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walkereng
walkereng, Consultant
Category: Structural Engineering
Satisfied Customers: 2593
Experience:  Over 30 years of Structural Engineering experience.
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Basically I'm looking opinion. I installed two

Customer Question

Hi,
Basically I’m looking for an opinion. I installed two skylights in my roof which required cutting one rafter which I properly headered off on the top and bottom where I cut the rafter. the whole for the skylight is less than 3 feet span. The roof rafters run 16 inches on center (rafters and joists are 2 x6 as is the ridge beam). My question is to accommodate the skylights I need to cut some floor joists which I’m not sure if they are structural or not. The interesting issue is the roof rafters and floor joists run in a different direction meaning they run perpendicular and I’m not quite sure with this construction. . From what I can tell above the joist attached the length of the rafters are collar ties and rafter ties (2 x 4 pine wood). I believe these keep the roof from spreading due to tension. I would not alter these. Im thinking if I can remove the joists maybe I can strengthen the other floor joists by sistering the joists to pick up the structural integrity that I will lose by removing other joists. I believe that the joists are just just for holding Sheetrock however im not sure. If the rafters and joists ran in one direction i would not have this issue. Just a FYI the house was built in 1970 and there are currently no structural issues but I’m concerned if I cut the joists to accommodate the skylights I might have issues.
Submitted: 1 year ago.
Category: Structural Engineering
Expert:  walkereng replied 1 year ago.
I can help
Do you have any pictures you can share with me?
I understand the issues with the roof rafters, but you use the term floor joists also, do you mean ceiling joists?
You can attach pictures to the Chat Window with the paperclip in the toolbar.
Customer: replied 1 year ago.
can i email them to you i have no waive to attach the pics. it seems like the attach buttons are missing
Expert:  walkereng replied 1 year ago.
Yes you can, try sending to***@******.***
Customer: replied 1 year ago.
im sending you three emails with pics let me know what you think. My intentions are to remove 3 of the ceiling joists. By removing i will cut a box arouund the skylight and sister the joists
Expert:  walkereng replied 1 year ago.
Thanks for all of the great pictures! That gives me a real good picture of your configuration.
I have one question, can you tell me the span lengths of your ceiling joists (distance from each supported end)?
You are correct with the 2x4 collar ties, they make the roof system kind of act like a simple truss and they are in tension. They help resist the outward force of the roof rafters trying to flatten out.
Expert:  walkereng replied 1 year ago.
Let me know when you have your ceiling joist span information, then I can let you know if you need a double or triple ceiling joist on each side of your sky light.
Expert:  walkereng replied 1 year ago.
Let me know when you come back online
Customer: replied 1 year ago.
Ridge beam= (One)= 2 x6 lumber 143 inches in length from foundation wall to middle
Ridge beam = (two) =2 x 6 lumber 172 inches in length from foundation wall to middle
Ridge beam support at both sides = 34 inches from under ridge beam to foundation. (ridge beam support= 2 x6 lumber)
Ceiling joists = 1 through 14 joists (2 x6 lumber) 16 inches on center 143 inch length from foundation wall to middle section.
Ceiling joists = 1 through 12 ( 2 x6) 16 inches on center 172 inch length from foundation to wall middle section.
Rafters= 2x6 16 inches on center from ridge to foundation wall = 142 inches . There are 8 rafters on one side and the other side of the middle section is 9 rafters.
Color ties measure 101 inches and are 1.5 inches from ridge beam. The collar ties are installed on every other rafter.
There is also a two by four that spans 142 inches and is connected to two rafters only.
When I say middle section it seems like the joists are attached with joist hangers to a middle piece of wood that looks like a truss but has plywood around it. The pics I sent you yesterday show the ceiling joists and how they connect to this middle section. The thickness of the middle section is a piece of two by 6 wood and then plywood on the back that is over 5/8 in thickness.
Customer: replied 1 year ago.
Ridge beam= (One)= 2 x6 lumber 143 inches in length from foundation wall to middle
Ridge beam = (two) =2 x 6 lumber 172 inches in length from foundation wall to middle
Ridge beam support at both sides = 34 inches from under ridge beam to foundation. (ridge beam support= 2 x6 lumber)
Ceiling joists = 1 through 14 joists (2 x6 lumber) 16 inches on center 143 inch length from foundation wall to middle section.
Ceiling joists = 1 through 12 ( 2 x6) 16 inches on center 172 inch length from foundation to wall middle section.
Rafters= 2x6 16 inches on center from ridge to foundation wall = 142 inches . There are 8 rafters on one side and the other side of the middle section is 9 rafters.
Collar ties measure 101 inches and are 1.5 inches from ridge beam. The collar ties are installed on every other rafter.
There is also a two by four that spans 142 inches and is connected to two rafters only.
When I say middle section it seems like the joists are attached with joist hangers to a middle piece of wood that looks like a truss but has plywood around it. The pics I sent you yesterday show the ceiling joists and how they connect to this middle section. The thickness of the middle section is a piece of two by 6 wood and then plywood on the back that is over 5/8 in thickness.
Customer: replied 1 year ago.
collar ties are 1.5 from ridge beam sorry fixing the 1.5 inches i said before all the measurements are good. Oh yeah just a fyi there is a piece of wood that spans across the cieling joist that seems to keep them together from spreading i would not cut near this piece at all.
Expert:  walkereng replied 1 year ago.
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 planning purposes only (general sizing and budgeting) and is based on the information provided by you. All loading cases considered are for vertical loads only, no lateral analysis has been completed. 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.
If you are going to cut into 3 ceiling joists, you will need to sister 2 additional 2x6's to each of the two outside ceiling joists, thus making a triple 2x6 beam on each side. You should also use a double 2x6 header at the top and bottom of the cut ceiling joists.
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Expert:  walkereng replied 1 year ago.
Let me know if you have any more questions.
Expert:  walkereng replied 1 year ago.
If you feel you have received a satisfactory answer to your question, click the Rating button that is appropriate. Experts are credited for each adequately Rated answer they provide. If you have additional questions, please let me know. Thanks

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