This post is a quick talk about leveling an existing subfloor to add a new subfloor on top of it. A very common issue with old homes is a condition known as sagging floor joists. Joists are the structural pieces of lumber usually 2”x10” sometimes bigger sometimes smaller that run the length of a floor from wall to wall. Subfloor is placed upon the joists and finish flooring goes on top of that. Joists can begin to sag over time for many different reasons but sometimes just from age and building methods at the time of construction.
This particular example is on the second story of a 150 year old brick home. The home was structurally sound and there was no visible water damage, termite damage, rot, or cracks in any of the joists. they were just sagging from age and weight. On the first floor the same conditions had already been addressed and some load bearing walls where built under the second floor to help re-secure the sagging joist to keep the sagging from occurring again after the repairs.
At this time it’s ready for the joist straightening techniques. The first step is to stretch strings down the lengths of the room perpendicular to the joists using a line-level to ensure that your repairs will be level. You do this in at least two spots near opposing walls sometimes more depending on the size of the space you’re working on. The next step is to stretch strings parallel to the joists directly on top of the joists and level them off of the perpendicular strings. You do this on every joist that is to be leveled. This allows you to visually see the dips in each joist space and also gives you a place to measure when you are ripping your “furring joists”. The furring joists are the wedges that will be cut to sit on top of the existing joists/subfloor that will bring the grade of the floor up to the strings. The furring joists should be cut from structural grade joist material IE 2x10 or something similar.
When measuring the furring joists its good to check the string in multiple places to make sure you are ripping them at the correct angle to match the sag in the floor. Some folks will even set the furring joist material next to the string and use a scribing technique to mark the string location before making the cuts. I prefer to use the measuring method to reduce the amount of steps involved thus saving time. Once the strips are measure, ripped, and checked against the strings for accuracy it is time to move to the next step of attaching them.
When attaching the furring strips it is always good to make sure the existing subfloor is clean of dirt and debris first. This allows your adhesive to stick to its best ability and also ensures your furring strips will be making direct contact with the subfloor. First run a solid bead of construction adhesive like Loctite PowerGrab down the length of the joists then set the furring strips on top of the adhesive. Next you want to use structural screws such as GRK R4 framing screws to secure the strips to the subfloor. It’s important to use screws long enough to penetrate all the way down through the subfloor and into the joists below. It’s also important to make sure they are framing grade screws as opposed to drywall screws or something of that nature.
Once all of the furring strips are attached to the tops of the joists its time to attach your new layer of subfloor just as you would on new construction. Typically I use 3/4 tongue and groove plywood for this. Again, I put a solid bead of construction adhesive on top of the furring strips just like below before putting the plywood down. Also you want to make sure that you are leaving a 1/8” gap between all intersections of plywood and a 1/4” gap from the outside walls. This allows for expansion and contraction to take place without causing buckling or anything in the middle of the floor. I also like to use GRK R4 framing screws to attach the plywood to the furring joists.
I hope this helps you when working with your historic home project in the future. And as always if you have any questions, concerns, or would just like to say hi please use the comment section below because I would love to hear from you! Thanks for reading!