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Frontage Increase and Frontage Perimeter

S

SteveZ

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Aug 13, 2019
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14
Location
Montana
We have a building where the allowable area as determined by IBC 2018 equation 5-3 would exceed the allowable area for the ground floor depending on how the value of 'F' in equation 5-5 is determined. The building would be below the allowable area if the full frontage increase were permitted.

My question pertains to decks and how they relate to the value of F in equation 5-5 - length of perimeter that fronts onto public way or open space. The definition of Building Area would seem to include these covered decks as they are underneath a horizontal projection of the roof or floor above (another deck or the roof covering the last deck on the top floor). Thus, it would follow that the outline of the decks would be included in both the building area AND and building perimeter with relation to equation 5-5.

In this R-2 building, there are several decks in a row projecting from the exterior wall facades. Would the projecting walls between adjacent decks not count toward the value of F because they do not front onto a public way with a width greater than 20' and instead would basically 'front' onto the adjacent deck? Thank you for the input!
 
Do you have a plan view or site plan that you can share?

FYI - As you are not a paid member, you would have to share via a link.
 
classicT said:
Do you have a plan view or site plan that you can share?

FYI - As you are not a paid member, you would have to share via a link.
Click to expand...
This is not our building but is similar with regards to this question. Assuming this building fronts on 4 sides for purposes of frontage increase (W=30'), would this building receive the full increase? The question area is where the building and decks jog in, continues across, and then jog back out again. Would the walls/decks that jog in/out not be included in the value of F in equation 5-5 because perpendicular to those walls is another part of the building?

This may be getting too in the weeds as it seems that those areas are essentially still open to the qualifying public way with a width greater than 20' perpendicular to the main façade.
https://chelseacommonsapt.com/floor-plans#building-plans-1
 
Per the design presented, I would measure from the face of the walls and the decks. In this case, I believe the intent of the code is to take the measurement form the planar element of the building side (the building face).

506.3.2 Minimum Frontage Distance
To qualify for an area factor increase based on frontage, the public way or open space adjacent to the building perimeter shall have a minimum distance (W) of 20 feet (6096 mm) measured at right angles from the building face to any of the following:
  1. The closest interior lot line.
  2. The entire width of a street, alley or public way.
  3. The exterior face of an adjacent building on the same property.
 
classicT said:
Per the design presented, I would measure from the face of the walls and the decks. In this case, I believe the intent of the code is to take the measurement form the planar element of the building side (the building face).

506.3.2 Minimum Frontage Distance
To qualify for an area factor increase based on frontage, the public way or open space adjacent to the building perimeter shall have a minimum distance (W) of 20 feet (6096 mm) measured at right angles from the building face to any of the following:
  1. The closest interior lot line.
  2. The entire width of a street, alley or public way.
  3. The exterior face of an adjacent building on the same property.
Click to expand...
"measured at right angles from the building face" is exactly what is confusing me here. Is it the facade (building face) as a whole with a width greater than 20' or are perpendicular measurements from individual walls to be considered? If it was the latter, where the plan jogs in/out and the wall is perpendicular to the lot line, it would seem to not be fronting and would not be included in the value of F. In the example presented, would F=P?
 
You would measure the average distance from the face of the building. You can do it as a weighted average. Say you have 100-ft that is 20-ft away and 50-ft that is 25-ft away. Then the calculation is (100'x20'+50'x25')/150'=21.66'.
 
Yes, I am familiar with the weighted average for W and associated calculation. Here I am wondering about the value of F in equation 5-4 and 5-5. What about the case of a U shaped building? It is the two inside faces of the U shape I am wondering about, but at a smaller scale. The courtyard for this building could conceivably be less than 20' so would the two inside faces not be included in the value of F if that were the case? To get back to the question, what about at the smaller scale of adjacent inside faces of decks or plan modulations of apartment buildings?
 
I apologize, but I am not following you and I think you are over complicating this.

F is going to simply be the length of the wall measured in a straight line. In the example I gave above, the value of F is 150-ft. Think of it as drawing a box around the whole building and then measuring the length of the sides of said box.
 
haha yes I think I might be overcomplicating too but its easy to sometimes get stuck on these little code nuances, especially when it makes a difference in the design direction. In the example I gave of the U shaped building, if you drew a box over the plan of that building, F would not equal P, even if the interior courtyard were greater than 20' wide between inside faces of the U, which seems incorrect. Would it be better to say that a length of wall does not count toward the value of F if it is obviously not adjacent to an open area greater than 20' perpendicular to an envelope drawn around the extents of the building?
 
I think when you're on the balconies, you're 'inside the perimeter'; don't count those distances. Both F and P say 'perimeter' rather than 'length of exterior walls' also, so I think that supports this stance. I also look at the case of the giant U shaped building- at the inside side-face of the U (or side of the balcony too) as you measure perpendicular to the lot line (it doesn't say "perpendicular to the face of the SAME building" so you're passing right thru it) you eventually cross another wall, that is also being measured the same direction, at a lesser distance. The point of frontage increase (as I see it) is to say "You get a bigger area if you're far away from property lines or other buildings", and it disregards your own building (except in the case of a structure bifurcated by some fire walls to make separate buildings?). So with that being the reason to measure frontage increases, including more distances into P than just the absolute perimeter makes F/P smaller, and thus less of an increase than should be allowed- like the distances to *itself* shouldn't be reducing it's increase.

I'm super late to this reply, but if anyone would want to comment, thanks!
 
I've always been conservative and measured the minimum distance from building face to property boundary or separation line to another building. I've always included roof overhangs but not (small, steel) canopies or balconies. YMMV
 
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