Finding the live load limits for non-standard floor joists in attic conversion

[bcm]

Hello,

Could anyone point me in the direction for learning the the live load capacity for 3x5 lumber? These are the actual dimensions of the joists. This is an old house from 1905. This type of joist isn't referenced in the IRC tables, but I'm guessing the live load rating could be close to double a modern day 2x6 (which is 1.5"x5 1/2"). I'm not able to find the actual span today, but I'm assuming they are going to span to the center of the house, so this might be around 11 feet.

I'm working with a house that had an attic renovated by a prior owner, presumably without permits. I want to put a bathroom in, but am being told the floor has to be able to support 40psf. So I'm trying to understand the limits of the floor before I go in too deep. Maybe I need an engineer to consult. The joists also have wide and thick planks running across them. I'm assuming this is original to the constriction. On top of that is a layer or 1/2" flooring put down by the last owner.

Thank you.

 

[mtlogcabin]

Joist span calculator | Floor - Ceiling - LearnFraming.com

Calculate the span for floor and ceiling joist. Select a load from the list, select the spacing, enter a span, and select a type of wood.

learnframing.com

Floor Joist Span TablesCalculator

How to read floor joist span tables (page includes a span calculator). On this page we will explain how to design with residential wood framing tables. This module is part of the design your own home tutorial.

www.the-house-plans-guide.com

 

[ADAguy]

Do you know the specie of the joists?
I did this with net 2x4's at 36" x 12'/space sheathed roof in my 1905 vs 1.5 x 3.5 today. cross section is 30% greater then todays. still hasen't sagged in 116 yrs.
What is your joist spacing?

 

[bcm]

The joist spacing is 16 inches.

I don't know the species of wood, but I it's very dense, and it's very dense and very dark in color. It's not like the typical framing lumber one might get a Home Depot.


BTW, mtlgcabin, that tool is really helpful!

 

[Mark K]

The IRC is limited in scope and if you do not like the answers an option is to use the IBC.

An engineer can calculate the capacity of your 3x5 joists, but he will need to know the species and grade of the members. If the grade and species are stamped on the members you will know what you have. Although unlikely, if the stresses are low enough he may find that grading is not a controlling issue.

First start by having the engineer calculate the loads and then the stresses in the joists. You will then be able to find out if even the most optimistic grading will work. If not you then explore adding members to provide the necessary capacity.

If the species and grade of the members is not known and this would make a difference consider hiring a lumber grader to visit your site and grade the members.

Your options are to either add members or to spend money and time to find out what you actually have. Your choice.

 

[ADAguy]

Point Mark, sistering additional similar sized members will minimize the spacing and the load.

 

[mtlogcabin]

I don't know the species of wood,

Depending on when the home was constructed sometimes you can narrow down the options by what was available in that area at the time.

 

[Genduct]

Hello,

Could anyone point me in the direction for learning the the live load capacity for 3x5 lumber? These are the actual dimensions of the joists. This is an old house from 1905. This type of joist isn't referenced in the IRC tables, but I'm guessing the live load rating could be close to double a modern day 2x6 (which is 1.5"x5 1/2"). I'm not able to find the actual span today, but I'm assuming they are going to span to the center of the house, so this might be around 11 feet.

I'm working with a house that had an attic renovated by a prior owner, presumably without permits. I want to put a bathroom in, but am being told the floor has to be able to support 40psf. So I'm trying to understand the limits of the floor before I go in too deep. Maybe I need an engineer to consult. The joists also have wide and thick planks running across them. I'm assuming this is original to the constriction. On top of that is a layer or 1/2" flooring put down by the last owner.

Thank you.

So double 2x6's on what centers for the 11 ft span?
yes a simple calc can give you the section modulas value for an equiventent value, but the lack of depth of the lumber would imply your HEAVY Bathroom could probably benefit from some aldded strength in the center of the span. 1/4 or 3/16 steel rips bolted of screwed to stiffen up the floor.

A simple calc for an enginer to do