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Cantilevered deck

Rick18071

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2015 IRC Table R507.5 has a " f. Cantilevered spans not exceeding the nominal depth of the joist are permitted." Does that mean you can't cantilever a 2x8 joist more then 8"?

What does this mean? I thought that cantilevers can span 1/4 of the span.
 

Joe.B

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Arcata Ca
2015 IRC Table R507.5 has a " f. Cantilevered spans not exceeding the nominal depth of the joist are permitted." Does that mean you can't cantilever a 2x8 joist more then 8"?

What does this mean? I thought that cantilevers can span 1/4 of the span.
Maybe it means if you exceed that number then you need an engineered design? Just guessing based on the residential code being a "prescriptive code" designed to allow builders to avoid engineering fees?
 

steveray

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R507.5 Deck joists. Maximum allowable spans for wood deck
joists, as shown in Figure R507.5, shall be in accordance with
Table R507.5. Deck joists shall be permitted to cantilever not
greater than one-fourth of the actual, adjacent joist span.


Maybe Glenn will chime in...I am thinking they are not considered cantilevers < joist depth....
 

Sifu

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I think that footnote means that a cantilever can exist up to the depth of the joist with no additional provisions or considerations, like Steveray just said, essentially just being treated like the joist. If you wish to have a larger one then you can use the back-span ratio provisions, which are mostly aligned the table. Could certainly be more clear. I bet Glen's book covers it. Sorry Glen, haven't bought it YET.

This from the code change proposal reason statement. Not sure it does what was intended.
The way cantilever lengths were displayed in the table. The current table is difficult to understand, and this revision more
clearly explains the two limitations, namely cantilevers are limited to joist span divided by 4 or the lengths in the table,
whichever is the lessor.
 

patrickjames

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Georgia, United States
This is out of the 2018 IRC.
FIGURE R507.6

TYPICAL DECK JOIST SPANS

TABLE R507.6

DECK JOIST SPANS FOR COMMON LUMBER SPECIES (ft. - in.)

SPECIESaSIZEALLOWABLE JOIST SPANbMAXIMUM CANTILEVERc, f
SPACING OF DECK JOISTS
(inches)
SPACING
OF DECK JOISTS WITH CANTILEVERSc

(inches)
121624121624
Southern pine2 × 69-119-07-71-31-41-6
2 × 813-111-109-82-12-32-5
2 × 1016-214-011-53-43-62-10
2 × 1218-016-613-64-64-23-4
Douglas fir-larchd,
hem-fird

spruce-pine-fird,
2 × 69-68-87-21-21-31-5
2 × 812-611-19-11-112-12-3
2 × 1015-813-711-13-13-52-9
2 × 1218-015-912-104-63-113-3
Redwood,
western cedars,

ponderosa pinee,

red pinee
2 × 68-108-07-01-01-11-2
2 × 811-810-78-81-81-102-0
2 × 1014-1113-010-72-82-102-8
2 × 1217-515-112-43-103-93-1

For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 pound per square foot = 0.0479 kPa, 1 pound = 0.454 kg.

  1. No. 2 grade with wet service factor.
  2. Ground snow load, live load = 40 psf, dead load = 10 psf, L/Δ = 360.
  3. Ground snow load, live load = 40 psf, dead load = 10 psf, L/Δ = 360 at main span, L/Δ = 180 at cantilever with a 220-pound point load applied to end.
  4. Includes incising factor.
  5. Northern species with no incising factor.
  6. Cantilevered spans not exceeding the nominal depth of the joist are permitted.
 

Glenn

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The 2015 was the first table to address deck spans and cantilevers. It's not easy to do. We revised it in the 2018, but it still had confusing parts. We revised it again in the 2021 it is by far the best yet. I encourage everyone to use the 2021 IRC Section 507 Deck provisions as an alternative.

Now, to answer your question. The 2015 max joist span table is split in two. One side is for joists "with no cantilever" and the other for "joists with cantilevers". In order to make it work out every time that the cantilever could be 1/4 the backspan, the span has to be limited. In some conditions, when the span gets near the max, the cantilever maximum begins to shrink again. This is the problem that has caused us to revise the table each edition.

The footnote about the cantilever being the up to the depth of the joist is saying that with a cantilever that small you can still use the side of the span table for "joists without cantilever".

Hopefully that makes sense, but I hope even more that people use the 2021 version of this table.

Now, if you are curious about what is confusing in this table in the 2018 IRC and how we fixed it in the 2021, well I happen to have that exact video from my On-Demand Courses as the sample video at www.BuildingCodeCollege.com The cantilever discussion starts at about 14 minutes. I can dig up my old video about the 2015 change to the 2018 if I'm asked.

I'm happy to answer any follow up questions. I've also got 12 hours of On-Demand deck code courses from the 2021 IRC available online.

www.DeckCodes.com
 
Last edited:

Glenn

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I think that footnote means that a cantilever can exist up to the depth of the joist with no additional provisions or considerations, like Steveray just said, essentially just being treated like the joist. If you wish to have a larger one then you can use the back-span ratio provisions, which are mostly aligned the table. Could certainly be more clear. I bet Glen's book covers it. Sorry Glen, haven't bought it YET.

This from the code change proposal reason statement. Not sure it does what was intended.
The way cantilever lengths were displayed in the table. The current table is difficult to understand, and this revision more
clearly explains the two limitations, namely cantilevers are limited to joist span divided by 4 or the lengths in the table,
whichever is the lessor.
Well they are doing a second printing, so it will be ready when you are! My book only explains and provides the 2018 to the 2021 IRC. I never wrote one on the 2015. It's been long overdue.
 

Rick18071

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Nov 28, 2009
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Location
Poconos/eastern PA
Thanks Glenn. Looks like the "f" should be next to "Spacing of deck joists with no cantilever" rather than next to the table heading. When next to the table heading it makes me think that it applies to "spacing of deck joists with cantilevers" too which now I see it doesn't.
 

Sifu

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Joined
Sep 3, 2011
Messages
1,631
The 2015 was the first table to address deck spans and cantilevers. It's not easy to do. We revised it in the 2018, but it still had confusing parts. We revised it again in the 2021 it is by far the best yet. I encourage everyone to use the 2021 IRC Section 507 Deck provisions as an alternative.

Now, to answer your question. The 2015 max joist span table is split in two. One side is for joists "with no cantilever" and the other for "joists with cantilevers". In order to make it work out every time that the cantilever could be 1/4 the backspan, the span has to be limited. In some conditions, when the span gets near the max, the cantilever maximum begins to shrink again. This is the problem that has caused us to revise the table each edition.

The footnote about the cantilever being the up to the depth of the joist is saying that with a cantilever that small you can still use the side of the span table for "joists without cantilever".

Hopefully that makes sense, but I hope even more that people use the 2021 version of this table.

Now, if you are curious about what is confusing in this table in the 2018 IRC and how we fixed it in the 2021, well I happen to have that exact video from my On-Demand Courses as the sample video at www.BuildingCodeCollege.com The cantilever discussion starts at about 14 minutes. I can dig up my old video about the 2015 change to the 2018 if I'm asked.

I'm happy to answer any follow up questions. I've also got 12 hours of On-Demand deck code courses from the 2021 IRC available online.

www.DeckCodes.com
You da man Glenn.
 
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