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Sagging Roof Rafters-Long-Sorry

Posted by Danvirsse (My Page) on
Mon, Apr 2, 12 at 7:36

Hi all.

Well winter is leaving us and my husband and I are embarking on the next phase of our 180 year old home repair/renovation/restoration. For those of you who haven't seen my prior posts, we started in the basement and have installed new support piers. From those we have straightened the 1st floor, re-framed the load bearing walls on the 1st floor and straightened the 2nd floor.
Now we're ready to tear off the at least 3!!! layers of shingles and install a new roof. The question is the all roof rafters have approximently 1/2" of sag in the 20'. There are existing knee walls at approximently 9' from the outside walls. The ridge is straight. Is this amount of sag acceptable or should we try to straighten them? I've read that very old wood sometimes won't straighten, but rather will lift off the outside walls when you try to jack.
Anyone have experience or input on this?


Follow-Up Postings:

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RE: Sagging Roof Rafters-Long-Sorry

I wouldn't call a 1/2" deviation from straight in 20 ft. a "sag" and I certainly would not try to straighten them. If the ridge is straight the roof is probably fine. What is the rafter size (exactly), spacing, span and slope? Are there any collar/rafter ties? How is the attic space used?


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RE: Sagging Roof Rafters-Long-Sorry

1/480 for deflection is not bad at all except for the roof is not loaded.

See what the building code in the area calls out for new work.

Simply sistering the old rafters may be more than enough.


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RE: Sagging Roof Rafters-Long-Sorry

Thanks for the input. The rafters are true 4" x 4" rough sawn oak. They are on approx. 36" centers with 1-1/4" thick x random width (most 12-24" wide) board for the sheeting. Since this is a cape style home, it will have 1 bedroom & a full bath. The pitch is 9/12. There are 1 x 8 collar ties.

Hope that covers all the questions.


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RE: Sagging Roof Rafters-Long-Sorry

Location, wind load, snow load, etc.

It would probably be best to hire a local engineer who can look over things, then possibly do the calculations needed.


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reRE: Sagging Roof Rafters-Long-Sorry

And the recommendation may well be do do nothing.

If it did not fall yet, it is likely fine.


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RE: Sagging Roof Rafters-Long-Sorry

Location (for snow) and span (horizontal dimension from exterior wall to ridge)are needed to determine the loading. The height of the ties from the top of the exterior walls would be useful too.


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RE: Sagging Roof Rafters-Long-Sorry

Find someone local.

oak is much stronger than the 'per se' softwood in the building code.

You have a structure the code does not address.

After an examination it very well may be the best thing is nothing, instead of paying thousands of dollars for stamped off plans and a fight with the AHJ.
While there are general strengths for things like oak, the AHJ can be areal PITA in forcing testing to verify that the rafters you have meet the assigned values.

The odds are very good that the rafters are significant stronger than even the oak available today.
Tread very carefully around the AHJ.


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RE: Sagging Roof Rafters-Long-Sorry

Let's see, the house is nearly 200 years old, hasn't moved to a harsher climate, and the ridge is straight--kneewalls help support the rafters about half-way to the ridge. You are removing all the loads of shingles to put on new...I'd say you don't need an engineer at all--you are reducing the weight of the roof, and all other conditions are the same.

What I would do, when replacing, is tell them to keep the random width boards as they are better than plywood, and nail new shingles (real or asphalt) over them. You might put some ice shield membrane on the lower sections near the edge to prevent ice dams, but that's the only thing I'd change.

The structure has lasted this long, foundation was the original problem, nothing with the roof--use some common sense people--you are reducing the load, so any stresses on the roof will be less than current ones--as long as all the old wood is intact and not decaying, don't mess with it. At least here in Ohio, I didn't need a permit to have my roof reshingled in 1995.


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RE: Sagging Roof Rafters-Long-Sorry

The weight of the roofing materials is a minor dead load and would not be an issue unless the design was already borderline; the important issue is the maximum live load from snow. I am assuming some kind of snow load only because the OP mentioned that "winter is leaving". I don't know if 20 ft is the rafter span, the width of the house or the length of the rafters and the kneewall at 9 ft from the outside wall is a mystery too. The OP has told us too little about the structure for anyone to recommend more than hiring a local engineer.


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RE: Sagging Roof Rafters-Long-Sorry

"The weight of the roofing materials is a minor dead load..."

Unless there are multiple layers.

I restored a 1920s craftsman house with cedar shingles and three layers of asphalt.

Cracks in the 3 cat plaster that went from ceiling to floor in many room that has opened wider at the bottom closed up at the bottom to hairlines and opened at the top as the load cam off.

Shingles are not all that light, especially higher quality thicker shingles.

Couple that with the large area being loaded, and you quickly have many thousands of pounds for a single layer.


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RE: Sagging Roof Rafters-Long-Sorry

Thank you all, especially columbusguy1. I guess I didn't ask my question clearly, but I seem to have gotten the answer anyway. Basically, I wanted to know if 1/2" of deflection/sag over the 20' long rafter was considered "alot" or "minor". We are not changing the framing and will be re-nailing the existing sheeting, so we are not really interested in what the current code calls for. The house has stood the test of time with no real maintenance since the 1930's. I would imagine it has already carried whatever snowload mother nature has in mind for our area. With your input and that of another forum site, I believe we will be leaving well enough alone and proceed with the tearoff and re-shingle.
Again thank you.


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RE: Sagging Roof Rafters-Long-Sorry

The dead load of a new single layer of asphalt fiberglass shingles might be 2 lbs/s.f. or less; a snow load might be 30 lbs/s.f. or more.

I was not recommending that you meet the building code only that you meet well established engineering practice while you have the opportunity to do so. Without the span or loading no one can do that for you but I can tell you that in northern climates the existing rafters are very likely to fail in a serious snow storm if the knee-walls or rafter/collar ties were moved.


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RE: Sagging Roof Rafters-Long-Sorry

"The dead load of a new single layer of asphalt fiberglass shingles might be 2 lbs/s.f. or less"

For thin cheap short life shingles.

Many are far more.

Actual snow loading into the structure also depends on pitch.

The local rules will specify the design load required.
The pitch will determine how the roof must be designed for the required load.


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RE: Sagging Roof Rafters-Long-Sorry

The most expensive CertainTeed triple laminated asphalt shingle weighs 3 lbs/s.f. so the weight of asphalt shingle roofing should not be a major factor in the design of a residential roof.

I would not reduce the regional ground snow load for a single family house roof unless the pitch was steeper than 10 in 12 and even then only if the roofing was smooth (metal). Laminate asphalt shingles can hold snow very well so counting on snow to slide before the roof fails is an unreasonable gamble.

The roof design load should be increased for obstructions that might cause drifting of snow.


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RE: Sagging Roof Rafters-Long-Sorry

And now two layers are up to around 6 PSF, not including the extra loads from valleys.

The normal design load is 10 PSF for a reason since even the sheathing must be included if you want to cut things tight.

You do not reduce the snow load, but how the load is applied to the slopped roof is very significant.
It is built in to many rafter tables, but those wil not work if you want to take advantage of having oak rafters.

You need to understand what the design loads are and how they act in the structure.

10 pounds on a 45 slope resolves into different structural loading than 10 pounds on a flat surface.


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RE: Sagging Roof Rafters-Long-Sorry

The span tables in building codes and "accepted engineering practice" assume that snow loads are applied to a sloped roof in the same manner they are applied to a flat roof.

Slope is a factor only if the roof were steep enough and smooth enough for snow to slide off (greater then 9 in 12 for asphalt roofing). The snow load would be increased where there are obstructions that could create snow drifts.

Image and video hosting by TinyPic

Here is a link that might be useful: Paul Fisette - Building & Construction Technology - U Mass Amherst


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RE: Sagging Roof Rafters-Long-Sorry

"The span tables in building codes and "accepted engineering practice" assume that snow loads are applied to a sloped roof in the same manner they are applied to a flat roof. "

And when the stresses in the roof members are computed the loads are resolved into vectors.

A 10 PSF load on a 45 degree roof is 7.07 pounds straight down and 7.07 pounds parallel to the rafter length.

A 10 PSF load on a flat roof is ten pounds straight down only.

These are the force vectors that are used to determine the rafter size required.

It is all rolled into the rafter tables so folks that do not understand statics and loading can look up rafter sizes required, and the requirements for ties to absorb the spreading loads created by the loading parallel to the rafter length.

It is simpler when the rafters are continuous from peak to eaves, and the ties are the ceiling joists at the bottom of the rafters.
It is a little more work when the collar ties are higher up since the rafters will flex and the ties will not absorb all the spreading load.

If you had ever taken a statics class you would understand how loads are treated to size structural element.


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RE: Sagging Roof Rafters-Long-Sorry

For the OP who might need to have this information: a rafter tie is normally in the bottom part of the rafter span and a collar tie is in the upper part. The best configuration is a rafter tie at the attic floor and a collar tie normally just above whatever head room is desired.

You always try to turn the forum into a glorification of your structural education. You are quite mistaken to assume that you are the only person on the forum who has had a formal education in the structural design of buildings. I studied structural design for 2 years in college before switching to architecture and was exempted from further study in graduate school but I chose to study it again for 3 years because the structural professor, Neal Mitchell, had just designed the structure for Le Corbusier's only building in North America, (The Carpenter Center at Harvard) and he had a well-deserved reputation as a passionate and innovative teacher. He was not much older than his students and we worked many nights and weekends on extracurricular design competitions.

Here is a link that might be useful: don't try this at home


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RE: Sagging Roof Rafters-Long-Sorry

"The best configuration is a rafter tie at the attic floor and a collar tie normally just above whatever head room is desired. "

Depending on the exact framing if you have a tie at the bottom of the rafters there may be no reason or advantage for one higher up, and they are often omitted.

If the rafters are fastened to a ridge beam a collar tie does almost nothing.

The collar tie is generally needed if there is no ridge beam.

"I studied structural design for 2 years in college before switching to architecture"

It sounds like you did not do well or it did not stay with you well if you do not understand how loads are applied to structures.

If you want to take advantage of having 'better than' materials you have to do the entire set of calculations.

The AHJ is likely to want a PE stamp on the documents, since many of them cannot actually check he structural design from scratch (they use the same rafter tables in soft woods since those are readily available and do not require careful analysis).


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RE: Sagging Roof Rafters-Long-Sorry

I switched to architecture in order to be able to design an entire building instead of a small part of it for the architects in charge. I have never regretted that decision and greatly enjoy dealing directly with owners and being the lead professional on projects.

Where I live both rafter ties and collar ties are required by the state building code for conventionally framed sloping rafters. Of course ties are not needed if there is a ridge beam but the OP did not mention one and I would not expect to find one in a traditional Cape.

R802.3.1 Ceiling joist and rafter connections.
"Ceiling joists and rafters shall be nailed to each other in accordance with Table R802.5.1(9), and the rafter shall be nailed to the top wall plate in accordance with Table R602.3(1). Ceiling joists shall be continuous or securely joined in accordance with Table R802.5.1(9) where they meet over interior partitions and are nailed to adjacent rafters to provide a continuous tie across the building when such joists are parallel to the rafters."

"Where ceiling joists are not connected to the rafters at the top wall plate, joists connected higher in the attic shall be installed as rafter ties, or rafter ties shall be installed to provide a continuous tie. Where ceiling joists are not parallel to rafters, rafter ties shall be installed. Rafter ties shall be a minimum of 2 inches by 4 inches (51 mm by 102 mm) (nominal), installed in accordance with the connection requirements in Table R802.5.1(9), or connections of equivalent capacities shall be provided ... Collar ties or ridge straps to resist wind uplift shall be connected in the upper third of the attic space in accordance with Table R602.3(1). Collar ties shall be a minimum of 1 inch by 4 inches (25 mm by 102 mm) (nominal), spaced not more than 4 feet (1219 mm) on center.


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RE: Sagging Roof Rafters-Long-Sorry

This discussion has passed its moment. The stress is too much.
Casey


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RE: Sagging Roof Rafters-Long-Sorry

Brickeyee has for many years consistently thrown everything I say back in my face in quotes as if he were the only person on the forum qualified to comment on structural matters.

Rather than submit to this childish nit-picking and ankle-biting I have decided to no longer participate in the Old House forum.


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RE: Sagging Roof Rafters-Long-Sorry

As the original poster of this, I'm disgusted with this whole thread. As way above average DIY's, we were trying to gain some insight as to whether or not our rafters had significant sag. We nowhere stated that we we changing any of the framing or room usages, yet brickeeye and renovator8 felt the need to turn this into some type one-up-manship match. Only columbusguy1 chose to give any reply that we could utilize.

Rather than renovator8 leaving this forum, it is time for both he and brickeeye to grow up and treat the OP with some respect. While I have no other experience with renovator8, brickeeye has managed to wreak havoc on some of my posts in other forums as well. Brickeeye, it is time for you to either tone it down or quit posting.


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RE: Sagging Roof Rafters-Long-Sorry

The bottom line is you have very little "sag". If you remove old roofs and re-nail wide plank sheathing you will be fine. Old houses built with wood as you described that have lasted for that many years need not be changed. Rot or insect damage would be the only reason in your case. If snow or ice storms haven't damaged it in 100 years then keep going with your project.


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