Roof bracing / pulling rafters together
Over the past few years a long crack has developed in my living room ceiling. It's about five feet long, surrounded by hairline cracks and is about a foot from the the wall is lined up with the wall. After three visits from two different structural engineers I think the cause of the cracks has been identified.
Most roofs use attic joist that run parallel to the rafters but in my roof the attic joists are perpendicular to the rafters. 2X4s are laid flat and nailed across the attic rafters to provide bracing against the outward force of the roof pressing down.
My roof had three layers of shingles bearing down and we believe this excess weight has over time caused the wall to be pushed out a fraction of an inch which has caused cracks in the ceiling. The area above the crack does not have a 2X4 brace immediately over the attic joists. I had the old roof torn off last year and now there is just one layer of shingles so the excess weight should not a problem any more.
I'd like to repair the crack in the ceiling. The structural engineer suggested laying another 2X4 over the joists in the affected area. That should hopefully prevent any further spreading out. He also suggested installing rafter ties which I am planning on doing.
The engineer also suggested the option of attaching a cable to two opposing rafters and using a cable puller to attempt to pull the rafter together just enough to close or partially close the ceiling crack. The crack is about 1/16th of an inch wide.
I'm interested in closing the crack but am at a loss as to how to go about connecting a cable to the opposing rafters. The connection point would need to be at the bottom of each rafter. The force needed to close the crack could be great. Perhaps great enough to rip the connection point out of the rafter and cause serious damage the rafter. There would also be the danger of the cable snapping and causing serious bodily harm.
Maybe I should just leave well enough alone and simply put the 2X4 bracing in and hope the problem will stop. But I can't help think that it would be better to first pull in the rafter some and then install the bracing beam while it's under tension.
What would you do? My diagram is not complete but it hopefully is to explain the situation.
Link to larger diagram of below image: http://s114920633.onlinehome.us/misc/rafter2.jpg
The force opening the crack can be calc'd using vectors.
Using 6# per sq. ft. for 3 layers of shingles, 2' rafter spacing using 2x4 rafters, 6' high roof at the center and 20' wide roof, I get 270# pulling the wall outward at each rafter. A steel cable 0.140" dia. will break at 4x this value.
You could use a storm door snubber spring as a homemade spring balance
and a pulley system giving you a 2x or 4x mech. advantage to actually measure what it takes to close this crack.
If you plan to pull walls back in with a winch, I would think that it would be wise to free the ends of the existing collar ties (the flat 2X4's), before attempting to draw the walls in.
Otherwise, you will compressing these and they will want to pop back out.
Once the walls are pulled in, renail the 'ties' in fresh nail holes.
Anchoring the winch hooks is a bit of a problem. Drilling a hole in the rafter and bolting in a clevis comes to my mind.
The engineer's idea was to apply the force to just the rafter that is above ceiling location where the crack first appeared.
I can understand how the force could be calculated. However a gable beam is intersecting the problem area so that seems like it would make it tricky to calculate the forces. If I were a mathematician I might attempt to figure this out. Is there some software that can do the math for me?
Diagram of outside of home: http://s114920633.onlinehome.us/misc/house1.jpg
The clevis fastener sounds good. I'm just wondering what affect drilling a hole into the bottom of a rafter will have on the strength of the rafter. And maybe I'm paranoid but it seems like it could be possible for the rafter to split if the force is too great. Maybe a mending plate could be placed on either side of the rafter to help prevent the possibility of a split ? http://www.strongtie.com/products/connectors/MP.asp
At some point I might just want to bail out and have a pro take over for this task. But I'm wondering who would be experienced in something like this? A roofer? A general contractor? Would a pro typically have some type of insurance that would protect me if something went wrong like the rafters being damaged?
I forgot snow loading at 30 # per sq. ft. or so.
OK, I have the roof pitch at rise:run = 1:2.5. What's the width at the bottom of the gable? Counting the attic floor rafters it's 32'.
I'll assume that the direction of the primary force pulling the crack apart is at right angles to the crack axis.
To pull that back in with the valley rafter there is going to be really tough if you pull it at all. If this were my home I would stabilize what is there so it couldn't move any further and repair the ceiling. I had a couple of houses spread in the framing stage and it was really hard to get them back with three hoists and jacking the ridge in three places. Look down the side of your house at the cornice and see how crooked it is. That will give you an idea of how much the house has spread.
The rest of the drawing would help........
The crack is between drywall pieces?
Because of it's proximity to the wall, is there a smaller piece (1' rip) nailed to the shorter ceiling joists, and a larger piece, 8' or 12' nailed to the common house ceiling joists? Is that a building ell where the roof framing has a valley? If so, the wall is not moving outward against 20' of right-angle wall with shear sheathing, abutting. I would sight for straight the other side of the house's fascia board (opposite of the crack on main roof). Without proper rafter ties at construction, the other wall could have moved outward when the roofing material was stocked. Then drywall applied in it’s spread condition with the latest movement because of the loading with the layers of old roofing added, and no ties to hold it stable. The weakest spot would be the short ceiling joists where the larger drywall is fastened to the main room ceiling—8’ or 12’ long. The two end nails/screws into the short c.j. would give, allowing a crack to open. The shorter drywall ceiling is completely fastened to the short c.j.’s only, so the won’t move. Perhaps drywall layout started on the other side of the house and ended with the short-1’ wide piece at the crack.
You really need rafter ties (1x4) installed ON EDGE, fastened to opposing rafters per minimum code. Did the S.E. tell you this? NOT flat 2x's which do nothing as rafter ties because of the connection at the rafters. These are required every 4' when ceiling joists run perpendicular to the rafters, as yours do. The tie/rafter connection is critical, to prevent spreading: http://www.engineersedge.com/civil_e...onnections.htm
Sec. 2326.12.6. Rafter ties. Rafters shall be nailed to adjacent ceiling joists to form a continuous tie between exterior walls when such joists are parallel to the rafters. Where not parallel, rafters shall be tied to 1-inch by 4-inch (25 mm by 102 mm) (nominal) minimum-size crossties. Rafter ties shall be spaced not more than 4 feet (1219 mm) on center.
I don't recommend cable: http://www.eng-tips.com/viewthread.cfm?qid=105386
Be safe, Gary
Wow. Getting lots of smart people helping out here. Thanks !
The base of the gable is 28' wide.
I'm not 100% certain but I think the cracks occur where the gypsum boards are joining. This home was build in 1954 using gypsum boards covered with a plaster veneer for the walls/ceilings. These boards are not that wide, maybe 18" or 24", and somewhere between 6' and 8' long.
I've lived in the home for five years and the problem was either covered up or did not exist when I moved in.
I put a level against on the interior surface of the outside facing walls to see if they were deflecting outward. But my level is only 3' long so it's hard to get an accurate reading for the entire 8' horizontal wall surface. If they are deflected then it's off only a little bit.
It took me many hours to get the drawing to it's present state. I tried to make it as accurate as possible but that has proved to be a difficult undertaking. Finishing it up will take allot of time. I'm using Google Sketch Up so I guess I can't complain too much since it's free. I did have to take some guesses at how the rafters come to rest on the outside walls since that is not really visible from inside the attic. It would be ideal if I could make complete and accurate and then some of you could view it in 3D using Google Sketch up.
GBR in WA - I need to read through your post and the referenced links a few times to digest it all (if I can!). The SE did tell me about needing rafter ties which I believe meet the requirements you stated. The flat 2X4 was an idea he came up with to help prevent further spreading in the problem area. My diagram shows the present locations of the flat 2X4s which I presume were part of the original construction of the home.
Assuming 30 PSF snow loading, 6 PSF asphalt loading, 15' 2x4 rafters and 1/2" plywood sheathing
I get 1430# of horizontal force that your wall has to resist at each rafter end to keep the roof from collapsing.
|All times are GMT -5. The time now is 07:46 AM.|