Floor Span Sizing Calculation of Old Hardwood Timbers

Hello Everyone,

I am an electrical contractor by trade in the process of purchasing a 1780’s Federal Colonial timber framed home located in New Jersey. The years and previous owners haven’t been so kind to the structure. My primary concern is a 20’ x 20’ living room area that sits over a full basement and has two bedrooms above. The ceiling has 6 1/2” deflection at the midspan which is evident at the floor above as well. The ceiling/floor joists are all hand hewn hardwood timbers that measure minimum 4” wide x 8” height. The spacing is approximately 19” on center. The wood appears to be an Oak or Chestnut species.

I recently hired a structural engineer to evaluate my concerns. I received his report the other night and have questioned the recommendations. His recommendations are to sister new 2x8s to the existing ceiling joists. Although, I realize that the existing lumber has not been professionally identified or graded; I am having a difficult time accepting his recommendations as adequate. In my 20+ years of construction experience, I typically see 12” deep joists or greater for such spans without supports. I have not found published span tables that deal with old growth timber beams. Additionally, modern span tables typically reference structural, #1, & #2 Southern pine or Douglas Fir, or TGI joists.
I have contacted him over my concerns and have not received an answer back that supports his recommendations. Please let me know if my concerns about his recommendations are warranted or there is some complex mathematical formula that qualifies such work. Thank you,
Brian

 

[NorthCivil]

if you don't trust him, hire a second engineer for a second opinion. you wont get any meaningful input here. impossible to fully analyze a problem without inspecting in person.

If your goal is to reduce the deflection, give the engineer your limit you are trying to acheive, and he can make a recomendation that will target that deflection limit.

 

[SlideRuleEra]

Please let me know if my concerns about his recommendations are warranted...

IMHO, your concerns are warranted. As suggested, hire another engineer.

http://www.SlideRuleEra.net

 

[bridgebuster]

6 1/2” is a lot of deflection. Are there bearing walls running along the joists?Anyway, sistering will increase the joist moment of inertia by 80% but won’t improve the deflection unless you jack the load out first. Actually, you wouldn’t be able to add the reinforcement in the deflected condition.

Ask your engineer what will be the deflection after the retrofit. The calculation is simple. It should be around l/360 to prevent cracking the ceiling.

 

[HouseBoy]

KRM,
I'm not completely sure what your objections are but I can see many myself and I agree with the others that a second opinion is in order.

Let me offer some insights:

I'm not sure if there is a plaster/drywall ceiling or if the timbers are exposed.

The existing joists are relatively shallow and that is the chief aspect that is contributing to the deflection that you see. Also, the very long time is a factor, not necessarily because the wood is old but because the load has existed for a very long time and wood "creeps" under load and this is especially prevalent for very long spans (with shallow joist depths).

I don't particularly like the recommendation of adding 2x8's for a few reasons;
Definitely not very attractive and very much out of character for the existing conditions (if it's an exposed condition).
Again, very shallow members are not going to do well in a deflection situation.
The existing deflection is going to be complicated to coordinate with any new straight pieces. "Pushing" that much creep deflection out of the existing joists is going to be a challenge.
Although it is manageable, and some reduction of the existing deflection may be possible, it takes some real skill to pull it off well.

I'm not clear on what your "ideal" solution would be. For example, if you are open to the idea of closing off the existing timber joists with a drywall or plaster ceiling, that could fit well with one solution such as adding 1 or 2 2x12's to each joist. Engineered wood might be appropriate (depending on the details).
Alternatively, if there is a practical way to support a beam at the center or two beams at the 1/3 points of the existing joists, you might like to add perpendicular beams in the ceiling. Given the proportions that I am guessing will be most fitting, the perpendicular beam(s) would probably want to be steel and (presumably) be wrapped with some sort of timber.

Again, effective solutions will have much to do with knowing what aesthetics are acceptable.

I think that finding someone who is experienced in dealing with old residential buildings is going to be key.

Good luck with all.

 

[oldestguy]

I once lived in an old house with such a situation. The bed in the bed room upstairs had to be placed so you have your feet in a lower area and head higher. Walking across the floor was like walking on springs. Regardless "it worked". We lived with it and recently I saw a photo of the old house and all still is there the same. If you can live with it don't change anything. If not, bending those old beams back to straight is not easy. It may take something like keeping them straight by a steel strap under each, plus sisters. The question should be "keep them as is" or "correct the sag". The suggested fix is "keep them same".

 

[KRM ELECTRIC]

Thank you to all for their comments and input!

The existing ceiling is plaster and wood lath attached to the underside of the 4 x 8 timber ceiling joists. My wife would love to expose those old joists; but some obvious structural repairs are required. I have entertained the idea of installing a new TGI ceiling system and then having the old timbers turned into decorative box beams to mimic the look of an open ceiling. I am awaiting a second reply from the engineer that did a site visit and definitely feel that I may need to find another professional moving forward. The first red flag was the engineer never measured anything prior to making his recommendations. Thank you again,
Brian

 

[HouseBoy]

OP, You're probably going get two camps on this one. I am not going to be a fan if OG's suggestion to "keep the same" for this reason:
Assuming that the member sizes are full size 4" x 8" actual one might find that the "code required" loading creates a stress level that is marginally acceptable. I think that one will need to squint too hard to see that. The thing I don't like about it is that the code required loading is not realistically there and the joists have still crept into a significant deflection. That alone tells me that the members are too highly stressed.

This is aside from considerations of preventing plaster or drywall cracking (again, not sure what s in place currently). That is another issue that could affect the needed repair recommendations.

Full disclosure, my family stays every summer for a week or two in a small privately owned cabin in New Hampshire that was built in 1928. There is no way that the 2x4 framed roof with single 2x8 valley and ridge boards should be able to withstand the ordinary snow loading (volume ceiling. No insulation or ceiling tying members at all. (I lie on my back every year and stare at this framing and wonder how it keeps working...) So I know old houses sometimes stand up out of habit but I just think your ceiling/floor as a bit too far gone. There is some risk that you are taking. Some engineers will feel differently about that.... (just sayin').

 

[emmgjld]

Having lived in and recommended repairs for several old residences (70-100+ years old), I understand both schools of thought expressed above. Modern codes and standards accept very little deflection & utilize relatively new growth lumber. Repair is dictated but, as you have found, verification with computations is difficult to aquire. Sistering old lumber is a common repair to recommend but actually accomplishing it requires time and some imagination. Remember this lumber has been in this deflected state for many years and is going to resist straightening efforts. The last older residence I worked on required incremental jacking for a period of over 12 months before I could 'lock it' in place. The lumber was NOT happy with me! Plus, sistering has the downside of unsightly, unless some real effort is expended.

Old growth lumber characteristics and a mentality which accepts a lot of deflection will require some looking into the structure, especially connections, and determining just how well it is (or is not) performing. Some repairs may be required but, if it is working, then let it work! I enjoy this type of work, including the interesting foundation solutions which have been employed. To me, sure beats the use of tables and codes!

 

[KRM ELECTRIC]

Update:
My engineer responded back and indicated that his approach was based up the existing joists being actual 2x6 at 12” centers. He will be recalculating his numbers.

Bridge buster,
The entire frame of the house is heavy timbers constructed in bents using mortise and tenon joints with pegs. There are no load bearing walls sitting on the ceiling joists that are deflected. The space above it must have been unfinished originally and was later partitioned off with 3/4” bead board to form two bedrooms. The bead board was applied to the outside walls, ceilings, and installed by themselves for the dividing wall. The attic space above has 4x4 hand hewn timbers as roof rafters spaced 24” on center. Typical of the age, there is no ridge board or beam. There is a cedar shake roof attached to 1x3 skip strapping with another slate roof on top of it. The roof is being replaced in a month with new 3/4” CDX decking and a standing seam metal roof.
I am an old house guy at heart and am trying to respect the original structure and still make it safe and work for the family.

 

[JStructsteel]

Did you ask for your money back?

How does he know they are 2x6 if you say they are 4x8 and he never measured?

Is this a 20' clear span?

Anything about jacking up the floor? 6.5" must feel like walking on a bowl upstairs!

 

[Ron247]

First, I would get a 2nd structural opinion if I was uncomfortable with the 1st one.

As far as the sag and any proposed corrections, I would be cautious before attempting to lift anything in a structure that old. I have done a lot of work on old houses both as a carpenter and an engineer.
1. Any lifting should be done slowly and with constant monitoring of all areas that could be potentially affected. I am not saying don't attempt lifting or correcting, I am just saying to pay very close attention.

2. Some of the sag is from creep since this is a very old structure. Creep for the most part is a lengthening of the beam. When you attempt to raise it, you a straightening out a member that possibly was not that long when it was first installed. As noted in #1, straightening the longer beam "too much" could shove the walls apart. If the roof sets on the same wall but does not want to let the wall move, you get a new problem. Creep is generally negligible in comparison to direct sag from load, but it is not non-existent. You have a really old building.

3. The amount of sag is more related to E than any other property of the wood. I do not work with the species you noted, but SYP does not have a huge E range as compared to its Fb range. For the most part, the grade of the lumber is not that big of a deal. Nothing has broke so far I am assuming.

4. One thing beams, joists and columns do is they keep items spaced apart. The joists and beams keep the walls separated over time. I have found that over time, these walls, floors etc may want to move towards or away from each other. The joist, beams and columns help prevent this. Once your joists sagged, any inward push from the walls becomes a P-Delta moment which can make your sag increase. For anyone who has ever cut a wood main beam in an old building, they most likely experience one of two things. The blade in the saw kept binding, so they repeatedly pull it out and start cutting again. Or, they don't quite get completely cut through the material when there is a "pop" and the rest of the beam breaks apart on its own. One was inward compression and the other was tension. I have see this happen a LOT.

 

[NorthCivil]

>"The first red flag was the engineer never measured anything prior to making his recommendations."

I wouldn't call that a red flag necessarily. After a month of experience working on alterations to timber frame buildings, I could call out beam or joist sizes with a glance. After about a year of experience, I could easily call out a typical span to the nearest foot by eye.

If your engineer thought 4x8's were 2x6's he may not have the experience. Though I believe there is some confusion. what he is recomending is likely sistering 2x6's to the 4x8s.

 

[Ron247]

If you have 4x8s at 19" oc that span 20' and he is proposing you add two 2x8s to that, the design may not be that far out of bounds if that is a bedroom above. That gives you close to an 8x8 at 19" oc. I would still be careful in the lifting though.

 

[racookpe1978]

To eliminate (or even make adequate and tolerable!) a 6-1/2 drop across 20 foot) CANNOT be donme by sistering the existing joists.

Sistering might prevent further drooping and sagging, but cannot "lift up" the existing curve. (6-1/2 sag is more than 1/2 the nominal 2x12 height of a high-strength modern floor!).

So you need to expect to "anchor (lock inplace) the outer rim of your upper floor. Jack up S-L-O-W-L-Y the middle 1/4 of the drooping floor so minimize cracking (over several weeks I'd think) from below while watching for plaster cracks and odd cracking across the upper walls. Then, decide how you're going treat the lower rough lumber. I'd like to recommend they be preserved and visible since the house is a "treasure" but you can reinforce with an arch "old iron" looking steel arch or multiple ribs from below, If the "room below" can tolerate it, keep 2x or 4x posts below to maintain height and reduce load.

It appears that you want the large room above, and the lower is "just a room" (or even a finished basement) that is not the center of the building's visitors.

 

[RWW0002]

I would recommend sending a sample of your timbers to the USDA to determine the species. They will do it for free and it is handy information both for design, and just for general knowledge about the historic structure.

https://www.fpl.fs.fed.us/research/centers/woodanatomy/wood_idfactsheet.php

Also, the timbers can be field-graded by your engineer to get a better idea of the properties of the existing wood. See grading protocol and general information about historic timber in the national park service document at the following link.

https://www.ncptt.nps.gov/blog/a-gr...er-and-timber-in-historic-structures-2009-01/

I have used the species and grading information along with some historic data (near the time of construction) for timber design to analyze historic beams and timbers in the past. It is surprising how strong some of the old timbers can be.

All this being said, the deflection/creep of the existing beams definitely point to some span issues. If you are wanting to leave the beams exposed and do not want ugly sistering, another option for strengthening the floor may be to construct a simple truss with stays and a tension rod below the existing beam. This would be something for your engineer to design and recommend, but I thought I would throw it our as an option for a little more "historic" look for floor strengthening than sistered finished lumber (especially if you ahve plenty of headroom).

 

[Ron247]

If final appearance is an issue, I would consider steel flitch plates on each side of the 4x8s. They would look "more period" than 2x8s sistered.

 

[BridgeSmith]

I like the idea presented by RWW0002, if you have the headroom, as noted. I might suggest a system with turnbuckles or some other means of gradually restoring it to the original position. The flooring above may also resist the change, as well, so you may have to go slowly for that reason. As others have noted, those timbers did not get into the shape they're in overnight; it took years for them to get where they are, and everything has conformed to it over time. If you want to keep them, you'll want to restore them to their original shape slowly, probably over several weeks, at least. The advantage of a system like this is that the forces are applied 'internally' to the system, so it doesn't apply force to other parts of the structure, like trying to jack the beams up.

If you opt to shore them up with posts, etc., I would expect it to take at least several months, if not a year or more, to get those timbers straight again without significant stress on the beams and exerting significant forces on the structure (both vertical and horizontal). If the ends of the beams aren't held down securely, you could create bigger problems than a sagging floor by trying to correct it quickly.

There may also be the option to simply replace the beams with new rough-cut timbers, if the end support configuration makes it feasible. The subfloor and floor above may not like the immediate change of shape, though, depending on what they are.

Edit: I'd like to add, I'm an 'old house guy' too, and I love that you're putting the effort into restoring one. Don't give up; there's a solution to everything. It just takes some time to work through it and get there.

 

[XR250]

I have not read thru all the posts, but I usually kerf the timbers in 3 or 4 places, jack them and bolt steel plates or channels to the sides. The kerfing allows them to be easily jacked back into almost place.

 

[mmarlow]

XR250,

For my reference.. Can you explain what Kerfing is? I did a quick google search, and its not quite clearing it up.

Thank you,
Morgan

-MMARLOW EIT