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Discussion Starter · #1 ·
I'm replacing the asphalt shingle roof on my 650 sq ft, 86 year old house. The existing sheathing consists of 1x6 boards. I am replacing the really bad ones. But the rafters are 30 inches apart. I am want to put plywood over the top of the existing 1x6's. I am worried that the extra weight from plywood would create a problem for the old house.
Does anyone know what plywood size is best?
Would H clips help with the sheathing's structure?
How do I deal with the problem of the 8ft plywood not aligning with the rafters?
Any other suggestions or hints?

Thank you,
Ray
 

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Where are you located?
Snow load?
What size rafters?
What is the span?

I'd install new rafters in the middle of the 30" distance between the existing rafters
 

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Discussion Starter · #3 ·
I live in northern California. Snow is not an issue here, just an occasional light flurry once every few years-thank goodness. The rafters are made of 2x4's. They are 18' in length from the ridge to the eave. There are some 1x6 boards attached to the top and bottom of the rafters. From what I could see there seems to be only one per rafter about 1/3 from the roof ridge. I assume they were put on there at some point to support the roof deck. The slope is 4 inches per foot.

Thanks
 

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That is way beyond current codes
Unless you have trusses installed?
Are the ceiling joists in the attic also 2x4?
Any signs of sagging anywhere?

18' length may equal 14' actual span
You are looking at maybe just over a 2x8 to get a 14' span for a rafter

The Span is the distance between the inside wall & the ridge
labelled "half span" in this pic

 

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Discussion Starter · #5 ·
I'm not sure if we had codes here in 1923 when the house was built. :(

I'll try to explain the roof's construction as best I can-My apologies for lacking the vocabulary and roofing insight.
-The roof is a simple gable form. Not even a dormer to work around.
-The wiring is the old knob and tube type. And it is often covered with insulation. So I am not able to see much or poke at the house's skeleton.
-Each half of the roof is 30' x 17'8" (30 feet at the ridge). Both halves are the same (except for the back porch that continues another 3' x 12' ).
-The eaves have about a 30 inch overhang all the way around the house.
-The ceiling joists seem to be 2x4. I can't see if they are joined anywhere or not.
-The house's foot print is 30'x24' (24' corresponds to the 30' ridge in the center of the house).
-The wall support is two 2x6 boards. The rafter is secured to the wall edge by a large dado(?) type cut in the 2x4.
-Fortunately there doesn't seem to be any sagging except along the rake edge where the eaves were repaired by a previous owner.
-The half span would be 15 feet.
-The ridge beam is supported at the gable ends of the house and the rafters. The rafters are supported in part by the 1x6 boards cut at a slight angle connecting the ceiling joists to the rafters. The 1x6 sheathing should also provide support for the rafters.

Can adding new rafters in between the old ones be done economically and without disturbing the existing guts i.e. electrical and existing roof? Can you think of an alternative?

And to think, I wanted a little bungalow.
Thank you again for your help.
 

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It sounds like you have a truss system
These are engineered to for more support then a normal stick framed 2x4 roof
So you may not need any more rafters
And if it has not sagged in 86 years......and no snow :thumbsup:

Some roofs use 1/2" plywood as sheathing, I like 3/4"
You have 1x6 - even thicker - more support
Is there a reason you want the plywood?

 

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Without talking to your local code department first, the 1" x 6" plank boards offer additional rigidity to the structure, even though the rafter spacing is unorthodox and seemingly too wide for current code requirements.

If the remaining 1" x 6" plank boards, after you do the replacement required of defective ones, are not significantly deflected or pose a structural issue, and do not have a 1" to 2" gap in between each board, you should be able to apply the new shingles directly on top of the existing dimensional deck boards, without any concerns.

But check with your code department first, if that is acceptable, which it should be.

Ed
 

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Discussion Starter · #8 ·
Thanks for your help and insight!
As for plywood vs osb I think that weight is my main concern. Then there might be better water resistance, structural integrity, and nail gripping traits. The extra cost of plywood is not a big issue on such a small house.

The existing 1x6 board sheathing is not smooth enough (for me anyway). Even after replacing the bad boards there can be a height difference of up to 1/4 inch in the old vs new planks.
 
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