I’m in the process of major renovation on my 100+ year old house. Stripped walls and ceiling on a small room addition on the second floor and discovered an intereresting (at least for me) roof structure. Room is ~10ft x 13ft, gable roof. Ridge is the 10′ dimension running between the gable (13′ wide) and a wall part of the rest of the house. Eave walls are 7’3″ high, platform construction (cause it’s addition, rest of house balloon), pitch about 7 in 12. Rafters full 2×4 about 24″ OC, sheathing old/full 1″ pine. Cathedral ceiling, no ties and -here’s the fun part: no ridge beam. Only the skimpiest ridge board, 1x, splintered and not even continuous between gables. Construction of this room was around 1923, found signature of guys who built in inside wall cavities. Inspection reveals no spreading of eave walls.
Q is how come ? Is it the fact that the ridge dimension is short, and eaves not too high. More important Q is what can I do, or do I need to beef up the roof before proceeding? Already decided to beef up rafters to 2×6 (add new ones in the wider spans under old/weak boards, plus laminate/strap existing ones (PL + screws)) add a few simpsons here and there… 2×6 gives me a little more room for insulation. Roof is new, so all work is being done from inside. Wanna keep the cathedral ceiling look, no collar ties or cables. I can add a ridge beam if I have to, only if I have to. Room did fine for over 80 years, with lots of snow on the roof on many occasions (Boston area). I already talked to a structural engineer and have some thoughts, but would surely appreciate any additional input. Thanks.
Replies
I'd say it's largely because the room is so small.
If it was a lot bogger, you would have likely had problems with it.
You're only given a little spark of madness. You mustn't lose it. [Robin Williams]
At least two reasons that I can think of,
1 - because it is so small
and
2 - because it is so small
with regards to #1 -
the roof area that loads to ridge is only 10 x 13 for a total of 130 sq ft. Figure a combined live and dead load of 50#/sf and you see there was never more than 6500# on it.
Then in regards to #2 -
since the plane of each side opf the roof was partially supported at the ends, some load was transferred there. So I doubt that there was more than a theoretical maximum of 2000-2500 pounds translating into outward thrust, half on each side of the roof.
I don't have much doubt that the top plates ( and maybe soffit assenbly) of these walls accepting the load had enough strength to resist that thousand pounds or so of horizontal thrust. Since these are the members you are forced to rely on, you should inspect their condition carefully while they are openned up to check for defects. Also be sure not to splinter them up by making them nail poor when you add the sistered rafters.
A couple other tricks you could use to beef it up:
You could have rafter brakets made up to bolt the tops of the rafters together if they meet exactly opposite each other. These would extend down each side maybe 12-16"and cup themselves around the bottom of each in U fashin abnd be welded together at top to match the pitch.
or
frame so that you have a hip cieling instead of a gabled cathedral. That way, you can place kickers up from side walls at gable ends to the ridge to add some support
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balloon framing and plank siding can add lotsa strength to old houses.
I know he said the addition isn't balloon ... but with the rest of the house being ....
might be tied in differently somewhere.
also might be a bit of the exterior siding helping out ...
depending how it's all tied into the original house.
can't picture this on in my mind ...
but ... my advice ... when it comes to old structures ...
if it ain't broke don't fix it. And ya better be sure to put back what ya take out.
sometimes that crooked board over there is the one cross brace holding the whole gable side together!
Jeff
Jeff
Right. When I undo things, I take a lot of coffee breaks to just stp back, study, and get to know the old place
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
I agree that the size of the structure is the main reason for strength, 10' of sidewall 7'3'' high. A method to get a little strength is to gusset opposeing rafters together with plywood. [PL/screws] The gussets will project below the ridge, flatening the top of the cathrdral . You mentioned that you would like to keep the cathedral, but the flat spot gives you a spot mount lite fixtures.
Also can skin gable wall with 1/2'' ply, [PL/screws] , may be overkill.
Lots of old houses in your area with no ridge beam at all. Some sag, but some do not.
With no ridge beam, snow load in center of roof conspires to pull ridge down, spreading rafters, as you expected. Rafters transfer this load to top plates, trying to bow walls. Top plates resist the lateral force. You can design a structure with adequate top plates to resist spreading force. This is why many historic structures with post & beam can get away with no ridge beam. The top beams resist the lateral load from the roof.
Also, in order for the rafters in the center to move downward when the ridge sags, the sheathing boards have to give accordingly. The wide? 1 inch pine sheathing boards have a lot of strength in this direction, particularly if they are continuous for the full 10 feet. This load is spread across to other rafters, and ultimately to the endwalls.
You could further enhance strength in this dimension by screwing a plywood 'shear' panel to the underside of the rafters.
80 years of field testing proves that their design worked, so you may not want to mess with it.
Thank you all for your feedback and ideas, I'm very greatfull.
Here's some comments back:
1) Yes we all agree that mostly the small size of the structure kept it up. Additional factors that probably contributed on teh plus side: thick sheathing on top, thick side planking (mentioned by a couple folks) and 2~3 layers of siding. I just removed aluminum (yuk), discovered cedar shingles in great shape underneath, and while replacing windows discovered cedar clapboard underneath shingles. These walls are thick.
2) comments on some of the ways to improve: most folks also agree if it ain't broke don't fix it. But for discussions sake:
- stiffening various "panels" (roof or walls) to resist the gravitational & twisting forces, using plywood firmly attached to the framing. My BIL structural engineer suggested this (csnow too) underneath the rafters. He called it "diaphragm" (i think something else when i hear this word), I think of this more as a "torsion box" construction, stiff. My BIL said that just the sheetrok/ceiling is already providing a lot of stifness, and that even a layer of luan would greatly stiffen things further. Makes sense, think of airplane wings, light structure, thin skin, yet great stifness. I respect the guy, owns/runs a reputable firm restoring/shoring up old structures in this part of NE.
Interesting that Kid suggested doing this to the gable (my brother the architect did too). I didn't think this mattered as much, but the whole thing is a system and if one wants to do everything possible, no doubt it would help.
3) another idea was to make the 2 opposing rafters act more like a single V member that will resist the 2 ends from spreading, either metal (piffin) or beefy ply(kid)... great ideas offered, but I'll stop rambling and digest what can be implemented...
last thing: I very much appreciate the comments about taking one's time to figure things out when it comes to remodeling old work. I've had this problem on my mind for many months now (been doing other things), and didn't want to proceed till I felt comfortable understanding it. Just discovered this forum a couple weeks ago, and I'm glad. Thanks again.