How Much Glue Is Enough?

The Classic Huntboard article in Popular Woodworking June 2009  (issue #176) includes traditional and non-traditional construction methods. The traditional methods involve mortise-and-tenon construction where the case back and sides meet the legs. That’s cross-grain construction.

The photo on page 45 (shown above) shows how the joint is divided and the caption explains why. In the text, when the time comes to assemble the case, I write, “First, add glue to the leg mortises for the back, then spread glue on the tenons and slip the back into the mortises.” If you read this to assume that I add glue to all the mortises and tenons before I slip the joint together, you read it correctly. However, there are many woodworkers who don’t agree with that method. A synopsis of friendly e-mail exchange is posted below.

A reader wrote: “It’s entirely possible that I missed it, but I didn’t see any comments by Glen as to whether a reader should glue all three mortises on the case sides, or glue the bottom and let the top two float, or glue the top mortise, pin the bottom one (and elongate the hole in the tenon for the pin), and let the middle one float (this last option would be my choice). Obviously, this affects how the piece will age – if all three mortises are glued, it’s likely that the case sides will crack.  While historically accurate, that result might greatly disturb some readers, especially newbies.”

My response was: “I’ve built many pieces of furniture (from huntboards to highboys and lowboys) that have case sides mortised into the legs. As a novice woodworker, I glued the sides securely into the legs without any thought of wood movement. You would think the case sides would or could crack given that arrangement. However, I have yet to see this happen to any of the pieces. Today I practice the same method , I fully glue the tenons and do not worry about wood movement issues and the possibility of cracks.

If you calculate the total movement of the sides of the huntboard, you’ll arrive at a maximum potential movement of around a 1/8″. It is my contention that the glues we use and the “give” of the woods are forgiving enough to counteract any movement. Add in the idea that most homes do not experience extreme changes in humidity and I don’t see this as an issue.”

OK, here are my thoughts on this issue. A board moves from the center outward when affected by seasonal changes. The middle mortise-and-tenon joint on the huntboard will not lose its hold. The top and bottom joints are the ones under stress as the wood expands and contracts. In accordance, the wood movement of 1/8″ overall is reduced to half , 1/16″ from the center of the board to the top and a 1/16″ from the center to the bottom. I believe the wood is resilient enough to handle this movement.

On a 36″ or wider tabletop that has breadboard ends, I work differently. I know the top is going to move. I’ve seen it happen over the course of a day or two, let alone during seasonal changes. In this scenario, I glue the center tenon joint and peg the remaining mortise and tenons after I enlarge the holes to allow for movement.

I asked Chuck Bender , a fellow woodworker with 30 years of furniture construction behind him , for his take on this issue. His answer was “In all these years, having made plenty of sideboards and huntboards, I can’t recall any that seriously cracked.” He glues the mortise-and-tenon joint fully and has experienced no major problems.

Chuck did correct me on one item though. His wrote, “During the ‘period’ the humidity levels ranged pretty widely but the change from the low to the high was rather gradual. In today’s homes, we can go from open windows and doors on an early October day with a reasonably high humidity to the next morning having the forced hot air system making the house 90Ã?° Fahrenheit with virtually no humidity. It’s that kind of nonsense that kills furniture.”

I have to agree.

Additionally, I think a bigger problem , and possibly the cause of case side cracks over and above the glue question , is how tenons are fit. If tenons are fit too tightly from top to bottom and, as a result, there isn’t enough room for expansion and contraction, your work is much more prone to issues such as cracks. As the wood moves and the glues creep, stresses build and something has to give.

Do you agree? Disagree? Let’s hear your take on this issue. Leave a comment. Also, if you a photo of a project you’ve built that has developed a crack, send me the photo and I’ll post it. There’s no better way to learn than from experience.

- Glen D. Huey

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10 thoughts on “How Much Glue Is Enough?

  1. Chuck Bender

    Folks, wood moves. In all the years I’ve been working on period pieces, many of which are museum quality, I’ve never seen one that wasn’t still susceptible to changes in humidity. I hear LOTS of woodworkers talk about temperature change. As we say here in the mid-Atlantic states "It’s not the heat, it’s the humidity." If you buy a perfect period Philadelphia highboy from the third quarter of the 18th Century, and put it in a house that is kept very warm and dry, you’re asking for wood movement troubles.

    For centuries case joinery has had to deal with cross grain construction. Realistically, you can’t copy period pieces without having some cross grain construction.

    With Glen’s huntboard, he left room in each of the tenons on the case side for expansion and contraction. If he were to try to leave the joint dry over a portion of the case side, where should he do it? If he glues the upper portion of the joint, the bottom will shrink taking the drawer runners with it causing drawer problems. If he glues the bottom of the joint, the top of the side will shrink causing stress where he has the top attached. From a "commercial" stance, both of these alternatives leave the piece susceptible a broken joint if someone drags the piece around the house because the leg isn’t firmly affixed. The little bit of wiggle room in the joint, combined with the leverage from the leg scraping against a floor (or worse…a carpet) is enough to cause a complete joint failure. I know because I’ve fixed pieces with this problem.

    Someone mentioned draw boring. Almost all my sideboards, huntboards, highboys and lowboys have had the same construction as Glen’s huntboard and nearly all have been held together with pegs that were draw bored. In period, draw boring was used because clamps were expensive and few in a shop. It is a method of construction that allows the builder to work with few, if any, clamps. To this day I have only had a few pieces develop minor cracks. Even if a side were to crack from leg to leg, because of the construction method used, it wouldn’t be a major structural problem because the strength of the overall joint would still be intact. To me, and my customers, trading off genuine, solid period construction for contemporary theory on wood movement just isn’t worth it.

    I do attach table tops, lowboy, huntboard and sideboard tops with buttons in most cases. This method does not change the structural integrity of the overall piece. In the end, I’d rather fix a small comsetic crack than try to re-attach a broken leg.

  2. Tim Crossan

    A question for the experts.
    my son has a chest that is made entirely from aromatic cedar. The top cracked and was repaired by someone said to be knowledgable in furntitue building.
    The ends and the front of the top/lid have a molding that is glued and nailed. In that the top is only about 17" wide could the cause be the cedat and not the molding?
    From the conversation here it would seem that the assembly should have survived. I am wondering if the cedar is too unstable to withstand the captivity of the molding.

    Tim C

  3. Chris F

    I think gluing all three is asking for trouble. I repaired a 20" wide red oak table top which appeared to have split because a strip of molding had been glued across the end.

    Also, people travel a lot more than they used to. If a piece is made in Arizona and gets shipped to Florida, there could be a lot of wood movement.

  4. Stephen Kirk

    I can take the middle of the road and say that if I can design/assemble with wood movement in mind, then I will. I too have seen pieces that didn’t allow for movement stay fine for years and years, while seeing properly designed pieces crack. And I’ve seen the opposite of both.

    These days I think owners and the environments the pieces are in are more demanding. Temperature swings occur faster and I think houses are drier than they used to be. Owners expect a lot more from the furniture, too, such as air tight dovetails, inside/outside finished, drawers that don’t stick even if it gets humid, flawless finishing, etc. Given these things, it’s probably best to decrease the chances of structural problems even if it’s not always necessary.

  5. Tom Volz

    1. Structurally the back tieing into the legs and a top with breadboard ends is the same.
    2. Finishing has a lot to do with expansion and contraction of the furniture component. Finish may be only on one side( e.g. do you finish the underside of a tabletop? or the inside of the carcase?) Glue also acts as a sealer, whereas no glue allows the wood to absorb moisture or dry out. If there is no glue on the ends of a tenon, the leg can act as a wick and draw moisture from the tenon to the tip and upper end of the leg (who seals the top or bottom end of a leg?) The finish doesn’t stop the moisture, but sure slows it way down.
    3. Table tops or backs that are multiple boards, and edge glued, are essentially sealed by the glue in the joints giving more stability to the piece.
    4. If an individual board has grain that is partially rift sawn transitioning to flat or it is flat sawn from near the center of the tree and is unfinished on one side, it is more likely to absorb/desorb moisture and crack.
    5. Some boards may be weak in a growth ring (inherent flaw) and pre-conditioned to crack or split. Or improper drying may have locked in stresses in the wood that may be worsened by where the tenon (or gap in the tenon) was placed.
    6. Structurally, gaps in the tenon are stress risers. If the tenon is unglued and can move in the leg mortise, it WILL move. If the fit of the tenon in the mortise is too tight, the back is more likely to crack from restricted movement.

    Overall, assuming that the boards will move and allowing for that movement is the best approach. In the case of the sideboard/ huntboard I would pin the tenon near the top of the leg and have the 2 lower pins in slots with no glue on the lower tenons, much like breadboard ends fixed at one end instead of the middle.

  6. David Barbee

    I see how that would strengthen the leg. Got a little to focused on wood movement there. I think most of what woodworkers are taught on wood movement leaves a little wiggle room. Type of wood, defects in wood, moisture content, finish, and environment play a significant role in the equation. So our accepted rules have been formed to allow some wiggle room. I have seen numerous antique pieces that had major wood movement issues with no cracks. What’s the secret? Personally, I think it’s mostly wood selection and a little luck. As they say, "only time will tell."

    A few years ago I built my mom a twin set of bookcases to replace some that my dad had built, probably, 20 years ago. So we had to remove the books and get rid of the two old bookcases. He had built his bookcases out of 1×12 white pine. The shelves were just butt joints with 3 screws. As we started pulling books off the shelves I noticed what appeared to be some cracking in one of the cases. After removing all the books I could see that one of the cases had had literally had cracked all the way around the case. The only thing holding it together was the shelves. The case standing next to it was identical but hadn’t cracked what so ever. The cases were built identical an shared the same environment. Why did only one crack? Natural selection maybe???

    David Barbee

    PS. I wonder if the fact that these are short length of wood helps? Less surface area to absorb moisture? Just a thought…

  7. Glen

    David,
    That is exactly the point of the entry – to challenge what is believed to be true. Like you, many woodworkers (myself included) are deluged with writings that profess to be the only answer. Yet, here are two furniture makers that have built many pieces and have found different results. That should make you think.

    Breaking the tenon into three sections is not done to affect the holding power of the joint. The divisions provide additional support for the joint in that the bridges between the mortises add strength. One long mortise would make the leg considerably weaker.

    Glen

  8. David Barbee

    Everything I have learned about wood movement tells me this is a no-no. Doesn’t look to me that this is any different than gluing on a breadboard end. I don’t really see how breaking the tenon into three sections helps you if you are gluing them. This looks like a good time for a drawbore to me.

    David Barbee

  9. Rob Millard

    I have made a few sideboards, one of which has cracked. The case depth was around 20”. Despite the reputation that mahogany has for stability, I took what precautions I could to prevent the cracking. The side had 4 tenons, of which only the center two were glued; the others rode in elongated mortises. A very slight concavity was planed into the leg, so the un-glued mortises would stay tight (think of a spring joint on an edge joint). While I didn’t totally buy into the idea, I applied clamps as shown in the Jeffery Greene book on 18th Century furniture, to pre-compress the sides. It takes a pretty historically savvy customer to accept a crack in a $7000 piece of furniture. Other customers who aren’t as knowledgeable are made aware of the possibility. This customer happens to go away for extended periods; and I wonder if the rapid changes in the interior environment didn’t play a part in the crack, because the other sideboards not exposed to such changes have not cracked and one sits very close to a forced hot air register.
    Rob Millard

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