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 (just a DIYer) 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 also plan to add 2 evenly spaced exposed rafter ties and collar ties every 2nd or 3rd rafter right up against ridge board.

Suggestions and comments are appreciated.

Keith

 

[nealtw]

Welcome to the site. S-P-F-S Spruce, pine, white fir, ?. don't know about second S.
Four season will require insulation and with that in mind I would go all the way to 2x10s the whole way which would allow venting. I would sister them right to the old rafters. The coller ties I think should be in the middle third of the span.

 

[keithmaryq]

Thanks Neal, your point on using 2x10s is well taken. I'll have to figure out how to sister on larger rafters and attach them to the new walls...all the while with the current walls in place to hold up the roof!

 

[nealtw]

Cut your rafters so that the level line of the birds mouth is 3.5" or 5.5" to match your outside wall,step the cut up so the tale of the rafter is 2x4 outside the wall. Support each rafter with a stud under it near the old wall and add a scab to hold it in place, remove the wall and sister the new rafters to old ones. Build the new wall under the new rafters. One side and then the other. When insulating you want to consider a ridge vent as each bay will be a closed cavity so regular roof top vents won't to the job.

 

[Wuzzat?]

Stacking equal height rafters on edge gives 8x the span resistance to bending but may not possible here and you'd need long bolts. Sistering just gives 2x.

 

[nealtw]

The 2x10s will be beside the 2x6s. 2x10s will do the work of rafter and the 2x6 are just going to hold the sheeting in place and will be nailed to the 2x10s

 

[keithmaryq]

Neal,

Thanks again for the help...I understand the process you laid out -- looking forward to getting to it...Keith

 

[BridgeMan]

Section Modulus of a 2 x 10 = 21.39 C.I.

Section Modulus of a 2 x 8 = 13.14 C.I.

Section Modulus of a 2 x 6 = 7.56 C.I.

Since the amount of any member's Section Modulus is a direct measure of its ability to resist applied loads, the above values show that sistering with 2 x 8s will just about double the strength of the support system that's in place with 2 x 6s now; using 2 x 10s would almost triple the available strength. Using 2 x 10s would be over-kill, in my opinion, and their size may present installation challenges under the sloped roof sheathing at the ends--removing too much "meat" with the heel cuts could compromise their shear resistance.

 

[nealtw]

Section Modulus of a 2 x 10 = 21.39 C.I.

Section Modulus of a 2 x 8 = 13.14 C.I.

Section Modulus of a 2 x 6 = 7.56 C.I.

Since the amount of any member's Section Modulus is a direct measure of its ability to resist applied loads, the above values show that sistering with 2 x 8s will just about double the strength of the support system that's in place with 2 x 6s now; using 2 x 10s would almost triple the available strength. Using 2 x 10s would be over-kill, in my opinion, and their size may present installation challenges under the sloped roof sheathing at the ends--removing too much "meat" with the heel cuts could compromise their shear resistance.

We build mostly with engineered trusses but on occasion we frame roof ceiling systems like this, the engineers alway call for 2x10. I think one of the reasons is that the birdsmouth can be cut to cover the width of a 2x6 wall with out loosing strength and still leave room for insulation and venting,
The birdsmouth is adjusted some time to help the tails line up with other roof section.
The real question here is where should the cross ties be situated.