have the following three qualities: 

The openings in the least dimension are at least a quarter inch or that 6 mm. That means punch-out holes have to be a quarter inch in diameter, or if we have mesh grating, then the width needs to be this minimum quarter inch wide. 

Now the thickness of the ceiling material is our #2 qualifier. The thickness of the ceiling material cannot exceed the least dimension of the opening. If we’re talking about mesh metal with quarter inch openings, then the ceiling cannot be more than a quarter inch thick. So, let’s say we have steel grating that’s a little more heavy duty. Let’s say the width of the openings are 1-1/2 inches wide, but the depth is 2 inches deep. Well, that’s a no-go, Houston. When we’re qualifying open-grid ceilings here, the depth of the ceiling cannot be greater than the minimum opening width. So, if we got an inch and a half wide, but we're two inches deep, not gonna work. We can't be more than the minimum opening with. 

Lastly, number 3, the openings have to be at least 70% of the ceiling area. This one can get a little tricky. Sometimes, creative folks AKA architects, look to substitute materials or try to achieve different looks along the ceiling. Well, if the ceiling isn’t at least 70% open, then it’s not an open-grid ceiling. Perforated metals can fall into this category. This is where we have a metal sheet and we punch holes into those openings. Those often are not 70% open. You can check the dimensions and openness if that’s a concern. Usually, there’s plenty of data on these by the metal fabricator or the material supplier. Worst case, we can always open up AutoCAD or Revit, draw it out, and take some area measurements. We have to be 70% open or more to qualify as an open-grid ceiling. 

Thanks, Joe. But why spend so much time on this one? 

Well, in the context of obstructed vs. unobstructed, what we’re finding here is that an open grid ceiling would fall under unobstructed construction. If we’re say, 60% open and don’t qualify as open-grid. Well, it may not in fact change all that much because we still may fall under smooth ceiling construction. 

So, at the end of the day, this might not be a huge deal breaker one way or another on our approach for sprinkler design. 

IMPLICATIONS

But this exact same definition and term comes up in two other areas of NFPA 13, and they have major implications. 

Do we locate sprinklers underneath an open-grid ceiling, or above? Does the open-grid ceiling qualify as an obstruction, or can water just flow through it? If we do have an open-grid ceiling, can it be located right next to the sprinkler, or does it have to be further down in elevation and away from a sprinkler so that the sprinkler pattern develops? 

Now we’re getting into some more complexity. 

SPRINKLER OMISSION LOCATIONS

NFPA 13 Section 9.2 addresses allowable sprinkler omission locations. This section is a big one for us, and we’ll park in this lot for quite a bit in future segments. 

One of the listed allowable omission areas is when we have an open-grid ceiling that complies with Section 9.3.10.  

Well, section 9.3.10 gives us this same open-grid ceiling definition as what we just looked at earlier, and then it goes further. There’s a lot of detail in here. Clearances between the sprinkler deflector and these open-grid ceilings can range from a minimum of 18 inches (450 mm) all the way up to 48 inches or 1.2 meters. What? A 48-inch clearance? Try telling that to an architect. (highlight definition) 

Well, in short, if these ceilings are too close to a sprinkler or obstruct too much area, it will affect the water from getting from a sprinkler to properly reaching the hazard. That's why these rules are in place. 

70% OPENNESS

Also, this 70% openness has kind of latched on as an engineering-judgement theme for sprinkler design. When we talked about open trellises or grating or structure that’s open to the sky, some look at this 70% openness as guidance on determining if and where sprinklers should be provided. 

STRUCTURES ABOVE

The last note before we wrap up this segment; if all we have in as open-grid ceiling, we’re unobstructed construction in accordance with this definition. The annex is telling us we're unobstructed construction. 

If, like we could very well have in many cases, some sort of solid structure is above the open-grid ceiling, well then we also have to consider the arrangement of that structure that’s above the ceiling. If that has solid steel beams, or coffered concrete decks, or thick trusses, then that will drive Obstructed Construction requirements for sprinkler spacing and locations up at that deck. 

So we can't just say, oh, we've got open grid ceilings or unobstructed construction and completely ignore the structure above. If the sprinklers are located up close to that deck where the structure is, that very well could still be obstructed construction. We'd have to comply with those rules. 

SUMMARY

So as a summary today, when we only have open grid ceilings with some kind of flat, smooth deck above, and the open-grid ceilings have minimum opening sizes, they are not too thick, and are at least 70% open, well then, our ceiling construction would be considered unobstructed construction. This then cues all the principles for sprinkler spacing and height that’s based on unobstructed construction. 

In our next segment, we’ll get into smooth ceiling construction.  

I’m Joe Meyer, this is MeyerFire University.