1x4 deck vs osb

#1

northern california. rain only, yearly totals are around 10-15 inches per year, rare freezes to 26 F. i have a 1940 ranch house built by a farmer and his uncle! front half is only 2x4 rafter 16’ span, about a 5/12 slope. the rafters have single props, no purlin. its always been wood shingle (1940-1986) or now cedar shake. im wanting to convert to comp shingle, worried about the added weight of re-roofing using osb decking plus shingles. currently the deck is 1x4 spaced 50%. shakes cannot be used in CA.

question: regardless of increased cost, is it viable to deck the roof by adding 1x4 between bats, provided there is a small gap for expansion? the existing 1x4 is in great condition, tho im sure hard as a rock.

alternatively, is the added weight of simply overdecking with osb not a big deal?

is a purlin support system the best answer?

is stripping the 1x4 and replacing with osb only (4 feet at a time) a better option?

when i consider how light the roof is, and how those 2x4 rafters have never bowed, it gets me thinking. back half is engineered truss. side gables have short rafter length.

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#2

The weight of 7/16 osb shouldn’t be a concern, I’ve seen old house with 4 and 5 layers of shingles on them with no bows. That old framing lumber is stronger than the crap you get today.

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#3

thanks. that was fast and much appreciated!

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#4

Put a layor of plywood over the existing g lath, do not try to fill in the gaps. You will end up with far too many gaps when you go to properly install the shingles.

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#5

thank you. thought about that too. so add bats under the unsupported edges of osb and put the new deck down on existing. do you think a radiant foil face on the osb is worth it if 50% has no air gap?

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#6

Make sure the ends of the sheeting land on rafters, you may need to cut most of the sheets for this to happen.

If you want to do it wrong like many and just throw your sheets up there with no regard for the joints breaking on rafters at least use screws on the ends or they will pop up.

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#7

thank you. this detail is included in the county building spec and i did see it. i plan on supporting each sheet with added bats if the horizonal edges do not end with support. i will find out how well the rafters are spaced on the vertical edges!

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#8

Fun part is driving the 5 billion cedar shake nails in before you deck it.

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#9

We do at least one of these jobs a month. Never fun espically if you have couple layors of shingles over the cedar. If it’s just cedar as someone said worst part is bamging down the nails.

Cut the sheets so all vertical seams over rafters. You may need to add a few rows of horizontal lath to so the seams break on lath. Use minimum of 10d 3" framing nails to nail into the rafters.

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#10

they used staples instead. with some previous repair areas i was able to flatten the staples with a roofers shovel. the staples are long and go thru the 1x4, at least 2 inches.

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#11

thank you. yes, looking at adding more bats or lath. the shingle nails are a galvanized head with what looks like a vinyl coat on the tip. on the 10d nails, do you rec. galv.?

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#12

i see you say framing nails, got it

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#13

I don’t know how your area is but the cities here require solidly sheathed decks as mentioned in the IRC. Stating that anything larger than 1/8" be decked over with plywood 3/8th CDX or 7/16 OSB. Depending on the shingles many manufacturers require the same covering if the spacing is greater than 1/4"

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#14

it is the same here. thanks

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#15

In my area most older cedar roof were built over lath with rafters on 24" centers. Due to the rafters spacing to meet code 5/8" is required.

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#16

OSB is cheap around here, $9. Why not consider a 26 gauge metal panel?. You could clean up the fasteners on the 1x4’s , put down a low E roll of underlayment This is only about 1/4" thick, with silver on one side and the white foam on the other. This gives an R factor of 8 and prevents sweating. Put your panel on and you don’t bother with the decking step.

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#17

i guess i would consider it! now that you mention it. thank you

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#18

mine is as well. what is standard spacing on rafters? 16"? makes sense the 5/8". thanks

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