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Discussion Starter · #1 · (Edited)
I ran this question past the flooring crew here first but would like a follow-on opinion:

I'm working on a project to retro-fit insulation between the ceiling and roof of the finished attic level of a 1920s Center Hall Colonial. This third level attic space is your typical narrow ceiling down the center, with slopes to either side, and short knee walls. The flooring in place is a single layer of tongue and groove (3/4") laid directly over 10" joists.

Almost all of the floor boards stop inside the crawl spaces just behind the knee walls - so those crawl spaces weren't accessible for working with insulation without placing 2' x 4' particle board sheets over the joists - which I did. But in the process, about every 4' I encountered a flooring board or two that extended all the way across the joists to the outside wall where the roof meets the top of the side walls.


In this photo you can see two such instances where those long flooring boards continue to the outside wall (this particular section of crawlspace hasn't yet received its 2'x4' particle board deck). What I'd been doing in the spaces I'd already laid particle board was to cut those extra lengths of floor board away so that I could lay those sheets side by side for a flat continuous surface - because those 3/4" boards are taller than the particle board sheets.

But now I'm wondering if those extended boards actually served some structural function in bracing the top of the walls where the roof framing joins it. Those boards were held in place by 4" or 5" nails that I pried up. Given the length of those nails - and how tight the space is where the floor boards meet the wall and roof - it's almost as if they had to be laid before the roof went on. A good bit of space was required to swing a hammer to pound those nails in. Also fascinating about those boards is that there are very few floor joints in all of the attic space due to how long all of the floor boards are. The first two you see in the photo run for 6' inside the crawl space, then another 5' and 9' respectively to the center of the room and past - with no joints.

So the big question: when I removed those floor boards did I defeat some structural purpose they were performing? I should also add that the roof is the original slate tiles. Do I need to add back something structural in the places I removed the flooring?

Many suspect that those flooring segments probably served a temporary function during construction, as maybe some sort of collar tie. I'll add that there are collar ties higher up in the roof framing:

 

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Building standards were a lot higher in those days than they are now.


Probably served as a walk board then and it was assumed they they helped stabilize the joists to prevent twisting as the joists fully dried.

I even have a few attic boards like that in my stick built attic (circa 1995).
 
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Stop cutting and recheck your framing. The 1x boards may be acting as collar ties because your floor joists aren't. That depends on the roof and the wall framing materials. If the rafters are bearing on a wall plate of big post type lumber, wall may be resisting the roof load, and I could say it's time tested. If not, you need those boards. I would replace the 1x with 2x6 and raise the ply floor with ply strips under every 16". The collar ties must tie with wall plate. If not a single piece, I would overlap the collar ties at least 3 joist bays because the joists are taking the load.
Is there anything else? A shed against that wall or some beams?
The 2nd photo shows rafter ties (search for the difference) without a ridge beam and not doing anything as collar ties.
 

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Discussion Starter · #5 ·
@carpdad Thank you for the detailed response! I'd like to add a bit more detail about the framing at the walls.



This photo shows the wall plate, with rafter ends resting on it at each side of the photo. The top of the first floor joist is at the bottom of the photo. I have not observed any lumber in the wall plate area larger than what's shown. These wall plates are along the long sides of the house (the front and the back), at the top of the second story, and there is nothing else against the front or back of the house.

Here's another view:



The long floor boards (that ran to the wall plate) that I've already removed were single boards, 3/4 in thick. The two paired boards shown in the photo in my earlier post are the exception. Everywhere else single boards were serving the function of rafter ties at floor level.

Having said that I'd like to explore more some options for restoring rafter tie function.

You suggested using 2x6 lumber, but I wonder, given that the single 3/4 in floor boards served this function so well for over 90 years, if 2x4s would be just as effective as what was there? I have a good bit of that on hand already. Here is a mock-up of what that might look like:



The 1x lumber under the OCB sheet pretty much levels this with the 2x4 and allows for a smoother work surface. For something like this, would I use nails to secure the rafter ties - or is there a structural screw strong enough for the task. The tight space at the wall plate has me favoring screws. But either way, they'd almost certainly have to be driven at an angle due to the roof deck limiting the space.

My other thought is to consider using steel strap or tension ties - which if feasible, provide a number of advantages. Already the head room in the crawls spaces is super tight - and raising the floor deck with 2x lumber would make it tighter. As well, the 2x would interfere with forming a tight ventilation channel at the wall plate and part way up the roof deck. Something like this:

Simpson Strong-Tie 7-Gauge 2-1/2 in. x 21-1/4 in. Heavy Strap Tie-HST2 - The Home Depot
http://www.homedepot.com/p/Simpson-...PIPHorizontal1_rr-_-100375090-_-100375093-_-N

or:

Simpson Strong-Tie 48 in. 12-Gauge Strap-MSTI48 - The Home Depot
http://www.homedepot.com/p/Simpson-...PIPHorizontal1_rr-_-204842395-_-100258180-_-N

Your thoughts on strap ties, and also the screws and/or nails for this or the 2x?
 

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Sorry, I forgot about the 2x that make up the ceiling and also acting as the rafter ties. The lumber used there are old (usually means better cuts of wood but can't depend on it) and looks like bigger chunks of lumber. I am not an engineer or even tried to look for such information. I just used what's available which means soft wood.
But the ceiling material, the rafters which look better than the usual 2x, even the size of the nails and the steeper angle of the roof, your 1x on the floor joists, all helped with holding up the roof without the usual collar ties. I can't decide what you should do. I tend to go over board for structure and that is why I'd use 2x6 and I don't know if 1x are ok except they were there and the roof is ok. Only thing I know is that the rafters have forces that want to push out the wall. Ceiling joists and collar ties resist this force and your structure doesn't seem to have the collar ties.
That does not mean you should restructure yours since it looks like it stood there for long time.
Only thing a non engineer like me can say is don't change things unless you go bigger.
 

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This thread has a number of unfortunate terminology problems which is obscuring the function of the various pieces of framing. First off, the photo shows you have ordinary roof framing using a ridge board, which is a non-structural element intended as a nailing surface for the rafters. The large boards which are about a third of the way down from the peak to the floor level are collar ties, which are intended to equalize roof uplift pressure during high wind events. Your collar ties are heavier than used in modern framing, and may be sufficiently strong that they are assisting in resisting wall spread, but that is not the normal function of collar ties.

The floor boards you removed, or are thinking of removing, appear to be rafter ties, which are typically floor level boards intended to resist wall spreading due to outward thrust exerted by the rafters. In most houses, floor joists perform this function, but in your framing it seems the extended 1xs serve that function. If that is in fact the case, you cannot safely remove those boards, as I did not see any other elements to resist wall spreading. The collar ties by themselves are not sufficient to resist wall spreading.
 

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Discussion Starter · #9 ·
Thank you @Daniel Holzman - and everyone, for the very helpful information, including informative links!

What was simply an effort to put some decking boards over floor joists, to give me a deck to work from as I put insulation in between the rafters, got complicated very quickly! In my wildest imagination I never expected that floor boards would double as primary structural elements. I didn't even realize that floor boards were made that long!

But a nagging feeling about their purpose stayed with me and led to me asking for guidance in a couple of different forums - even after all of the early responses speculated that that they likely weren't structural. What surprises me most is that no one who has answered anywhere I've asked has actually encountered this specific framing system - even though there's a lot of old housing stock out there. Go figure.

But amazingly, about 6 or 7 lengths of single 3/4" floor boards at the front of the house, and 4 or 5 at the back (stairway is at the back side) worked flawlessly as rafter ties for nearly a century - with a slate roof above them! And that was to cover about 16 rafter bays at the front - about 12 at the back!

I'm expecting that beefier 2x4s - laid from wall plate to the 4th joist away (where the knee wall begins and the interior flooring ties into that 4th joist) - will be an upgrade to those old floor boards. I'll use construction screws - GRK and TimberLOK - to make the connections. May even go overkill and add some steel strap ties where the 2x4 ties meet the 4th joist/flooring.

Thanks again!
 

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Sorry about the names.
But on the other hand, calling them ties don't make them ties. It's a bit of self deception to say 1x were somehow strong enough to be ties.
Your roof sheathing, good old wood t&g, could be part of the structure. I don't know what it is called, but you can have a skin&bones (sheathing+framing) structure that is self reinforcing.
BTW, I had 2 story house which was balloon framed with 1x6 sheathing. Somebody cut the 1st floor wall and added a shed. Did not add a header. That 12' span was held by the corner post and mid wall and the rows of 1x sheathing. The wall of sheathing was the header. Roof was on that span.:smile: The mid wall had a basement beam and a post somewhere near. I put a double header on the studs all bolted together and it passed the inspection. Mine is a different story but your house has something that keeps the roof upright - probably something or things that is better than those 1x. It is also probably a narrow wall, allowing the top plate to resist better. It is interesting house.
 

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Discussion Starter · #11 ·
The roof sheathing is skip sheathing - heavy lumber similar to the joists - though the flooring at that level is tongue-and-groove. And yes, as you've no doubt already gathered, I'm still incredulous that those single boards were doing that job as rafter ties. Being someone who likes to understand how things work, I really would like to know definitively how the framing system is functioning here.

I found the discussion at the link below helpful in understanding that my roof slope of 9:12 produces much less outward thrust - and hence lower tension forces - than more shallow roofs:

https://www.nachi.org/collar-rafter-ties.htm?loadbetadesign=0

It also discusses that rafter ties are generally 2x4s or 2x6s - often spaced every other rafter - so you were correct with your 2x6 recommendation, even though I'm opting to go with 2x4s spaced at a maximum of every other rafter bay (which is still an upgrade).

I took another look at how the single floor board ties were connected to the wall plate and near as I can tell, I exaggerated the length of the nails used. Though they certainly felt like '4" or 5" nails" pulling them out in that confined space, it looks like they were closer to 3". I'd pulled them out of the boards so that I could safely stack them, but I think what I'm looking at now is one of the remaining nails.

You mentioned the walls being "narrow" and I'm not certain if you meant the span from wall plate to wall plate (27') or the thickness of wall just below the wall plate (6" rough measurement). The sheathing there is about 1" thick boards that are 12" wide, attached to 3"x4" studs:



But the house at the first story - and the gable end walls all the way up to the ridge board - is a very thick stone wall construction (just the interior of window sills are 15" deep). Because of the thickness of the stone I'm unable to see how the roof skip sheathing and rafters are affixed/terminated at the stone gable ends:





It'd be awesome if, as you surmise, there's something more to the structure that enables it to withstand expected wall spread forces. But at any rate, I'm confident that the new connections will be beefier and stronger than before.
 

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Discussion Starter · #12 ·
Update: I went with 2x6 prime Douglas Fir, adjacent to every other rafter, and used GRK construction screws to make the connections at the wall plate and across four rows of the perpendicular floor joists. Where the 2x6s met the floor boards at the fourth joist I also used construction screws to beef up the connection of the floor boards to that joist. So it's a good solid "system" from wall plate to wall plate.
 

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