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StructuralEng
StructuralEng, Consultant
Category: Structural Engineering
Satisfied Customers: 4831
Experience:  Structural Engineer
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how far can i span a rough cut douf fir 4X12? It will hold

Customer Question

how far can i span a rough cut douf fir 4X12? It will hold DF 4X6 rafters on 30" (7 total)centers to make a vaulted ceiling. design is 4X12X18' down the sides and a 4X12X25' (with 3 8X8 posts) along the front.
Submitted: 2 years ago.
Category: Structural Engineering
Expert:  StructuralEng replied 2 years ago.

StructuralEng : Ho
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 : Can you tell me the city and state, so I can look at snow loads?
StructuralEng : Also, what is te length of the rafters on each side of the ridge?
Customer:

its a basic patio cover 18X25 the outter corners are supported by 8X8 posts and inner corners sit on the wall plate 2x4's.

Customer:

tx....rafters are 14.5'

Customer:

rafters sit on the 4x12x18 witha 14' 6" span and form a vaulted ceiling with a 4/12 pitch

Customer:

sorry not 14'6" span thats length from beam to ridge.

StructuralEng : Is this a gable or hip roof?
Customer:

tie into hip roof but new structure is a gable

StructuralEng : So are te rafters spanning to yhe 18' beam or the 25' beam?
StructuralEng : What is the length of the ridge beam?
StructuralEng : Is the gable end going to be open or can you have a post to ground to support the ridge beam?
Customer:

ridge beam and rafter support beams are all 4x12x18 DF. There are 2 corner supports 8x8 holding the outter ends of the 18' and 25' beams. There is a center support ground to 25 beam 8x8 then a 4x4 fromt top 25' beam to ridge beam...open gable

Customer:

hello?

StructuralEng : Are you there?
StructuralEng : Can you see my text?
StructuralEng : Mac span for ridge beam is 9'.
StructuralEng : Max span is 11' for the eaves
StructuralEng : If you could rate my service, I would appreciate it.
StructuralEng : O would also recommend having a licensed PE take a look at how the ridge beam will tie into the existing house.
Customer:

i have cedar already designed and spanning further than that...how is the distancd shorter for DF than cedar? fir is stronger

StructuralEng : Was the cedar designed by a structural engineer?
StructuralEng : I don't believe that cedar checks out on paper for spans longer than what I've shown
Customer:

i hope so I hade an archetectural firm desing my home

Customer:

ok I am not real good on the span charts but I thought it was 15+ feet for 2x12

StructuralEng : 4x12 cedar beams with 12.5' of trib with spanning more than 12'? Just something else to think about - Doug-Fir is stronger under equal conditions, but they're not equal conditions
StructuralEng : Right, 15' is about right for a 2x12, but that's at 16" oc, so it had 1.333' feet of trib, not 12.5 feet of trib. That's a huge difference.
StructuralEng : Additionally, it's possible they used prescriptive requirements.
Customer:

trib?

StructuralEng : The width of floor/roof that is directly supported by the beam.
Customer:

whats the 12.5' trib? sorry like i said Im not the great on those charts or terminomlogy

Customer:

gotcha

Customer:

so the max span for the ridge is 9'? my design is nearly double....failure?

StructuralEng : Yes, I would not double the span. You can push past 9' a little if you don't mind some extra deflection, but it manifests itself in a lot I different ways, which is why I try to limit it
StructuralEng : The 12.5' of trib comes from a horizontal dimension of 12.5' on each side o the ridge (from ridge to eave). 6.25' on each side goes to the ridge
Customer:

how are these long spans being made in the new home designs using the popular rough cut lumber? I know I see them without engineered beams

StructuralEng : The dirty secret is that 99% of things built in and for single family residential construction is done with prescriptive code requirements and not actually engineered
StructuralEng, Consultant
Satisfied Customers: 4831
Experience: Structural Engineer
StructuralEng and other Structural Engineering Specialists are ready to help you

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