Maximum beam spans based on joist cantilever length

Here's a clip from my On-Demand Course "Down the Load Path" available anytime at www.BuildingCodeCollege.com. The new 2015 and 2018 IRC beam span tables confused a lot of folks with much shorter maximum spans than we were all used to. This video explains why it appears that way, but isn't, and how the 2021 IRC was modified to fix the issue.

[@marcmercado2085]

Wishing you had more content, but anyways. I’ve watched your deck and codes and they’re very helpful. So thank you.

[@bikramsidhu962]

Thank you for makejng this knowledgeable video, it's very helpful

[@billg00001]

Thanks for pointing out the assumed max joist cantilever assumption for the beam span table. Context is everything in applying the data from the span table.

[@buildadeck7244]

Thanks Glenn

[@boots7859]

Thank you, great tutorial.

[@tylersmith293]

Super helpful.

[@vigilantezack]

To be a little more precise here. You interpolated at 7' effective joist to get a beam span of 8'2" which allows the draft to work. But the actual joist span was supposed to be 10' x .66 which was 6' 7". In other words, a little lower than the interpolated 7'. If we interpolate a little more accurately, the beam span works out to be more like 7'11" which would technically fail the desired 8' span. So my question is, how accurate does this really have to be? What tolerance do the inspectors give for something like this? If they were a mean grumpy inspector, they could fail you at 8' because the max is 7'11"? The way I calculated this is that 6'7" is about 29% between 6 and 8 feet effective joist span. So interpolating the same percentage of beam span between 8'9 and 7'7" is about 7'11". What am I missing?

[@gelndx5]

Disclaimer, I'm not an inspector or tradesman, but i think you are reversing the logic here. In the example, for every 1" DECREASE in effective joist span, there would be a .583333" INCREASE in maximum beam span. So 6'7" is a 17" decrease from the 8' effective joist span, which would therefore increase the max beam span by 9.9" in the table under the 8' row, resulting in a max beam spam of 7'7" + 10" = 8'5"..... so i think your 3" variance you interpolated is the correct difference between the 7' example and 6'7" actual, but you need to increase the beam span by that amount to find the max beam span, not decrease.... Let me know if my logic works here, as i am designing a deck now for my house, and want to ensure i have this right.

[@zackwallace1794]

@@gelndx5 You may be right, something felt off, I just couldn't wrap my head around it. I'd rather use a precise number than "kinda guess the middle" and hope the inspector doesn't kill my entire project over 1".

[@timothytimothycoza]

What beams to use when the beam span is 26 foot? (building a 26 x 26 foot free standing deck, but post are only located on the edges of the deck = imagine a grid of 9 post but center post missing, as underneath deck will be used as a "garage") What adjustment could I make for this to work? inserting any extra post in the center of the garage will not work

[@shawnbadanjek9561]

Your joists will need to be steel wide flange or steel truss beam.

[@triumfdula8830]

Can anyone give an answer, making a horizontal roof with 5 x 30 cm wood boards (30 cm being vertically) would the boards in 5 meters length bend from it's own weight and snow ?

[@Braindead154]

Hi Glenn, I’ve got an L shaped house and I’m planning on building a rectangular deck on the inside portion of my L -shaped house. I can’t find any resources that document how having the support of an extra ledger impacts the span table and sizing. Do you have any guidance here? Thanks!

[@gelndx5]

From R507.5 2021 IRC, "beams shall be permitted to cantilever at each end up to one-fourth of the actual beam span". The diagram in this section shows optional cantilever at both ends of beam, HOWEVER, it also shows three posts. I am building a freestanding above ground pool deck with only two posts per beam, and want to cantilever each end, so it would appear the 'combined' cantilever length can only be 1/4 the span length. IE, if my beam span is 8', I could cantilever 12" at each end, giving me a total cantilever of 24", and not be allowed lets say, to cantilever 16" at each end. Is this correct? In the drawing in the code, the middle post essentially creates a cantilever at only one end of the span..... hence my question here.

[@GlennMathewson]

Sorry for my delayed reply. Probably not helpful anymore, you can cantilever each end up to one fourth the span between posts.

[@ManuelMartinez-nh2kp]

Is this apply for California state?

[@GlennMathewson]

I’m not sure. California amends a lot of the codes but I doubt they changed this one. That’s a guess though.

[@kbsanders]

Cliffhanger