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# Architecture/Raising and strenghting floor beams

Question
I have an unfinished attic floor that I would like to turn into a second floor room. It is framed out in 2x8's with a maximum span of 15'.  The room next to it is already finished and the floor sits 4" higher, so I would like to floors to match up without a step.

The question is, can I lay 2x4 lumber perpendicularly across the 2x8 beams to have increase the rigidity of the floor? I believe my only other option is to build everything up be sistering in 2x12 beams to add strength and height, but the price is much higher than the 2x4 option.

Thanks for your expertise - any other ideas are certainly appreciated!

Doug,

I need to make several assumptions about the existing conditions.  So far, I know that your ceiling joists (soon to be floor joists) are 2 x 8 and 15' long.  Let's assume that they are spaced at 16" on center.  Let's assume that a drywall ceiling will be applied directly to the bottom of these ceiling joists - which limits your allowable deflection before the ceiling cracks.

The short answer is that you will need nothing less than a 2 x 12 joist.

Theoretically, you could apply glue to the top edge of the existing 2 x 8 and then firmly attach a 2 x 4 to the top of this for the entire 15' length.  That would give you a solid depth of 7.25 + 3.5 = 10.75 inches acting together as one "beam" spaced every 16 inches on center.  But that would yield a calculated section modulus of 28.89 inches^3 and a moment of inertia of 155 inches^4.  What is needed according to the building-code-prescribed design loads is a section modulus of 29.71 inches^3 and a moment of inertia of 134.55 inches^4.  Thus if the new 2 x 4 were to act in perfect union with the existing 2 x 8, the results would be slightly less than adequate.  Unfortunately, a series of 2 x 4 joists running perpendicular with a series of 2 x 8 joists would not act together in perfect union but more like a blanket glued to the top surface of a trampoline = wasted material with no real strength.

The required moment of inertia to resist deflection in this case is 134.55 inches^4
A 2 x 8 acting alone has a moment of inertia of 48 inches^4
A 2 x 4 acting alone has a moment of inertia of 3 inches^4
Stacking these structural layers produces 48 + 3 = 51 inches^4 which is less than the required 134.55 inches^4.

You have a common question.  Unfortunately, the sum of the parts does not equal the entire depth of the joist and you need a joist of sufficient depth.

Let me know if you have a sketch of the problem or any other questions.

Thank you,

Richard Burton, AIA
Registered Architect
ICC Certified Plan Reviewer
NFPA Certified Fire Plan Examiner

Disclaimer:
I am not a licensed structural engineer and nothing above implies that I am trying to take work away from those structural engineers who would otherwise take offense that I would do their work for free.  Instead, I recommend that you hire one or several structural engineers to carefully examine your existing conditions and recommend that you tear everything down and build with reinforced concrete... designed by a structural engineer.
Questioner's Rating
 Rating(1-10) Knowledgeability = 10 Clarity of Response = 10 Politeness = 10 Comment I'm only pissed that the bastard didn't tell me what I wanted to hear... but did tell me what I expected. Full marks all around for being a great, no nonsense guy!

Architecture

Volunteer

#### Richard Burton AIA

##### Experience

Here in the midwest, we design and build "green" in ways that make sense. My construction methods prioritize weather resistance, ease of maintenance and durability. While a graduate student in San Diego, I taught drafting and history of architecture. After working ten years for other architecture firms, I have started my firm Arrow Architecture in 2008. More than half of my work involves commercial office buildings. But my portfolio of work also includes custom homes, residential additions, home remodels, and second story additions.

Organizations
President-Elect of American Institute of Architects - Lincoln, NE

Education/Credentials
B.S. Architecture - (Interior Design) University of Nebraska Lincoln 1997 Master of Architecture - (Urban Design and Professional Practice) NewSchool of Architecture - San Diego 2004 Graduated Summa Cum Laude ICC Certified Building Plans Examiner

Awards and Honors
AIA Henry Adams Medal and Certificate