Aggregate Venting

I am constructing a house not subject to code inspection in a remote rural area. There are 21 total fixture units serviced by two 2" vents extending through the roof. The vents are more than ample to service the fixtures. Although not required by the IRC, I just discovered that UPC requires that the total venting area be not less than the area of the building drain size. Thus a 4" building drain would require 12.57 sq. in. and a 3" drain 7.07 sq. in. However, my aggregate vent area totals 6.28 sq. in. As my vents are now set, it would be very difficult to enlarge my aggregate venting.

My building drain leads to a 4" sewer drain to a septic tank. Although I could run a 3" building drain, I would prefer to run a 4" drain. However, this would require another large vent under the UPC. Since the IRC is silent on aggregate venting, I am confused and wonder what problems, if any, would I incur by running a 4" drain with a 6.28" aggregate vent area? Would it be better to run a 3" main drain? Any suggestions or comments would be appreciated.

vents

Two 2" vents would still not equal a 3" sewer. You are asking about whether something is legal or if it will work. These two conditions are not always equivalent.

is this house already plumbed?

If not , go with a 4" stack like the rest of the country. are you worried about $50 ?

Vents

Thanks for the replies. I regret my lack of clarity in the post.

I am more interested in the practical adverse consequences as legality is not an issue. Moreover, since IRC is silent on the subject, aggregate venting would also not appear to be a legal serious issue where that code is applied. I also note that IRC and UPC differ in a number of plumbing matters.

Unfortunately, owing to the nature of my construction, (timber frame with SIP panels) it is not feasible to replace the existing vents or easily add another vent, as there would be if I had conventional stud framing. The roof is on, vents are in, dry wall in place. I increased the size of the vents at the roof for frost concerns.

I am assuming the only practical adverse consequence that the UPC is concerned about is slow draining and then only when the main building drain is completely full. If that is the case, owing to the relatively small number of separately vented fixtures, I am also assuming I should not have a problem with the existing 2" vents. But, as I am not a plumber, I have no experience nor expertise in this matter.

thank's for the explanation

my 2 cents leave it be. It'll drain fine. No insp. is necessary,you said

congrads. and enjoy your new home Tool

Thanks

Thanks again for your thoughtfulness.

In my area, (Tampa Bay) local codes require house vents to be 2" with one 3" vent. We usually place the 3" vent off the kitchen sink. Regards, Tom