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On an 85 year old house there are pressure treated 1x4s running horizontally across the rafters, and the plywood sheathing is laid on top of those 1x4s.

the 1x4s between the rafters and the sheathing

I have to replace some of these 1x4s but there were no pressure treated 8ft 1x4s available at my local HD, so I bought untreated white pine 8ft 1x4s to use instead, and sprayed them with Concrobium (trademark) to prevent mold.

Will that be OK, or is pressure treated essential?

(I was thinking perhaps the reason for pressure treated 1x4s was to stand up to water damage that would rot plywood edges.)

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    I'd be shocked if they were pressure treated 85 years ago, particularly since they are not black with creosote, which was the most common preservative treatment at that time...
    – Ecnerwal
    Mar 1 '20 at 12:45
  • @Ecnerwal - Then the reddish tinge is just the natural redwood color, redwood being the predominant building material of the time in CA. In which case the pine is perfectly adequate as a replacement. If you copy paste your comment as an answer I'll select it. Mar 1 '20 at 14:41
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    Redwood turns gray with age. But I agree with ecnerwal, creosote was the standard into the 70’s what you may have is old growth fur looking at the quality that was cants (junk wood for the day that is the outer slabs when squaring up quarter sawn lumber). Normally pressure treated is not used inside the house envelope. Some chemicals used in the past were quite toxic and today any pressure treated sold in CA causes cancer according to prop 65.
    – Ed Beal
    Mar 1 '20 at 19:35
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    From looking at your picture it looks like you have two roof slopes that come down toward each other. (Right where you have that shop vac sitting). If it were me I would build up a structure on top of the roof to create two additional slopes to eliminate that trough. Such construction as showing there is an invitation for leaks.
    – Michael Karas
    Mar 1 '20 at 20:47
  • @MichaelKaras You are a prophet - a leak is exactly why I stripped it down and removed some planks close to the valley. Some of the plywood and "skip sheathing" planks (hope that's the right term) were not in good shape. Fortunately the rafters were solid - I applied hydrogen peroxide and then used a wire brush to remove mold stains - now the rafters look fine. - About the cricket, I was instead thinking of a galvanized metal valley with flashing expoxied on at either end so the water would escape safely to the adjacent sloping valley. Because a cricket is challenging. Mar 4 '20 at 18:33
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Keep in mind that those boards that you see under the plywood sheathing were originally spaced out as they are so that they could accommodate the installation of either wood shake shingles or tile type roof. The boards were surely never treated lumber at the time your house was built 85 years ago. (1935 places your house build in the pre-WW2 years).

The most common lumber used at the time was rough sawn and may very well have been more than the standard 3/4" thick. White pine boards should be fine for replacement being as you are re-installing plywood over them. The white pine boards may very well not have the same strength as the original lumber but the plywood can take care of the difference. The difference in thickness should only be a problem if a plywood splice happens at the same rafter where new and old board butt up to each other. It the thickness difference is significant you may want to shim under the new pine board where plywood splices may occur.

Note that it has been my observation that roofers that are covering spaced board with plywood sometimes do not put much energy in trying to splice the plywood on the rafter centers so shimming may be a moot point if you are following that scheme.

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