Repairing damage to joists

[branchlake]

Hello - I have owned a 1930s Edwardian semi-detached house for about eight months. It is a very standard layout: when you walk in the front door, the stairs go up to the first floor along the outside wall, the corridor next to the stairs goes back to the kitchen, and the front and rear lounge are off to one side.

The house still has its original floorboards. There is a substantial crawl space under the house (2ft under the corridor and stairs portion, deepening to 4ft under the lounges), but this crawl space is tricky to access - you have to go under the stairs and the hatch is tight.

When I first moved into the house, I noticed that some of the floorboards at the base of the stairs felt a bit bouncy - they make noise when you walk on top of them (I can only describe it as a sort of "banging" or a "crunching" noise) and they don't feel 100% sturdy - not ready to collapse, but not rock-solid either.

I have two goals with the house: I want to insulate the ground floor from underneath and I want to put in a new floor. The couple up the street (identical house) actually just put another layer of flooring right on top of the old, original floorboard and I might give this a shot, too.

The other day I finally got into the crawl space and inspected the joists that were making the funny noises. I saw what looked like water damage (there was a wavy line of discolouration, kind of silvery on the wood) probably about 18in or so long on three or four of the joists. There must have been a leak or a flood at some time in the past. From knocking on the joists, however, they don't feel rotten. They feel as sturdy as ever, but they sound a bit dodgy when trodden upon.

Thanks for getting through all of this! Here's my question: I've read in several cases how people have just reinforced the joists by bolting supplementary joists to both sides of the ones that are already there.

Is this solution practical and straightforward to do? Will these additional joists not be too heavy? Should I get a professional in to do this or is it easy enough to do on one's own? If the surveyor neglected to pick this up (I paid for a full structural survey), would I have cause to raise a legal case against him?

Many thanks and kind regards

[Nestor]

If the wood feels solid, and you don't see any cracks or breaks in the wood, then my guess would be that the "spongy" feel of the floor in that area is not due to the joists, but due to the subfloor or floor boards.

A bit of discolouration on the joists is no indication that they're rotted. What you saw might have been slime from insects (slugs) crawling on the wood.

Here's how they build a floor:

IN THE BEGINNING was the concrete or stone foundation.
On top of the foundation they set floor joists and use blocking to keep the floor joists vertical.
On top of the floor joists they nail down 1X6 boards or glued and screwed down 3/4 inch T&G plywood to make the "sub-floor".
On top of the subfloor they built all the interior and exterior walls of the house.
Then, they put down 1/4, 5/16 or 3/8 inch "underlayment" in each room of the house.
Then they did everything else, and finally installed the finish flooring over the underlayment in each room.

That's just the general game plan. Some houses have 3/4 inch OSB subfloor and 1/2 inch particle board underlayment, and I expect that some houses have floor boards instead of underlayment in each room.

Now, if you feel a softness to the floor in certain areas, it's not necessarily the floor joists that are weak there. It could be that the subfloor is warped a bit there, and when you walk over the floor you press that subfloor down flat. It also could be that the underlayment is lifting up from the subfloor in that area.

I don't know whether you have any underlayment under your floor boards or not, but if your joists don't make any noise when you push up on them from below, then I kinda doubt they're the source of the noise/feel when you step on them from above. I would suspect that you have a gap between your joists and subfloor, or between the subfloor and whatever is above that (underlayment or hardwood floor boards) that's causing the spongy/crunchy feel.

PS: Most people don't know it, but floor squeeks are caused by the subfloor moving up and down. You see, years ago it was common to simply nail the subfloor down to the floor joists. The problem was that the lumber used for the floor joists was often still too wet to use or it had gotten wet at the job site by being rained on. Lumber shrinks as it dries, and the result is that a 2X12 floor joist can shrink a good 1/4 inch in height (from 11 1/2 inch tall to 11 1/4 inch tall) as the joist dries up. The result was that a gap would open up between the top of the floor joist and the underside of the subfloor. When you walked over the floor in those spots, the subfloor would be pressed down, and the squeek you heard was due to the subfloor and nail rubbing on one another (and vibrating as a result). Modern practice is to "glue and screw" the subfloor down to the joists to prevent a gap from developing between them, thereby eliminating the possibility of squeeks developing in the floor.

PS2: Wood cells are shaped like drinking straws that are closed off at their ends. Typically a wood cells is about 80 times as long as it is in diameter, so it's shaped like a very long drinking straw. When a tree is alive, both the cell interiors and the wood cell walls are full of a liquid which is mostly water. Once the tree is cut down and the wood starts to dry out, the ONLY change that can be observed in the wood is that it becomes lighter as the water inside the wood cells evaporates. Only after the water inside the wood cells evaporates, the water inside the wood cell walls starts to evaporate, and the result is that the cells walls become thinner and stiffer (just in the same way that an orange peel gets thinner and stiffer as it dries out). The result is a dimensional change in the wood as all the cell walls become thinner as well as a strengthening of the wood as those cell walls become stiffer. That is, the wood shrinks and gets stronger as it dries out. However, because that shrinkage is entirely due to the cell walls becoming thinner, and because wood cells are shaped like long, thin drinking straws, the shrinkage across the grain is very much larger than the shrinkage of the wood along it's grain.

And, that process is totally reversible. If you get wood wet, then the water will initially be absorbed into the wood cell walls, and the wood will swell up again. It won't be until the cell walls are saturated with moisture and further water is absorbed into the cell interior space that the wood will stop swelling as it absorbs more water.

It's that shrinking of wood as it dries that causes floor squeeks and drywall nail pops in walls and ceilings.

[Nestor]

To answer your question, yes, it's common to "sister" damaged floor joists, and that's done by attaching another joist to the side of the damage joist with nails or screws.

However, what's not as commonly known or commonly done is to strengthen your existing joist. By gluing a 2 inch thick by 1 1/2 inch wide piece of clear fit to the bottom of a fir floor joist, you can convert a 2X8 into a 2X10 or a 2X10 into a 2X12.

All that is required is that:
1.) The wood you add is as strong or stronger than the wood the joist is made of, and,
2.) That the glued joint is as strong or stronger than the wood the joist is made of (so that the wood would break before the glued joint does). And, I expect that LePage's PL Premium construction adhesive would fit the bill here.

And, considering that steel is 15 to 20 times stronger than wood, you can also strengthen floor joists by gluing steel strapping to the sides of the joists instead of sistering the joist. Again, the only requirement is that the glued joint between the wood and the steel be as strong or stronger than the wood (as it goes without saying that the steel will be stronger than the wood). Assuming mild steel is 20 times stronger than wood, you would theoretically get the same increase in rigidity by gluing a 5/64th by 11 1/2 inch wide strip of steel to the side of a fir 2X12 as you would by sistering the joist.

But, because such a thin strip of metal would tend to twist easily, a better design would be to glue something like 1 1/2 inch wide by 1/8 inch steel strapping to the sides of the joist at top and bottom on both sides, thereby making a very narrow "I" beam. Then, in order for the joist to bend in either direction, not only the wood at the top or bottom of the joist, but also the steel in those locations, would have to stretch. Because steel is very strong in tension, it resists stretching very much more than wood, and that's where the rigidity of such a modified floor joist would come from.

I don't believe the cause of your floor problems are due to the floor joists, but if it turns out that it is, keep in mind that sistering your joist(s) is not your only option. You can use steel and LePage's PL Premium construction adhesive to strengthen your existing joists as well. If you go this route, the requirement is that the glued joint between the steel and the wood doesn't slip at all, and so you need to use a glue that forms a bond between the wood and the steel that's as strong or stronger than the wood.

And, you can pop into the Engineering Faculty at your local university and ask any prof you find there if what I'm saying is correct, and he'll confirm it. Or, pop into the Engineering Library and ask any 2nd year or later student, and he/she'll confirm it too.

[pushkins]

Adding sister joists is a fairly straight forward project, you want the new joists to be the full length of the existing joists, add a good amount of adhesive to the existing joist and slide in the new joist making sure it's sitting on the ledger, the new joist should be toe nailed into the girder. Once they are side by side either nail ( I prefer screws) every 12" on center in rows of three (upper, middle, lower). Make sure the joists pull up tight to each other

Also note that using glue alone would not pass a city inspection, while glue should be used in sistering methods, code requires that a mechanical means of fastening be also used (nails, bolts etc...). Using glue alone for a chemical connection between steel and wood is also not advisable.