What options do I have to repair this chair?

Question

I have a 100 +/- year old oak dining set with chair seats that have become quite brittle.

Over the years planks on the chairs have split and I have glued them back together. At one point I added two 1/8" x 1 1/4" aluminum bars screwed to the underside to provide additional support, but this has had limited success and the small screws have popped out at places over time. Recently, two seats broke and I am now seeking a better way to reinforce them. My first thought was to provide a wood cross member support at the metal bar locations secured to be part of the seat. In this approach, the reinforcement pieces provide additional stiffness to the seat, but do not connect directly to the legs. For this reason, the stiffeners must be well attached to the seat to transfer any loads back to the seat at the corners where the seat rests on the legs. Although this can be accomplished with continuous gluing, there is the problem with the different expansion and contraction due to grain direction of the seat and stiffeners. If I attach the stiffeners to the seat with screws in slotted connections at each end of the stiffener, I am placing all the weight from the center of the seat on the two end screws that secure the cross member. The seat is 7/8" (or less at seat impression) thick and so a lot on load is placed on a small area of a very old oak seat where the screws penetrate.

How can I refine this concept or is there an alternative means of support that I have not considered?

EDIT: Issues raised in several answers led to adding more info.

I double checked and three chairs have cracked and not one crack is at a glue line. I added a photo of another chair to illustrate the cracking. In addition, these chairs have been well treated. No standing, limited use and I try to use other chairs with heavier guests. The wood has split itself. I believe the screws in the aluminum straps may have failed because they do not provide adequate vertical support. As the chair is loaded, the seat sags and places lateral load on the screws causing them to work loose over time (they only penetrate the wood about 1/4"). Ast Pace suggested that the triple leg braces might make the legs more resistance to movement in the seat. That's interesting, but I'm not sure how to determine if it is so and would be substantial enough to cause the wood to split.

Answer 1

As the picture of the underside of the chair isn't showing currently I'm not entirely sure I have the right picture in my head about what's going on underneath, so I may revise this once the link to the pic is fixed.

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It seems you have multiple issues you need to address here, the first with the screws popping loose from their holes and then the bigger problem, the wood or glue lines splitting on the seat.

Loose screws
With the screws, once the hole has begun to flake the hold is on a downward spiral if the screw is driven and removed multiple times and only a repair at the site is going to solve the problem. There most famous fix for stripped or loose screw holes is possibly inserting one or more matchsticks (or toothpicks, or just slivers of wood) into the hole and then driving the screw back, the new wood bridging the gaps and providing something for the screw to bite into. This can work well if the stripping is light, but I think there's a limit to what it can do once the hole is really chewed up even if you do the extra measure and apply glue to the matchsticks/slivers.

Covering the matchsticks with glue to help the process along points to what I think is the better fix in troublesome cases: actually filling the hole with filler. The simplest fill is just a mix of wood glue, sawdust (actual sawing waste) and a little finer dust to fill in the smaller gaps. You can also use something like Durham's Water Putty, which can be tinted to match the wood colour if needed. Another option is to make a filler from epoxy, mixed to a stiffish paste with sanding dust.

A more modern version of the matchstick trick is to insert a solid-core copper wire into the hole, then drive the screw back in. I haven't tried this myself but it seems to work very well and as it's so fast you might like to give it a shot.

Another option open to woodworkers is to drill out the hole and then glue in a dowel or wood plug. Dowels do seem to work OK in practice, but wood plugs would be preferable as the hold of screws in long grain is much greater than in end grain as on a dowel.

The seats
Despite what you've said in the Comments below in this picture at least the failure is on a glue line, it's not the wood itself cracking. This is what I'd expect, as in-service cracks along the grain on a chair seat I think would be nearly impossible with the chair used normally, i.e. assuming nobody stood in the centre of the seat to change a lightbulb. While glue joints can be "as strong as the wood itself" as the saying goes (actually stronger in most cases if done right) old glue joints can be the weak spot and it certainly appears that this is the case here.

It would be best to remove the seats entirely from the chairs if that's possible. Then assess every glue joint and repair any that are failing in one operation, after which you're assured the seats aren't going to fail and you can focus on the rest of the structure of the chair.

If the original is bonded with hide glue and if you are doing the repair with hide glue then you don't have to be that OCD about cleaning the joint faces before adding fresh glue and clamping up, although obviously remove any remaining glue of a noticeable thickness. Note: if you warm the wood first it makes a big difference in how well hide glue bonds. This is especially important when working with it during colder months. Running a hairdryer along both joint faces prior to glue application will do the trick.

If on the other hand you are doing the repair with any PVA-type glue (white or yellow, same deal) it doesn't matter if the chair was originally glued with hide glue, PVA or something else, the joint faces must be scrupulously cleaned to achieve a proper bond. If the glue doesn't have direct access to the wood an inferior bond will be formed — don't underestimate this, it's the difference between the joint being literally stronger than the wood (yes, even with oak) and being capable of splitting apart.

Note on clamp pressure:

• With hide glue moderate clamp pressure should be sufficient.

• With PVA glues very firm clamp pressure is the order of the day. Although you can get away with less (most woodworkers actually under-clamp) for the strongest bond to form you want to squeeze out all excess from the joint, resulting in a glue line only a thou or less.

Answer 2

Certainly, the effects of wood expansion and contraction caused by changes in humidity are exacerbated by the aluminum straps.

It appears to me that the problem with the cracking seats might also be caused by overloading. Even a merely overweight person sitting on the seat would not cause any harm because the weight would be nicely distributed over most of the area of the seat. However, a man of fairly normal weight standing on the chair, using it as a stool for reaching high places, might create a load that would cause the seat to break. This would be especially true if the seat were already weakened by the stress of drying wood pulling against the unforgiving strapping.

My suggestion is to remove the straps, carefully glue the seats back together, ignore cross braces for the reasons that you have already stated, and avoid overloading the chairs.

The big fallacy in my comments is addressing the original failing of the seats before the metal straps were installed. It seems to me that the legs would be flexible enough to allow the seat to expand and contract without cracking, but when they are triply reinforced with three rails, perhaps they are stiff enough to cause a problem.

Answer 3

OK, it took me a while to get around to it, but I finally attempted a repair to these chair seats. First I gently pried open each crack in the seat and injected Tightbond-II glue using a hypodermic type glue needle (20mm) which worked fine for getting lots of glue into the tight gaps. Standard clamping closed all the cracks. In some cases some gouges remained where some splinters had broken off (as wide as 1/8" wide and deep). I filled these with a mix of fine sawdust mixed with the oil varnish finish for the chairs. After drying for a day I sanded the surfaces smooth and applied a new coat of varnish over the entire seat. Finally, to provide additional support I added 3/4" x 1 1/2" stiffeners (see picture) screwed into the seat from the bottom. The screw holes are over-sized so that some seat expansion will be accommodated. The reinforcement strips are secured to the seat near the connection points to the legs to transfer the loads from the strips to the legs with minimum stress on the seat itself.