Sub Floor Support

[Jason Kay]

I have to replace a piece of water damaged subfloor in my bathroom. Before I cut out the subfloor, I know I'm going to have to block the joists to create a "ledge" for the new subfloor patch to sit on. However I know I have engineered I joists. The pic attached is a view from my basement to my first floor. I know the joists are the same as where I need to fix the sub floor. How do I add perpendicular support between the I joists...how do I go about adding parallel (sister?) support to these I beams? I can't drill into the web part of the I joist...I can only use the flange part to drill into. Everything I found online shows non-I beam style joists. Thanks.

 

[wwhitney]

How large any area do you need to replace?

For the edges parallel to the joists, the simplest thing would be to cut out the subfloor to the midline of the joist top flange, then secure the new and old edges of the subfloor to the top flange. Don't cut into the top flange (a light scoring would be OK).

For the edges perpendicular to the joists, that was originally a T+G plywood joint. So the simplest option is to use a floating splice plate to span a butt joint in the subfloor. E.g. a piece of subfloor that's maybe 4" wide, and a little shorter than the joist bay width, with 3 or 4 screws on each side of the butt joint, perhaps with some glue or construction adhesive.

Make sure your replacement subfloor has the grain oriented perpendicular to the joists, like the original.

Cheers, Wayne

 

[Sponsor]

 

[Jason Kay]

How large any area do you need to replace?

For the edges parallel to the joists, the simplest thing would be to cut out the subfloor to the midline of the joist top flange, then secure the new and old edges of the subfloor to the top flange. Don't cut into the top flange (a light scoring would be OK).

For the edges perpendicular to the joists, that was originally a T+G plywood joint. So the simplest option is to use a floating splice plate to span a butt joint in the subfloor. E.g. a piece of subfloor that's maybe 4" wide, and a little shorter than the joist bay width, with 3 or 4 screws on each side of the butt joint, perhaps with some glue or construction adhesive.

Make sure your replacement subfloor has the grain oriented perpendicular to the joists, like the original.

Cheers, Wayne

Probably a few feet wide. Not sure yet. I've a visual person....is this what you're describing? Is a 2x4 better than plywood for this? Thanks.

 

[wwhitney]

No, no connection between the splice plate and the joists. The splice plate is just a subfloor to subfloor connection for the edges of the patch that are perpendicular to the joists. Plywood is definitely better for that.

Cheers, Wayne

 

[Jason Kay]

No, no connection between the splice plate and the joists. The splice plate is just a subfloor to subfloor connection for the edges of the patch that are perpendicular to the joists. Plywood is definitely better for that.

Cheers, Wayne

I appreciate the help, but I can't visualize what you're saying. Could you sketch it out?

 

[wwhitney]

Yeah, was working on that. Here's a crude Paint sketch, where black is the original I-joists/subfloor, red is the subfloor patch, blue is splice plate, and green is fastener. The top is an elevation (section), the bottom is a overhead view (plan view) where the original subfloor is omitted for clarity. Obviously not to scale, and your patch may be bigger, but the perimeter details of the subfloor patch are what matter.

Cheers, Wayne

 

[Jason Kay]

Yeah, was working on that. Here's a crude Paint sketch, where black is the original I-joists/subfloor, red is the subfloor patch, blue is splice plate, and green is fastener. The top is an elevation (section), the bottom is a overhead view (plan view) where the original subfloor is omitted for clarity. Obviously not to scale, and your patch may be bigger, but the perimeter details of the subfloor patch are what matter.

Cheers, Wayne
View attachment 79853

Ah, I see. Thank you. One other wrinkle...I haven't yet opened the sub floor, but there' the possibility that I don't have access to the top flange of one of the I joists, because it may be under a wall. If that's the case, I have to somehow build out the web section of the I Joist to rest the patch on. Suggestions on that

 

[wwhitney]

Going back to your original question, I see that your flanges are solid sawn 2x material. So you could use flatwise 2x4 blocks between top flanges, connected only to the top flanges of the joists, with two toe screws to each flange, into the narrow edge of the flatwise 2x4. The edges of old and new subfloor (perpendicular to the joists) would still get screwed to the 2x block, and I'm not sure if that adds any advantage compared to the floating splice plate that is not attached to the blocks. I guess it would let you use longer screws.

Either way would be fine for perpendicular to joists edge supports. The floating splice plates idea has the advantage of only requiring access from above.

Cheers, Wayne

 

[wwhitney]

Ah, I see. Thank you. One other wrinkle...I haven't yet opened the sub floor, but there' the possibility that I don't have access to the top flange of one of the I joists, because it may be under a wall. If that's the case, I have to somehow build out the web section of the I Joist to rest the patch on. Suggestions on that

I take it you're talking about a wall parallel to the joists. That would be more hassle to deal with, so it's worth cutting back only to the mid line of the first joist within the room, and carefully considering before trying to replace the subfloor between that joist and the wall. Give it a good screwdriver durometer test, compared to the undamaged subfloor elsewhere, and replace it only if it seems pretty bad.

If you have to go past the first joist under the room, is the wall load bearing? If it is, there should be cross wise blocks already to support the load bearing wall. Then with respect to the those blocks, you can just handle edge using the "perpendicular to the joists" detail, relative to the blocks.

If it's not load bearing, then you'll basically need to add your own blocks, which will also support the wall in that area. The flatwise 2x4 blocking to the I-joist top flanges would be a good way to do that, 24" o.c. or more frequently, with one at each corner of your cutout. Then again handle that edge with the "perpendicular to the joists" detail, relative to your new blocks.

Cheers, Wayne

 

[Jason Kay]

I take it you're talking about a wall parallel to the joists. That would be more hassle to deal with, so it's worth cutting back only to the mid line of the first joist within the room, and carefully considering before trying to replace the subfloor between that joist and the wall. Give it a good screwdriver durometer test, compared to the undamaged subfloor elsewhere, and replace it only if it seems pretty bad.

If you have to go past the first joist under the room, is the wall load bearing? If it is, there should be cross wise blocks already to support the load bearing wall. Then with respect to the those blocks, you can just handle edge using the "perpendicular to the joists" detail, relative to the blocks.

If it's not load bearing, then you'll basically need to add your own blocks, which will also support the wall in that area. The flatwise 2x4 blocking to the I-joist top flanges would be a good way to do that, 24" o.c. or more frequently, with one at each corner of your cutout. Then again handle that edge with the "perpendicular to the joists" detail, relative to your new blocks.

Cheers, Wayne

Is this what you mean (not a 2x4 but same idea)? The top of the 2x4 gets screwed into the top flange. I should be able to get a screw or 2 into the bottom flange as well.

 

[wwhitney]

By flatwise 2x4 I mean something spanning between joists, attached to the top flanges only.

So if your joists are 16" o.c. and have flanges 2.5"wide (guessing), then by flatwise 2x4 I mean a piece of solid sawn lumber 1.5" x 3.5" x 13.5" long, with one 3.5" x 13.5" face in contact with the subfloor, and the 1.5" thickness coinciding with the the 1.5" height of your 2x I-joist top flange. Each end of each 1.5" x 13.5" face gets a toe screw into the joist top flange, 4 screws total.

In other words, the same as the blue splice plates, but 2x4 and long enough to hit the joist top flanges snuggly.

Cheers, Wayne

 

[Jason Kay]

By flatwise 2x4 I mean something spanning between joists, attached to the top flanges only.

So if your joists are 16" o.c. and have flanges 2.5"wide (guessing), then by flatwise 2x4 I mean a piece of solid sawn lumber 1.5" x 3.5" x 13.5" long, with one 3.5" x 13.5" face in contact with the subfloor, and the 1.5" thickness coinciding with the the 1.5" height of your 2x I-joist top flange. Each end of each 1.5" x 13.5" face gets a toe screw into the joist top flange, 4 screws total.

In other words, the same as the blue splice plates, but 2x4 and long enough to hit the joist top flanges snuggly.

Cheers, Wayne

I'll try again. Here's what I pieced together based on your suggestions. The left side joist is definitely under a wall. So the patch plywood (the highlighted yellow dotted line) will lay over these 2x4 blocks which run perpendicular. These 2x4 each get toe screwed into the top flange. The top, bottom and left sides of the patch will be screwed into theses 2x4 blocks. On the right side, it will lay over the I-joist. Edit: Nothing will touch the bottom flanges. Am I close???

 

[Reach4]

Another thing you could consider (at least for that wall area) is to cut out an area, leaving 1.75 of existing subfloor that is not in really bad shape around the perimeter. Screw and glue one half of a 2x4 under the remaining subfloor, and put the new subfloor plywood on the other half. Glue adds a lot of quiet strength, but you would not want that to get in the way of dropping in the new plywood. I am not a pro.

Just an alternative. Another idea would be to have pieces of wood on the bottom piece next to the web. The wood would go up nearly vertical to the subfloor next to the top piece of joist. Then attach a cleat to counteract any force pushing that new wood away from the top of the joist. No big stress. I would not want to push that joist bottom wood away from the center web.

 

[Jason Kay]

Another thing you could consider (at least for that wall area) is to cut out an area, leaving 1.75 of existing subfloor that is not in really bad shape around the perimeter. Screw and glue one half of a 2x4 under the remaining subfloor, and put the new subfloor plywood on the other half. Glue adds a lot of quiet strength, but you would not want that to get in the way of dropping in the new plywood. I am not a pro.

Just an alternative. Another idea would be to have pieces of wood on the bottom piece next to the web. The wood would go up nearly vertical to the subfloor next to the top piece of joist. Then attach a cleat to counteract any force pushing that new wood away from the top of the joist. No big stress. I would not want to push that joist bottom wood away from the center web.

This is under the toilet area...this doesn't sound like it would be strong enough. Don't want someone to fall through the floor while sitting on the toilet

 

[wwhitney]

Am I close???

Yes, that matches what I described. Two things are missing, though: "treat the edge like a perpendicular to the joists edge" means you need to support the left edge of your subfloor patch between the flatwise 2x4 blocks, either with plywood splice plates or with more flatwise 2x4 blocks. And the edges of the existing subfloor at the top and bottom (in the drawing) edges of the patch should also get screwed to the flatwise 2x4s; and so those flatwise 2x4s should be centered along the seam. A couple further comments:

The two toe screws connecting each flatwise 2x4 to the I joist solid sawn top flanges, that seems like an OK connection, not super. Someone might have a suggestion for a better detail there. [E.g. if you have a pocket hole jig, you could use some face pocket screws.] Also, if you look up the TrusJoist TJI installation manual (if it exists online), it might have a better detail. Using an upright block seems like overkill. For a header between two joists carrying a cut joist, the TJI manual shows padding out the web flush with the flange and using joist hangers, also overkill for this.

Also, if the edge of your subfloor cut is really very close to that joist under wall, you could just screw a member (e.g. a 2x2) lengthwise along the edge of the top flange to in effect make the flange wider, so its right side is now past the edge of your cutout. Then you can treat the left side of the patch the same as the right side, you don't need the intermediate 2x4 flatwise block, and the top and bottom 2x4 flatwise blocks could remain or be replaced with the plywood splice plates. IMO it would be worth cutting out some drywall to get your subfloor cut closer to the edge of the flange, if necessary.

Cheers, Wayne

 

[Jason Kay]

Yes, that matches what I described. Two things are missing, though: "treat the edge like a perpendicular to the joists edge" means you need to support the left edge of your subfloor patch between the flatwise 2x4 blocks, either with plywood splice plates or with more flatwise 2x4 blocks. And the edges of the existing subfloor at the top and bottom (in the drawing) edges of the patch should also get screwed to the flatwise 2x4s; and so those flatwise 2x4s should be centered along the seam. A couple further comments:

The two toe screws connecting each flatwise 2x4 to the I joist solid sawn top flanges, that seems like an OK connection, not super. Someone might have a suggestion for a better detail there. [E.g. if you have a pocket hole jig, you could use some face pocket screws.] Also, if you look up the TrusJoist TJI installation manual (if it exists online), it might have a better detail. Using an upright block seems like overkill. For a header between two joists carrying a cut joist, the TJI manual shows padding out the web flush with the flange and using joist hangers, also overkill for this.

Also, if the edge of your subfloor cut is really very close to that joist under wall, you could just screw a member (e.g. a 2x2) lengthwise along the edge of the top flange to in effect make the flange wider, so its right side is now past the edge of your cutout. Then you can treat the left side of the patch the same as the right side, you don't need the intermediate 2x4 flatwise block, and the top and bottom 2x4 flatwise blocks could remain or be replaced with the plywood splice plates. IMO it would be worth cutting out some drywall to get your subfloor cut closer to the edge of the flange, if necessary.

Cheers, Wayne

Final (?) revised drawing. I added 2 2x4s to support the left side of the patch. So I screw these new 2x4 (or plywood) into the top flange. Then I can lay the patch on top of this and screw the entire length of the left side into all these 2x4s I added.

 

[wwhitney]

Yeah that ought to work. Although it's not clear to me how you'd attached the (2) 2x4s you just added (running up down in the drawing) to the joist flanges or to the left-right 2x4s, so maybe plywood splice plates would be better. And it seems like a lot of blocking for a 36" x 15" patch, I'll give a little thought to whether there's an easier way.

What's the exact dimension (left-right in your drawing) between the roomward edge of the wall plate and the roomward edge of the joist flange?

Cheers, Wayne

 

[Jason Kay]

Yeah that ought to work. Although it's not clear to me how you'd attached the (2) 2x4s you just added (running up down in the drawing) to the joist flanges or to the left-right 2x4s, so maybe plywood splice plates would be better. And it seems like a lot of blocking for a 36" x 15" patch, I'll give a little thought to whether there's an easier way.

What's the exact dimension (left-right in your drawing) between the roomward edge of the wall plate and the roomward edge of the joist flange?

Cheers, Wayne

You're right...if I can do that, I wouldn't have this problem in the first place. Your suggestion of 2x2 may be easier since I can screw them into the flange from the right side and not from the top. If I can do a 2x2 all the way down the left side, I don't need the horizontal (in the picture) 2x4s going perpendicular to the joists. Opening in about 1'x3'.

 

[Reach4]

I infer that you see negligible value to construction adhesive or Gorilla glue in addition to screws.

 

[Jason Kay]

Opened the floor and this is the situation. The back joist is about 10 inches under the next rooms so don’t have access to it. Can I cut a piece of ply wood in a “L” shape and glue and screw it under the existing sub floor to create a ledge for the patch?