Historical house roofing

[WARose]

I was looking at the roof of a house (from the attic side) a few days back and noticed the "sheathing" (spanning between the trusses) was essentially a bunch of 2x4 (nailers; laid horizontal) back to back.

I've noticed this on a bunch of older homes (this was built in 1968). What year would you say most had switched from this to plywood?

 

[IRstuff]

2x4 seems a bit odd; I think the most common arrangement is more like 1x4, which was prevalent for at least wood shake. Most re-roofing jobs put full-sheet plywood down, so circa 1996-ish, which is when we re-roofed our shakes to aluminum shingles. It appears to be more of a fire code requirement than structural, since the aluminum is only 0.5 lb/sq.ft., which is way lighter than the shake or cement tiles that might have required the additional support.

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[NorthCivil]

Never seen 2x4 myself, but have seen a lot of nominal 1x4, (about 3/4" thick)

here is an interesting article. It appears plywood didnt really start getting manufactured on a huge scale around north america until after the 2nd world war.

https://www.apawood.org/apas-history

I would imagine that the factor that lead to plywood taking over from wood strips would have been the point in time where raw timber prices increased to where timber strips were more expensive than plywood. that would have involved a number of factors, including raw timber prices, labour prices, adhesive technology and cost, the state of automation of the mills, as well as transport costs.

the shift probably took place over 2 decades or more across north america.

 

[SlideRuleEra]

WARose - In SC, where southern pine lumber comes from, I'd say the 1960s is about right. Have to agree with the others, 2x4s is unusual.

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[BridgeSmith]

"I would imagine that the factor that lead to plywood taking over from wood strips would have been the point in time where raw timber prices increased to where timber strips were more expensive than plywood."

In bridge construction, the shift has been (and continues to be) in the relative economy of labor vs. material. 40 years ago, bridge plate girders used 5/16" thick webs with stiffeners every few feet to minimize the amount of steel used. Today, we wouldn't even dream of doing such a thing because the labor involved in adding all those stiffeners would make the girder 5 times more expensive than using a 1/2" or even 5/8" thick web without stiffeners. The shift from wood boards to plywood may have been at least partially be for the same reason. Even now, 1x4 furring is about the same cost per square foot as 5/8" exterior grade plywood, but almost nobody uses boards because slapping down plywood is faster.

 

[Ron]

Usually, prior to plywood and OSB usage, the sheathing was 1x6 members. For heavier, low slope systems, 2x4 or 2x6 T&G was common, but that was for built-up roof systems. I re-roofed my previous residence, built in 1974, and it had 1x6 sheathing. More commonly, I've found plywood in 1972 thru now and OSB from about 1980 thru now.

 

[WARose]

WARose - In SC, where southern pine lumber comes from, I'd say the 1960s is about right. Have to agree with the others, 2x4s is unusual.

Now that i think about it....I've probably seen 1x4's a lot too. I just always note that back to back spacing (i.e. no gap). For some roofs, there will be a spacing between the nailers.

I guess in the case of the house I looked at.....they had a lot of 2x4's left over from a old job or something.

At least you know it's not going anywhere.

 

[hokie66]

Interesting the way roof construction has evolved in different parts of the world. Maybe I am wrong, but I think North America is the only part of the world where solid sheathing is used. In most other places, just enough battens are provided to support (and tie down) the roofing.

 

[BridgeSmith]

My house had much that was overdone/oversized like that, simply because it was a DIY project where the previous owner didn't know anything about engineering or construction, so he didn't know what was needed, and wanted to make sure it was strong enough. (When in doubt, make it stout, right?) As a structural engineer, I can look at the 1x shelf spanning 30" (10" below the next shelf) and know that it's more than adequate for whatever was going to be on it, without the 2x2 under the edge, but apparently the last guy wasn't so sure.

 

[Guastavino]

Not to rain on anyone's parade, but in terms of diaphragm strength and lateral capacity, 2x4s are not nearly as good as plywood or OSB. At least looking at it from an NDS numbers approach. Unless it's diagonally installed.

But if you're asking me to walk on the roof, I'm gonna feel a lot better about it.

But, i think in about 48 hours we are going to know a lot more about wood roof performance thanks to Hurricane Michael...hope those people are safe because that wind is real...

 

[dhengr]

WARose:
I assume you are talking about 2x4’s edge to edge, a solid layer, not spaced 16-20” o/c as nailers, that’s a light timber roof decking system. What do the trusses look like and what’s their span and spacing? Unless the 2x4’s are edge nailed to each other, as well as two nails to each truss, they don’t offer much in the way of diaphragm action. I would say the transition period from shiplap board sheathing (or similar) to plywood was the mid 60’s, varying a little for availability and different regions of the country.

 

[Tmoose]

What part of the country and world is this house?
What is the lumber used for the truss, especially the top chord ?
What is the truss spacing ?
What was the 1968 roof material?

=.===========
This comment does not apply to the OP as described.

I think wood roofing (shakes, shingles) requires spaced sheathing in one form or another in areas that are not dry or even arid .

https://www.fpl.fs.fed.us/documnts/fplgtr/fpl_gtr201.pdf

 

[oldestguy]

Ever been in an old house that had cedar shingles, possibly topped over later with asphalt shingles? Those were secured to 1 x 4's spaced so the nailing hit them. My son's old house also has the rafters made of 2 x 4's rough cut. Snow country of Wisconsin. 1 x 1 roof slope.