Fire Damage to Roof Framing

[zero1238]

I recently inspected some fire damage in the attic of a very old home. The roof was framed with 2"x6" rafters at 20" on center with a single 2"x6" ridge. On one side of the roof, there was apparent fire damage with some of the rafters and the ridge experiencing charring. I can see the rafters being easily sistered but I'm not quite sure what to do with the ridge. It would be extremely difficult to replace it but sistering it also doesn't seem ideal since the integrity is somewhat lost. Open to any suggestions on this one as I want to take constructibility into account.

 

[SWComposites]

looks like it was already partially repaired. ??

 

[zero1238]

Some of the rafters, yes, but they didn't do a good job. Nothing was done to the ridge.

 

[jhnblgr]

Appears to be a ridge board and not a beam. Which is in violation of design standards since its depth doesn’t exceed the rafters. Best option is to replace rafters by opening up soffit and sliding them up in and attach to new rafter ties. Remove ridge board would be difficult, install a new board below and sister on both sides of the rafters

 

[ANE91]

NDS includes fire-design provisions that allow you to use average strength values instead of reference design values, which are based off a 5% exclusion limit. Ironically, fire-damaged members run "stronger" on paper, but NDS forbids exceeding the original strength, obviously.

The knee-jerk reaction to charring is always replace or sister. Usually not a big deal; sawn lumber is cheap. Sometimes it's not so cheap, as in your situation. You would be surprised to find how often the depth of char is limited to less than the outer 1/4-inch or so of wood. I always take a knife and scrape down to bright wood.

It's a ridge board; it's there for its stiffness, not its strength. Unless you're willing to go through some onerous process to establish that the ridge board can still serve as a reaction surface for opposing rafters in the worst-case scenario (e.g., mid-way between rafters), then it needs to be replaced.

 

[rlmengineer]

I recommend replacing any member charred over 1/8” deep and any sistered rafters need to be full length. Ridge boards would not be necessary if the rafters are directly opposite each other, but one of your photos shows the new sistered rafters with nothing on the opposite side. Not knowing the shear strength of the fire damaged ridge board that alone would be something I would write up.

 

[zero1238]

I agree with many of these points. What I think I'm going to end up doing is have them sister all of the rathers on each side, add another ridge ply beneath the existing ridge, and then add collar ties between each rafter paid and snug it up under the new ridge. I can't see this going anywhere if we take this route. I'm also probably going to have them post down to walls beneath the ridge.

 

[gte447f]

Zero1238, I do not understand the reasoning for your proposal to install a new ridge ply underneath the existing ridge board. What purpose would this new member serve? What you have here appears to be a simple ridge "board", not a ridge "beam". A ridge board does not support any gravity load in the vertical direction and is not under any bending stress about its major axis, or its minor axis for that matter if the rafters are in pairs opposite one another). Installing a new ply below the existing ridge board would seem to serve no purpose. Help me understand.

Considering this appears to be a simple ridge "board", I do not see how you can effectively repair or reinforce it. If you decide that the damage has compromised its intended function, then you should call for its replacement. It might be a pain, but fixing jacked up stuff is not supposed to be easy. Shore the rafters in place (the collar ties might actually accomplish this temporary function) and hack out the old ridge by whatever means necessary and replace it. If other stuff gets damaged in the process then it has to be repaired/replaced also.

Also, I'm curious how old is this fire damage and how long ago were the previous repairs performed?