Does Sistering Rafters Increase Maximum Span

[keithmaryq]

Project:
Expanding size of existing screen room to 15' x 23', and enclosing to make 4-season room.

Details:
Screen room is 12' 6"' x 13' 6" with 3.25:12 pitch gable roof that extends off rear of home so that 3 walls are exterior. Screen room is integral to large decked areas on both sides of screen room. Construction is good quality, and saving existing roof is desired.

Work To Date:
Installed 12 footings and piers to support 3 new 23' beams extending perpendicular from house, and added floor joists for extended length of 4-season room.

Planned Work:
Add decking and sub floor to expanded area, and then construct 3 new walls while leaving existing walls in place (to support existing roof).

Problem:
Although the current rafters (2x6, 16OC) are long enough to accommodate the extended walls, I failed to account for several issues:
1. The current rafter bird mouth notches will weaken the rafters once I remove the current walls.
2. Cutting new bird mouth notches in the current rafters (24) won't be fun given their location.
3. It seems the new wall span may be at or beyond the maximum span for the current rafters -- I can't tell the wood species, but they are stamped S-P-F S-Dry No1 on some and S-P-F S-Dry 2 on others.

Possible Solution:
Sister new 2x8 rafters onto the existing rafters to increase allowable span and overcome the weakening old bird mouth notches. This method also allows for new bird mouth notches to easily be cut on the saw horses -- but I haven't determined how to notch or where to cut off existing rafters.

Note: I'm also planning to add 2 evenly spaced and exposed rafter ties, as well as collar ties every 2nd or 3rd rafter as high as possible.

Suggestions and comments are appreciated.

Keith

[James Eggert]

Re: Does Sistering Rafters Increase Maximum Span

As informative as the description is, it at the same time is vague regarding layout.

So, provide a sketch, and no one can even suggest span corrections WHEN WE DON'T KNOW WHERE YOU ARE. That's why there is a profile, who you are , where you are, what you do, etc

I'm not a ^&^%mindreader

[James Eggert]

Re: Does Sistering Rafters Increase Maximum Span

Does sistering.....? The question asks, and the answer is "maybe"?

As a Building Official I'm not accepting a sistered joist situation unless it's provided by a design professional. Do I know when it';s correct, yes, but I see way too many screw-ups by the unknowledgeable who attempt structural changes based on how much they weigh.

[keithmaryq]

Re: Does Sistering Rafters Increase Maximum Span

Jim,

Points taken...I'm in North Carolina on the Inner Banks...profile location updated. I'd love to say snow load isn't an issue, but if you tune in to the current news you'll see we just got 6 inches of snow -- a real rare occurrence in these parts. I'll have to work on a sketch...

thanks again...

[Michael Kallis]

Re: Does Sistering Rafters Increase Maximum Span

S-P-F = spruce-pine-fir
#1 & #2 grade have the same allowed rafter span so that number becomes irrelevant in this particular case.

You're attempting to engineer a structural modification. I can guarantee you your building department will not accept your engineering nor will they accept anyone's opinion from this forum in place of engineering. If you want to modify the existing rafters you will have to engage the services of a licensed public engineer. If you decide to move forward with engineering, I would advise hiring one that operates in your county or city.

All that aside; I think the cost of engineering, additional labor required, and material costs to have this engineered is going to cost more than removing the existing roof and building a new one.