FHB Logo Facebook LinkedIn Email Pinterest Twitter X Instagram Tiktok YouTube Plus Icon Close Icon Navigation Search Icon Navigation Search Icon Arrow Down Icon Video Guide Icon Article Guide Icon Modal Close Icon Guide Search Icon Skip to content
Subscribe
Log In
  • How-To
  • Design
  • Tools & Materials
  • Restoration
  • Videos
  • Blogs
  • Forum
  • Magazine
  • Members
  • FHB House
  • Podcast
Log In

Discussion Forum

Discussion Forum

Beam Strength

JaggedFinish | Posted in Construction Techniques on February 12, 2014 09:14am

Hello All,

Can anyone show me the correct way to figure out the actual strength differences for a rosboro treated X-beam glulam 3.5″x9.5″x20′ and a 3 ply #2 treated 2″x12″ beam? I have attached some information on the glulam beam. I am curious to see how much the strength differs. I currently have 4 – 3ply 2×12 beams for a full cantilevered deck.

Thanks

Reply
  • X
  • facebook
  • linkedin
  • pinterest
  • email
  • add to favorites Log in or Sign up to save your favorite articles

Replies

  1. User avater
    Mike_Mahan | Feb 12, 2014 11:53pm | #1

    Need to know the span, the

    Need to know the span and the amount of cantilever. What do you want to know about the strength: uniform loads, point loads, how maximum loads compare?

    1. JaggedFinish | Feb 13, 2014 11:35am | #5

      Reply

      mike mahan wrote:

      Need to know the span and the amount of cantilever. What do you want to know about the strength: uniform loads, point loads, how maximum loads compare?

      How much of a difference a glulam woulkd make in my sutuation. Catlievered 8ft right now, with a 20ft beam overall. I want to replace the bent one for good measure. These are about 30 years old.

      1. User avater
        Mike_Mahan | Feb 13, 2014 10:09am | #2

        Spacing

        What is the layout of the beams i.e. the tributary area.

        I think that the 3 2x12s are going to prove stronger due to the extra depth.

        1. JaggedFinish | Feb 13, 2014 11:34am | #4

          mike mahan wrote:

          What is the layout of the beams i.e. the tributary area.

          I think that the 3 2x12s are going to prove stronger due to the extra depth.

  2. jimblodgett | Feb 13, 2014 10:25am | #3

    Do all the strength comparisons you want, heck check steel box beams, I say put a post under there. 

    "8' cantaliever"?  On an exterior beam?  Are you freakin' nuts?  This is a disaster waiting to happen.  I see details like this and shake my head.  Did somebody think they were going to outsmart gravity with the cantaliever and mother nature with that little cap over the endgrain of that beam(s)?

    "Don't want to add posts"?  "Want to keep unobstructed access?/Views?" No matter.  Do it. 

    1. cussnu2 | Feb 13, 2014 03:38pm | #6

      an 8ft cantaliever is one hell of a teeter toter!  You have to start the question not only the beam but the massive load being put on the fulcrum point at the foundation and the massive upward force being put on the opposite end of the cantaliever.  You're putting a very fat kid way out from the fulcrum point and its going to take a whole lot of weight on the other end (all exceptionally tied together) to keep that fat kid in the air.  Normal construction at the interior end isn't going to be enough.  You would have to have a way to spread the upward force over an extensive area or find some way to tie the interior end to the ground...not unlike a huge bridge abutment with all its weight counter balancing the bridge hanging out over the water.

  3. User avater
    Mike_Mahan | Feb 13, 2014 09:37pm | #7

    Assuming the beams are 8' oc., the glue lam is undersized by 600% and the 3 2x12s are undersized by 300%. You'd need a glue lam 20 inches deep or 4 2x16s.

    1. JaggedFinish | Feb 14, 2014 10:50am | #9

      Math

      Mike,

      Thanks for your input, could you show your math for figuring this out? Also, what psf load are you using? Could flitch plates be an option?

      -JF

      1. User avater
        Mike_Mahan | Feb 14, 2014 10:47am | #8

        Beams

        Beam design works by trial and error. You don't plug in requirements and get a beam size. You plug in beam sizes and get a yes or no.

        Beam 1 = the gluelam you specified

        Beam 2 = the solid 2Xs you specified

        Beam 3 = the minimum gluelam that works

        Beam 4 = the minimum build up 2x beam that works

        See the first image. span.jpg

        The span supporting the cantilever is 12' with a tributary area of 8' (as if they are 8' oc). Uniform load of 50psf. (These calcs come out the same for a restrained beam with no load on this part of the span.)

        The cantilever is 8' with a tributary area of 8'. Uniform load 50psf.

        See the image calc.jpg to see the pass/fail specs on the various beams.

        Sorry, but I can't share the software.

        I don't think you've got any good option other than posts. Doubling the number of beams doesn't help that much. Normally a deck cantilevers on joists that are 16" or 24" oc.

        1. JaggedFinish | Feb 14, 2014 10:54am | #10

          Mike, Wow, that is some

          Double Post

        2. JaggedFinish | Feb 14, 2014 11:48am | #11

          Mike,

          Wow, that is some great software you have there, can you share the name? Do you have the ability to do I-beams in the software? If i am stuck with the opening from the house at 11.25"x4.5" that limits the beam sizes. What if we moved to a steel beam such as a W10x49?

          Thanks -JF

          1. User avater
            Mike_Mahan | Feb 14, 2014 01:06pm | #12

            I've never done much with steel, but I ran the calcs. In order to keep under 11.5 depth you'll need three 1/2 flitch plates or a C10 30. The software doesn't do other shapes easily. You've got a lot of issues with restraint and bearing. Time to talk to a real engineer. I only use software for basic design then I take it to an engineer and tell him what I think will work based on the preliminary calcs.

            Good luck.

  4. JaggedFinish | Feb 15, 2014 05:06pm | #13

    Mike, Thanks for your

    Mike,

    Im going to run a quick beam deflection calulation, using cantilever uniform load, assuming each beam takes 1/4 of the entire load, is this correct? W10x19 Beam

    Load*Area

    60PSF * 230ft = 13800lb/ft

    13800lb/4 = 3450lb/ft per beam

    3450 lb/ft = 288 lb/in

    Max Deflection = (288*96) / (8*29000000*96.3)

    Max Deflection = 1.093 inch - Fail not lower then 1/360 of 96

    -JF

    1. User avater
      Mike_Mahan | Feb 16, 2014 03:59pm | #14

      Where'd the picture go?

    2. DanH | Feb 16, 2014 04:19pm | #15

      I always thought beam strength was inversely proportional to the square of the distance from the source.

  5. JaggedFinish | Feb 16, 2014 05:00pm | #16

    Mike,

    Drew this up in CAD for a better representation,

    Outside beams cantilever 8 ft, while inner beams canilever 5 ft.

    I am now trying to figure out the load on each beam, using the area reference in the attached picture. Is this correct?

    If so, then the see attached excel picture.

    -JF

  6. User avater
    Mike_Mahan | Feb 16, 2014 08:06pm | #17

    here ya go

    see attached.

  7. JaggedFinish | Feb 16, 2014 09:56pm | #18

    Do you know the math behind generating the %'s for Area, Ix, and Sx? Id like to build it into the excel spreadsheet.

    Would I be able to purchase the software you have somewhere?

    -JF

    1. User avater
      Mike_Mahan | Feb 17, 2014 05:15am | #19

      http://www.archforms.com/

  8. JaggedFinish | Feb 17, 2014 10:33pm | #20

    Mike, Thanks for the link,

    Mike,

    Does the beam actually see any load on the inside of the house? I ask because there is cinderblocks under the beam for that whole legth, im under the impression the beam is not under any load, as any force put on the beam will be applied in the opposite direction by the blocks, so it would be equalibrium. Therefor it would not be loaded like a simple supported beam, and the only load would be on the cantilever.

    Thanks!

    -JF

  9. DanH | Feb 18, 2014 06:20pm | #21

    Of course, there are cantilevered bridges with about a million tons of concrete for a counterweight.  They don't need to observe the "one to three" rule.

    1. cussnu2 | Feb 19, 2014 02:33pm | #22

      and I noted that in a previous post.  The bottom line is its not just about the cant being supported its also about the magnified upward force that is placed on the interior portion of the beam past the fulcrum point.  Having a cinder block wall underneath the beam does nothing unless the beam is correctly tied to the wall and the wall correctly tied to the foundation....and even then the weight of that wall and foundation may not be enough to counteract the multiplied upward force placed on the beam.

      Most times you can overbuild something cheaply enough that you don't need to worry about it.   But this case is definitely a trip to engineer land.

  10. JaggedFinish | Feb 20, 2014 05:39pm | #23

    Pictures

    Here are the pictures for how they are ancored in the house. I plan on reducing the cantilever down to 6ft. I have not seen any decking cenilevered like this before. The beams all have 12ft of block under them, the outside beams hold up the second level for the first 12ft of the house, the inner beams have 4-5 ft of house over them, seen in the pictures.

Log in or create an account to post a comment.

Sign up Log in

Become a member and get full access to FineHomebuilding.com

Video Shorts

Categories

  • Business
  • Code Questions
  • Construction Techniques
  • Energy, Heating & Insulation
  • General Discussion
  • Help/Work Wanted
  • Photo Gallery
  • Reader Classified
  • Tools for Home Building

Discussion Forum

Recent Posts and Replies

  • |
  • |
  • |
  • |
  • |
  • |
View More Create Post

Up Next

Video Shorts

Featured Story

Fire-Resistant Landscaping and Home Design Details

These defensive details give homes a better chances of surviving wildfires.

Featured Video

Builder’s Advocate: An Interview With Viewrail

Learn more about affordable, modern floating stairs, from design to manufacturing to installation.

Related Stories

  • Old House Air-Sealing Basics
  • A Drip-Free, Through-Window Heat Pump
  • Insulation for Homes in the Wildland Urban Interface
  • An Impressive Air-to-Water Heat Pump

Highlights

Fine Homebuilding All Access
Fine Homebuilding Podcast
Tool Tech
Plus, get an extra 20% off with code GIFT20

"I have learned so much thanks to the searchable articles on the FHB website. I can confidently say that I expect to be a life-long subscriber." - M.K.

Get home building tips, offers, and expert advice in your inbox

Signing you up...

This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
See all newsletters
See all newsletters

Fine Homebuilding Magazine

  • Issue 332 - July 2025
    • Custom Built-ins With Job-Site Tools
    • Fight House Fires Through Design
    • Making the Move to Multifamily
  • Issue 331 - June 2025
    • A More Resilient Roof
    • Tool Test: You Need a Drywall Sander
    • Ducted vs. Ductless Heat Pumps
  • Issue 330 - April/May 2025
    • Deck Details for Durability
    • FAQs on HPWHs
    • 10 Tips for a Long-Lasting Paint Job
  • Old House Journal – August 2025
    • Designing the Perfect Garden Gate
    • Old House Air-Sealing Basics
  • Issue 329 - Feb/Mar 2025
    • Smart Foundation for a Small Addition
    • A Kominka Comes West
    • Making Small Kitchens Work

Fine Home Building

Newsletter Sign-up

  • Fine Homebuilding

    Home building tips, offers, and expert advice in your inbox.

  • Green Building Advisor

    Building science and energy efficiency advice, plus special offers, in your inbox.

  • Old House Journal

    Repair, renovation, and restoration tips, plus special offers, in your inbox.

Signing you up...

This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
See all newsletters

Follow

  • Fine Homebuilding

    Dig into cutting-edge approaches and decades of proven solutions with total access to our experts and tradespeople.

    Start Free Trial Now
    • Facebook
    • Instagram
    • X
    • LinkedIn
  • GBA Prime

    Get instant access to the latest developments in green building, research, and reports from the field.

    Start Free Trial Now
    • Facebook
    • YouTube
  • Old House Journal

    Learn how to restore, repair, update, and decorate your home.

    Subscribe Now
    • Facebook
    • Instagram
    • X
  • Fine Homebuilding

    Dig into cutting-edge approaches and decades of proven solutions with total access to our experts and tradespeople.

    Start Free Trial Now
    • Facebook
    • Instagram
    • X
    • LinkedIn
  • GBA Prime

    Get instant access to the latest developments in green building, research, and reports from the field.

    Start Free Trial Now
    • Facebook
    • YouTube
  • Old House Journal

    Learn how to restore, repair, update, and decorate your home.

    Subscribe Now
    • Facebook
    • Instagram
    • X

Membership & Magazine

  • Online Archive
  • Start Free Trial
  • Magazine Subscription
  • Magazine Renewal
  • Gift a Subscription
  • Customer Support
  • Privacy Preferences
  • About
  • Contact
  • Advertise
  • Careers
  • Terms of Use
  • Site Map
  • Do not sell or share my information
  • Privacy Policy
  • Accessibility
  • California Privacy Rights

© 2025 Active Interest Media. All rights reserved.

Fine Homebuilding receives a commission for items purchased through links on this site, including Amazon Associates and other affiliate advertising programs.

  • Home Group
  • Antique Trader
  • Arts & Crafts Homes
  • Bank Note Reporter
  • Cabin Life
  • Cuisine at Home
  • Fine Gardening
  • Fine Woodworking
  • Green Building Advisor
  • Garden Gate
  • Horticulture
  • Keep Craft Alive
  • Log Home Living
  • Military Trader/Vehicles
  • Numismatic News
  • Numismaster
  • Old Cars Weekly
  • Old House Journal
  • Period Homes
  • Popular Woodworking
  • Script
  • ShopNotes
  • Sports Collectors Digest
  • Threads
  • Timber Home Living
  • Traditional Building
  • Woodsmith
  • World Coin News
  • Writer's Digest
Active Interest Media logo
X
X
This is a dialog window which overlays the main content of the page. The modal window is a 'site map' of the most critical areas of the site. Pressing the Escape (ESC) button will close the modal and bring you back to where you were on the page.

Main Menu

  • How-To
  • Design
  • Tools & Materials
  • Video
  • Blogs
  • Forum
  • Project Guides
  • Reader Projects
  • Magazine
  • Members
  • FHB House

Podcasts

  • FHB Podcast
  • ProTalk

Webinars

  • Upcoming and On-Demand

Podcasts

  • FHB Podcast
  • ProTalk

Webinars

  • Upcoming and On-Demand

Popular Topics

  • Kitchens
  • Business
  • Bedrooms
  • Roofs
  • Architecture and Design
  • Green Building
  • Decks
  • Framing
  • Safety
  • Remodeling
  • Bathrooms
  • Windows
  • Tilework
  • Ceilings
  • HVAC

Magazine

  • Current Issue
  • Past Issues
  • Magazine Index
  • Subscribe
  • Online Archive
  • Author Guidelines

All Access

  • Member Home
  • Start Free Trial
  • Gift Membership

Online Learning

  • Courses
  • Project Guides
  • Reader Projects
  • Podcast

More

  • FHB Ambassadors
  • FHB House
  • Customer Support

Account

  • Log In
  • Join

Newsletter

Get home building tips, offers, and expert advice in your inbox

Signing you up...

This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
See all newsletters
See all newsletters

Follow

  • X
  • YouTube
  • instagram
  • facebook
  • pinterest
  • Tiktok

Join All Access

Become a member and get instant access to thousands of videos, how-tos, tool reviews, and design features.

Start Your Free Trial

Subscribe

FHB Magazine

Start your subscription today and save up to 70%

Subscribe

Enjoy unlimited access to Fine Homebuilding. Join Now

Already a member? Log in

We hope you’ve enjoyed your free articles. To keep reading, become a member today.

Get complete site access to expert advice, how-to videos, Code Check, and more, plus the print magazine.

Start your FREE trial

Already a member? Log in

Privacy Policy Update

We use cookies, pixels, script and other tracking technologies to analyze and improve our service, to improve and personalize content, and for advertising to you. We also share information about your use of our site with third-party social media, advertising and analytics partners. You can view our Privacy Policy here and our Terms of Use here.

Cookies

Analytics

These cookies help us track site metrics to improve our sites and provide a better user experience.

Advertising/Social Media

These cookies are used to serve advertisements aligned with your interests.

Essential

These cookies are required to provide basic functions like page navigation and access to secure areas of the website.

Delete My Data

Delete all cookies and associated data