Structural Engineering

Ask a Structural Engineering Question, Get an Answer ASAP!

i have a 20 foot roof joists on a flat roof they are 4.5

Customer Question

i have a 20 foot roof joists on a flat roof
they are 4.5 " x 11 1/4" (wx h0
and 5 foot on center
they span connecting on each end to metal I beams
what is the maxcimum O.C spacing i can use for doubled tongue and groove 3/4 inch plywood if it is to be used for the decking and the roof joist is required to carry 80 plf

Already Tried:
OTHER STRUCTURAL ENGINEERS

Submitted: 298 days and 23 hours ago.

Category: Structural Engineering

Value: $34

Status: CLOSED

Expert: StructuralEng replied 298 days and 23 hours ago.

StructuralEng : Hi

StructuralEng : I can help

StructuralEng : Let me read closer

StructuralEng : Before we get started I’d like to point out that a Professional Engineer’s standard of care typically includes a site visit to assess field conditions and get an overall understanding of the structure. This can obviously not be accomplished through the internet. The information provided here is meant for informational purposes only (general sizing and budgeting) and is based on the information provided by you. The information should be verified by a professional engineer who can visit the site to ensure that potentially important information has not been overlooked or omitted

StructuralEng : I'm driving at the moment. Can I get back to you in a couple hours?

Customer :

YES

ALL UNDERSTOOD

Customer :

StructuralEng : Thanks

StructuralEng : I assume you mean 80 psf, not plf?

StructuralEng : Are the joists clear spanning 20'? What is the 5' o.c. that you reference in the question?

StructuralEng :

If you can answer the above questions, I'll get you an answer pretty quickly. Also, if you happen to know the species and grade, that would be helpful.

Expert: StructuralEng replied 298 days and 4 hours ago.

Our chat has ended, but you can still continue to ask me questions here until you are satisfied with your answer. Come back to this page to view our conversation and any other new information.

What happens now?

If you haven’t already done so, please rate your answer above. Or, you can reply to me using the box below.

Customer replied 296 days and 18 hours ago.

THE JOISTS ARE CLEAR SPANNING 20 FEET

YES IT IS POUNDS PER SQUARE FEET

THE RAFTERS ARE OVER 80 YEARS OLD AND LOOK LIKE SOME SPECIES OF FIR

THE FIVE FOOT ON CENTER IS THE SPACING OF THE EXISTING ROOF JOISTS

WHICH ARE 4" X 11AND 1/4" BY 20 FEET LONG

THE CURRENT DECK USES TONGUE AND GROVE THAT IS 1&/2" BY 6"

CONSIDERABLE DAMAGE HAS OCCURRED AT THE ROOF PERIMEETER

ROTTING SEVERAL TIMBERS WHICH HAS RESULTED IN CITY VIOLATIONS

Accepted Answer

Expert: StructuralEng replied 296 days and 17 hours ago.

Assuming that you have APA Rated Structural I sheathing, 24" is the max for the plywood's sake, and that it kind of pushing it 19.2" is more reasonable.

If you've found my answer helpful, please click the green "accept" button. I'll be happy to answer additional questions on the topic. If you could provide feedback, I would appreciate it.