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baffles in attic which ones?


big dodo
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hello everyone im new to this site and i just wanted to start of by introduceing myself im hector from southern california. my dilema as of today starts at the attic. i am planning to install a roof vent along with intake openings at the eaves because my house has no openings at all.[:-bigeyes well my problem is this:half my house has a cathedral ceiling and the other has attic space. i was planning on putting a ridge vent all the way through both ceilings attic/cathdral. on the cathedral side can i get away with just opening the intake vent at the eaves? or do i need to take the drywall off and install baffles all along the rafters? the reason i say this is, because i had a section of drywall that was sagging a bit and when i removed it there was what looked like rot or termite damage what you guys think?? another question...im not sure but i think i have a bit of mold on the attic insulation is there a remedy for that?

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Hi,

Your rafters and roof deck are rotten. Now you know why proper ventilation is important. Get a competent contractor out there to strip the roof, replace the rotting components, treat what's left with a borate based fungicide, redeck it - this time with proper ventilation - and then reroof it. Do not try to shortcut the process or you'll be doing it again in a few years.

ONE TEAM - ONE FIGHT!!!

Mike

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Hi,

Your rafters and roof deck are rotten. Now you know why proper ventilation is important. Get a competent contractor out there to strip the roof, replace the rotting components, treat what's left with a borate based fungicide, redeck it - this time with proper ventilation - and then reroof it. Do not try to shortcut the process or you'll be doing it again in a few years.

ONE TEAM - ONE FIGHT!!!

Mike

That pretty much sums it up.

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P.S. Do not use a ridge vent in California. You will loose diphragm continuity of your roof. What that means is the house will be less capable of resisting earthquakes.

Hi,

This is a new one on me. How do they effectively ventilate cathedral ceilings done with conventional rafters and not scissor trusses in California if they don't use a ridge vent and eave vents? Do they stick a pot vent on either side of the ridge in every single rafter bay?

I've driven through California a few times and I've never noticed that having been done on any houses.

We have rules for seismic bracing here too and I've never seen where the use of ridge vents is said to weaken the roof diaphram. Does it really weaken it enough to be a concern?

References please.

Is this some kind of La-La Land specific rule - like the one that says fiberglass insulation has been proven to cause Cancer in California but nowhere else?

ONE TEAM - ONE FIGHT!!!

Mike

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P.S. Do not use a ridge vent in California. You will loose diphragm continuity of your roof. What that means is the house will be less capable of resisting earthquakes.

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201064122458_Untitled%20picture.png

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would something like this solve that problem? leaving plywood between the rafters say...4 inches on each side alternating on rafters...

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Hi,

Your rafters and roof deck are rotten. Now you know why proper ventilation is important. Get a competent contractor out there to strip the roof, replace the rotting components, treat what's left with a borate based fungicide, redeck it - this time with proper ventilation - and then reroof it. Do not try to shortcut the process or you'll be doing it again in a few years.

ONE TEAM - ONE FIGHT!!!

Mike

well that sucks!! [:-weepn]

so you are telling me i have to remove shingles, plywood and possibly ridge beam if rotted?...i wasnt planning on a complete re-roof at this time, rather i wanted to remove drywall off the cathedral ceilings and add ventilation and treat the rotted wood from underneath. and since there are 3 rooms with cathedral ceilings living room, dining room, bedroom, im just trying to contain it or keep it from spreading.(if possible) i dont have the funds to do everything at this point,just looking at my options.

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P.S. Do not use a ridge vent in California. You will loose diphragm continuity of your roof. What that means is the house will be less capable of resisting earthquakes.

Click to Enlarge
201064122458_Untitled%20picture.png

64.26 KB

would something like this solve that problem? leaving plywood between the rafters say...4 inches on each side alternating on rafters...

A structural engineer might accept htis after reviewing. It appears in my humble opinion to be an intereesting solution if it had additional blocking and nailing.

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P.S. Do not use a ridge vent in California. You will loose diphragm continuity of your roof. What that means is the house will be less capable of resisting earthquakes.

Hi,

This is a new one on me. How do they effectively ventilate cathedral ceilings done with conventional rafters and not scissor trusses in California if they don't use a ridge vent and eave vents? Do they stick a pot vent on either side of the ridge in every single rafter bay?

I've driven through California a few times and I've never noticed that having been done on any houses.

We have rules for seismic bracing here too and I've never seen where the use of ridge vents is said to weaken the roof diaphram. Does it really weaken it enough to be a concern?

References please.

Is this some kind of La-La Land specific rule - like the one that says fiberglass insulation has been proven to cause Cancer in California but nowhere else?

ONE TEAM - ONE FIGHT!!!

Mike

Nope it is national... CBC refers to ASCE/SEI 7-05 "Minimum design loads for buildings and other structures" 12.10 Diaphragms, Chords, and Collectors. 12.10.1 Diaphragm Design. Diaphragms shall be designed for both the shear and bending stresses resulting from design forces, at diaphragm discontinuities, such as openings and reentrant corners, the design shall ensure that the dissipation or transfer of edge (chord) forces combined with other forces in the diaphragm is within shear and tension capacity of the diaphragm.

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P.S. Do not use a ridge vent in California. You will loose diphragm continuity of your roof. What that means is the house will be less capable of resisting earthquakes.

Hi,

This is a new one on me. How do they effectively ventilate cathedral ceilings done with conventional rafters and not scissor trusses in California if they don't use a ridge vent and eave vents? Do they stick a pot vent on either side of the ridge in every single rafter bay?

I've driven through California a few times and I've never noticed that having been done on any houses.

We have rules for seismic bracing here too and I've never seen where the use of ridge vents is said to weaken the roof diaphram. Does it really weaken it enough to be a concern?

References please.

Is this some kind of La-La Land specific rule - like the one that says fiberglass insulation has been proven to cause Cancer in California but nowhere else?

ONE TEAM - ONE FIGHT!!!

Mike

Nope it is national... CBC refers to ASCE/SEI 7-05 "Minimum design loads for buildings and other structures" 12.10 Diaphragms, Chords, and Collectors. 12.10.1 Diaphragm Design. Diaphragms shall be designed for both the shear and bending stresses resulting from design forces, at diaphragm discontinuities, such as openings and reentrant corners, the design shall ensure that the dissipation or transfer of edge (chord) forces combined with other forces in the diaphragm is within shear and tension capacity of the diaphragm.

Thats fine but where in there does it say that a ridge vent can't be used and that a ridge vent weakens the diaphragm to the point where it can't do what it is supposed to do?

Where is the residential code reference?

ONE TEAM - ONE FIGHT!!!

Mike

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Hi,

Your rafters and roof deck are rotten. Now you know why proper ventilation is important. Get a competent contractor out there to strip the roof, replace the rotting components, treat what's left with a borate based fungicide, redeck it - this time with proper ventilation - and then reroof it. Do not try to shortcut the process or you'll be doing it again in a few years.

ONE TEAM - ONE FIGHT!!!

Mike

well that sucks!! [:-weepn]

so you are telling me i have to remove shingles, plywood and possibly ridge beam if rotted?...i wasnt planning on a complete re-roof at this time, rather i wanted to remove drywall off the cathedral ceilings and add ventilation and treat the rotted wood from underneath. and since there are 3 rooms with cathedral ceilings living room, dining room, bedroom, im just trying to contain it or keep it from spreading.(if possible) i dont have the funds to do everything at this point,just looking at my options.

Your roof is rotting and you have no way to know how far incipient rot spore has spread.

You can kill the existing spore and make the framing non-viable by drilling and inserting Impel rods in every single framing member throughout the entire roof and sister the severely rot-damaged ones with new lumber (which will also need to have Impel rods installed) and fix the ventilation, but your roof will not have the strength it was designed to have and you'll have to advise any buyers in a seller's disclosure form that the framing in the roof is a bunch of rotting sistered wood that's been treated with a fungicide. That's liable to scare a bunch of folks off.

Do it once; do it right the first time.

ONE TEAM - ONE FIGHT!!!

Mike

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Nope it is national... CBC refers to ASCE/SEI 7-05 "Minimum design loads for buildings and other structures" 12.10 Diaphragms, Chords, and Collectors. 12.10.1 Diaphragm Design. Diaphragms shall be designed for both the shear and bending stresses resulting from design forces, at diaphragm discontinuities, such as openings and reentrant corners, the design shall ensure that the dissipation or transfer of edge (chord) forces combined with other forces in the diaphragm is within shear and tension capacity of the diaphragm.Thats fine but where in there does it say that a ridge vent can't be used and that a ridge vent weakens the diaphragm to the point where it can't do what it is supposed to do?

Where is the residential code reference?

ONE TEAM - ONE FIGHT!!!

Mike

The site in question is in California and falls under the CBC which refers to the ASCE document. The SE's that I have spoken with on this issue all say that "it is a basic tennant described by 12.10.1"

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The CBC (2006 IBC) does not require roof diaphragms in conventional light frame construction. Diaphragm requirements are found in section 2305. Section 2301.2 gives the available choices of design methods for wood framing, and the sections which apply for conventional light-frame construction are 2304 and 2308, not 2305.

The roof in question is not constructed as a diaphragm. Notice the absence of any edge blocking in the second photo. The provisions of the CBC (2006 IBC) for conventional wood framing (2308) allow for lumber or plank sheathing, as well as structural sheathing. Table 2304.7(3) specifically allows wood panel roof sheathing without edge support, such as at a ridge vent or the other joints which are perpendicular to the rafters.

ASCE/SEI 7-05 states, in section 12.11.2.2.3, that the sheathing is not part of the design of any required ties or struts, not that it matters since it doesn't apply here.

Douglas Hansen

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The CBC (2006 IBC) does not require roof diaphragms in conventional light frame construction. Diaphragm requirements are found in section 2305. Section 2301.2 gives the available choices of design methods for wood framing, and the sections which apply for conventional light-frame construction are 2304 and 2308, not 2305.

I am quoting from the 2007 CBC which is a modified version of the 2006 IBC. Per table 2304.7(3) f. is modified by the 1/1/2009 suppliment 2305.1.5 2.4 "Wood structural panel sheathing in diaphragms shall have unsupported edges blocked." ... with that being said I believe my original comments are still correct.
The roof in question is not constructed as a diaphragm. Notice the absence of any edge blocking in the second photo. The provisions of the CBC (2006 IBC) for conventional wood framing (2308) allow for lumber or plank sheathing, as well as structural sheathing. Table 2304.7(3) specifically allows wood panel roof sheathing without edge support, such as at a ridge vent or the other joints which are perpendicular to the rafters.
Per the owners quote: "half my house has a cathedral ceiling and the other has attic space."

So by his statement how do you know that the roof was not constructed as a diaphragm, and why would you expect edge blocking visible in the picture?

ASCE/SEI 7-05 states, in section 12.11.2.2.3, that the sheathing is not part of the design of any required ties or struts, not that it matters since it doesn't apply here.

Douglas Hansen

So if 12.11.2.2.3 does not apply why did you assert it while we were dicussing chords?

My comments above.

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Hello Rocon

It sounds like you agree that the scope of 2301.2 does limit conventional framing to 2304 and 2308, and you are disputing whether the table in 2304 brings diaphragms (2305) into the equation. In case anyone without a CBC cares to follow the conversation, I will quote the scoping section here:

2301.2 General design requirements. The design of structural elements or systems, constructed partially or wholly of wood or wood-based products, shall be in accordance with one of the following methods:

1. Allowable stress design in accordance with Sections 2304, 2305 and 2306.

2. Load and resistance factor design in accordance with Sections 2304, 2305 and 2307.

3. Conventional light-frame construction in accordance with Sections 2304 and 2308.

Exception: Buildings designed in accordance with the provisions of the AF&PA WFCM shall be deemed to meet the requirements of the provisions of Section 2308.

Table 2304.7(3) gives us allowable spans for roof sheathing. It has two span columns. One is for sheathing “with edge supportâ€

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