# How to Sister I Joist



## datahoe (Dec 26, 2012)

Hello,

I'm new to DIY and looking for a little advice. I am placing a 175 gallon aquarium in my living room and need to reinforce the floor it will be. Below the living room is unfinished and I have access to the floor joists ( I joists that are 24 inch on center). The aquarium will have to go parallel with two joists along a load bearing wall. Others have recommended I sister the two I joists with 2x12 and I should be fine. Would screwing or nailing a 2x12 into the I joist compromise the joist? What would be the best and most cost effective way to reinforce the floor for an aquarium? The house was built in September 2012. Any advice or information would be greatly appreciated. I just don't want to do something that can affect my new home, in a negative way.


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## joecaption (Nov 30, 2011)

That would work if it can fit in the webbing of the joist.
Georgia Pacific suggest just using 3/4 plywood or OSB on both sides with constrution adhesive and nails.


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## datahoe (Dec 26, 2012)

joecaption said:


> That would work if it can fit in the webbing of the joist.
> Georgia Pacific suggest just using 3/4 plywood or OSB on both sides with constrution adhesive and nails.


Thank you. So would I need to have the plywood or OSB span the entire webbing? From wall to wall is approximately 12 feet.


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## woodworkbykirk (Sep 25, 2011)

for extra strength fill in between the top and bottom chord with plywood then sister along the full width of it with plywood as well. do it so its glued and screwed but when you fill between the chords maks sure you dont cut it so its deadly tight leave 1/8 - 3/16 gap as if you cut it tight you can spread the joist apart which desroys its integrity


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## hand drive (Apr 21, 2012)

exactly what woodworkbykirk mentioned except don't forget to stagger the plywood seams away from each other as far as possible. staggered seams will help make the span stronger


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## datahoe (Dec 26, 2012)

Thanks for the advice. Just so that I am clear, when you say "fill in between the top and bottom chord with plywood" you cutting plywood and screwing/nailing it to the webbing with adhesive? And to "sister along the full width as well" you mean add an additional piece of plywood on the outside screwing/nailing it to the top and bottom flange? Sorry for the questions but I want to make sure I do this right for this and future projects since this is my first real DIY project.


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## datahoe (Dec 26, 2012)

Thanks hand drive, I'm not sure what is meant by staggering the plywood seems. Can you clarify for me? Thanks again


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## sixeightten (Feb 10, 2009)

Staggering the plywood means to offset the plywood seams on each side of the joist. Plywood comes in 8' lengths, so you will have a splice on each side.


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## datahoe (Dec 26, 2012)

Thanks sixeightten....now it makes sense!


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## hand drive (Apr 21, 2012)

if you have a 10' span, an 8' piece of ply would be used with a 2' piece to finish it out.v next over lap the opposite direction putting the seam on the opposite side. then cut ply to go over those that is wide enough to cover the full width of the joist bottom cord to top cord...


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## woodworkbykirk (Sep 25, 2011)

most i joists have a 1" gap betwwen the web and the chords so i take 2 layers of 1/2" ply to fill it.. first starting with a 8' peice centered then fill in the ends. then for the 2nd layer just switch it.. start from the end then finish. then a layer of either 1/2" or 3/4 on the outside


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## Gary in WA (Mar 11, 2009)

Depends on the brand for the thickness of ply fillers. Some I's are rated at 1500#, others ? Offset nail the fasteners into the top/bottom chord as well, C-3; F-11 if sistering another I-joist. https://docs.google.com/viewer?a=v&...UmVBIf&sig=AHIEtbRRgFjvJNSa_DT9GHwNIkSbvrbL_A

I would not use a new solid wood joist as a sister in your case.

Gary


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## oodssoo (Jul 16, 2012)

datahoe said:


> Hello,
> 
> I'm new to DIY and looking for a little advice. I am placing a 175 gallon aquarium in my living room and need to reinforce the floor it will be. Below the living room is unfinished and I have access to the floor joists ( I joists that are 24 inch on center). The aquarium will have to go parallel with two joists along a load bearing wall. Others have recommended I sister the two I joists with 2x12 and I should be fine. Would screwing or nailing a 2x12 into the I joist compromise the joist? What would be the best and most cost effective way to reinforce the floor for an aquarium? The house was built in September 2012. Any advice or information would be greatly appreciated. I just don't want to do something that can affect my new home, in a negative way.


I have to question wether the proposed "sistering" method is legitimately proper for your case.

*175 gallons equals 1457.75 lbs.*

With my background in construction and structure engineering, I would recommend considering seriously building a weight bearing framed wall directly underneath the aquarium to permenantly transfer the weight from the floor joists to the concrete slab of your basement.

Otherwise, you will dent your floor joist system in that very spot, causing unwanted problems down the road.

Hope this helps.


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## datahoe (Dec 26, 2012)

Thanks all for your replies! I am beginning to gather a way to proceed with this project


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## datahoe (Dec 26, 2012)

oodssoo said:


> I have to question wether the proposed "sistering" method is legitimately proper for your case.
> 
> *175 gallons equals 1457.75 lbs.*
> 
> ...


oodssoo you have a valid point. I originally thought about having a framing wall built but due to the space I would be losing I shifted focus. Maybe I need to revisit that idea if this will cause some type of structural issues in the future. As i mentioned this is a new home and I hate to cause problems later down the line. Would applying the same suggestions as the others about filling the joist and strengthening it then using jack posts to transfer the weight to the basement slab work the same as building a frame? I still have a few weeks before the aquarium arrives to finish the project. Thanks again for your comments and recommendations


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## lewisthepilgrim (Dec 8, 2011)

You my friend need to install some POSTS. thats a BIG aquarium....alot of added weight on your floor joists. I would much rather see that weight be transferred to the basement underneath. personally I would rather OVERdo it on this one...


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## datahoe (Dec 26, 2012)

Thank you all for the responses. I think I will rather over do it on this one, especially since this is my first home and first real DIY project. So my plan based of of everyone suggestions is to sister the two i-joists below the aquarium as mentioned in the earlier replies. Then I will use the two jack posts I have and place them underneath the joist that is directly under the aquarium for added support and to transfer the weight to the basement floor. I already had purchased two jack posts and a 4x6. Would that be enough to transfer the weight down to the floor? Like I mentioned the aquarium will be about 8 inches from a load bearing wall and will be over one i-joist and 3/4 of the way to the second joist. Is there an approved method to transfer some of the weight to the second i-joist or would the sistering and then using the 4x6 and two jack posts suffice for this application? I will purchase the rest of the items needed this week so I can work on it over the coming weekend. Thanks again for helping me make this project work.


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## handyman_20772 (Jun 28, 2006)

You will probably need to dig and pour concrete for footers under those jack posts to properly transfer the load to the foundation of your house. This may may be required in your area...check with your local building dept. or a structural engineer.


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## Sean'o ATHS (Dec 16, 2012)

also consider upgrades in is tank size later...but look at it this way. If you frame a little room under you could add a sump /filtration etc to run underneath adding capacity ...:thumbsup:


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## Mel43 (Aug 28, 2021)

joecaption said:


> That would work if it can fit in the webbing of the joist.
> Georgia Pacific suggest just using 3/4 plywood or OSB on both sides with constrution adhesive and nails.





woodworkbykirk said:


> most i joists have a 1" gap betwwen the web and the chords so i take 2 layers of 1/2" ply to fill it.. first starting with a 8' peice centered then fill in the ends. then for the 2nd layer just switch it.. start from the end then finish. then a layer of either 1/2" or 3/4 on the outside
> [/QUOTE
> I have TDI I-joists, would the same work with them? The subfloor above the I-joists has water damage/rot approx. 2'x 3'. I watched youtube videos to learn how to fix the subfloor and think I need to add extra strength under all edges of the subfloor patch. So after strengthening the web, can I screw through it to secure a 2 x 4 between the I-joists to support the front and back ends of the patch?


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## ront02769 (Nov 28, 2008)

No need to tear up your concrete for footers. you are not supporting the entire floor above, the existing i joists are doing that.You are just bumping up their capacity. What is the size of the aquarium in feet and inches length, width and depth.


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