Pocket screws for holding top to apron

[Phil Thien]

One of my test projects built last year surprised me quite a bit over the winter.

I built a little cabinet so I could try knife hinges and test a theory about planing boards with twist, and the completed cabinet sat in our bedroom for months.

The doors are solid wood, with the grain running vertically. The gap between the two doors really opened up over the winter, more than doubling in width, due to wood movement. Next time I'll use a bit of trim to make the doors overlap, so the gap never shows.

ANYWAY, right now I'm building a table for my printer and scanner and I decided to use-up some walnut for the top (trying to keep to materials on hand). Had I used plywood or particle board, I'd have no concerns about using pocket screws to attach the top.

But now my concern is that, unless I limit my pocket screws to the center of the glued-up board, the screws may not accommodate the expansion/contraction of the top.

I'd like to use pocket screws into the top from all four apron pieces, will they allow the wood to move enough?

The top will be about 22" wide, 44" long.

[fredhargis]

I don't think they will. In any case it's not worth taking a chance. If you want some Z clips I can mail you a few (gratis).

[Splinter Puller]

I've used pocket screws in about a dozen tables without issue. I always ream out the hole in the apron a bit so that the screw has room to move. I leave them "snug" and not "super tight" also.

Note, The center screws I don't ream (or the front of a dresser top if you want the expansion to happen towards the rear)

[Alan S]

According to this table (more complete ones are found in Hoadley's Understanding Wood, and in the Forest Products Laboratory Wood Handbook (a free download)) if the wood in your house varies by 4% moisture content summer to winter, you would have about 1/4" of movement over the 22" width. If you keep the pockets away from the corners, bending of stretchers might accommodate that. Of course, if you tie down the middle, you have only half that at front and back to deal with.

Another approach is to counterbore the top of the stretcher to give the screw some room to bend, installing the screw straight up into the bottom edge of the stretcher.

[cputnam]

I'd go with Z clips or buttons

[JGrout]

I too would go with z clips

or if I had none and did not want to pick up some I would use slotted holes in the apron or shop built wooden clips.

IOW Pocket screws are not acceptable in this situation unless the holes are slotted to accommodate movement.....

[Steve N]

Z clips or figure 8's so you are assured you can move. I fear the pocket screws would lock down the top so the only movement would be destructive. That said I only used Z clips or 8's, so I have no experience with pocket screwed tops.

Z clips from underneath.



Figure 8's




You can also make ho made out of small wooden blocks. I have tried this, and had quite a bit of breakage of the foot. But they are really cheap

Article with more info.

[Kansas City Fireslayer]

I wouldn't risk it either. Go with a method that will allow movement. I just added solid wood shop made buttons to the 42.5" wide, 1.25" thick, and 76" long farm table top I made. The domino made quick work of the mortises and the blocks didn't take terribly long to make. I haven't fastened the top down yet but there will be plenty of strength that still allows plenty of cross grain movement.

For the bench, I will most likely use the metal table clamps that fit into a saw blade kerf. These will be quicker and still plenty strong to hold the top securely. The Illustrated cabinetmaking book by Houghton (I believe) has a very thorough section on fastening table tops with many different examples. He also provides explanation of which fastening methods are best suited for particular table tops. He also has a great section on different bread board ends. Another fastening system that takes forethought towards wood joinery and seasonal movement. It's the best book I could recommend to anyone not following a pre engineered furniture plan.

[John Mihich]

Seeing that pocket screws have been used since screws were invented I think it's a way to go. Just widen the holes in the aprons - end aprons only. You'll be fine. I have worked on a whole bunch of antiques that were done that way and the tops were just fine. I also use a form of z-clips - they work fine also. Just keep in mind the wood movement and allow for it no matter the choice you make.

It also depends on the wood you are using. The softer the wood - less likely to split - the wood will allow the screws to move. Oak is very non-forgiving; pine on the other hand you can almost glue the top to the aprons. Woods in between will act differently.

All what have been presented by others will work.

[JimReed@Tallahassee]

As we grow in our woodworking craft, we all learn how to apply lessons from the past. Your problem has been known for centuries but it continues to vex woodworkers. It all comes down to the fact that some wood joints must be firm and some must be loose. An experienced craftsman understands the properties of wood and uses techniques that give the resulting project a long life.

In my early days of woodworking, I rehabbed many tables because the antique shops were loaded with them and prices were rock bottom. Most of these were made between Civil War and WWI. Glue blocks were common. Luckily the hide glue failed and the blocks failed also. Splits were not that common. Nails and pocket screws were also common methods of attachment and almost every table I purchased had splits. If the top did not have screws to begin with, it got some when the glue blocks failed. I grew to hate screws and nails but I got really good at fixing splits. That experience taught me to use only Z clips. New furniture or antique repair--makes no difference because solid wood panels will split over time if they are too confined.

Case in point is my Ulmia workbench, which lives in an unheated garage. The top has a breadboard end and I watch it move 1/2 to 3/4 of an inch each season. The top remains stable and flat. You never know when your furniture will be stored in an unheated house or garage. Those pocket screws will hold the top too tight and send it to splitsville. So I encourage you to dig deep into that craftsman woodlore and find a way to allow movement of the top. Z clips are the easiest method I know but there are others. Good luck with your project.