How JustAnswer Works:
  • Ask an Expert
    Experts are full of valuable knowledge and are ready to help with any question. Credentials confirmed by a Fortune 500 verification firm.
  • Get a Professional Answer
    Via email, text message, or notification as you wait on our site. Ask follow up questions if you need to.
  • 100% Satisfaction Guarantee
    Rate the answer you receive.
Ask walkereng Your Own Question
walkereng, Consultant
Category: Structural Engineering
Satisfied Customers: 2833
Experience:  Over 30 years of Structural Engineering experience.
Type Your Structural Engineering Question Here...
walkereng is online now
A new question is answered every 9 seconds

I am looking size and spacing flat roof 20' with a slight

Customer Question

i am looking for rafter size and spacing for a flat roof 20' with a slight slope of 1 to 2'.
above the rafters will be 2 by 12 t&g, vapor barrier, 2-2"polyiso, 3/4" treated plywood and roll torchdown roll roofing. flat roof will be supported by an 8"cmu walls. i would like to make it strong enough so that i can walk on legally as i will likely have an ac unit there and an occasional guest. please let me know the difference in spacing and rafter size
Submitted: 1 year ago.
Category: Structural Engineering
Expert:  walkereng replied 1 year ago.

I can help

Expert:  walkereng replied 1 year ago.

Do you have a rough sketch or plan sheet you can share with me? I want to make sure I have your configuration correct.

The Chat Window has a button on it that allows you to attach files.

Customer: replied 1 year ago.
Expert:  walkereng replied 1 year ago.

I did not give you an answer yet? This is a Chat process where we Chat back and forth until I get the information I need to work out your answer, then once I do, you ask any questions you might have and then when you are happy with your answer, you accept it.

If you do not have any sketches, I will need to ask a few questions:

1. What city and state do you live in for possible snow load.

2. Are you looking for conventional lumber or engineered lumber rafters?

3. Is this for a residential occupancy or commercial occupancy?

Expert:  walkereng replied 1 year ago.

Let me know when you come back online

Customer: replied 1 year ago.
no snow 77630. #2 grade pine residential
Customer: replied 1 year ago.
above the 2by 12 would be 2 by 6 tongue and groove #2 pine. my plan was to leave this exposed from the bottom. the 2 2"polyiso after the water barrier should give me atleast 25 r value.
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.

You can use 2"x12" Southern Pine (#2 grade or better) spaced at 12" o.c. for your 20' span roof rafters.

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

Expert:  walkereng replied 1 year ago.

Let me know if you have any more questions

Customer: replied 1 year ago.
What is the number for a refund. I am not satisfied.
Expert:  walkereng replied 1 year ago.

You have not been charged for this question, but you may have been charged a deposit.

I will send a request to customer service for a refund

Expert:  walkereng replied 1 year ago.

Just sent the rquest