“Mortise” in bottom of porch post
davidmeiland
| Posted in Construction Techniques on
We have to install three pairs of posts… 10″ diameter peeled logs that are to install without visible hardware over a stone paver surface. Plan is to pour a slab with a “knife” of 1/4″ steel plate poking up at each post location. Each knife will be 3″ wide and 10″ tall. Then, we need to mortise into the bottom of the posts so that they can drop into place over the knife.
Question is, how best to make those mortises? They need to be nicely done and in the right places. This would be a great time to own a nifty Italian chain mortiser but alas I have not…
Replies
So you're talking about mortising a 1/4" x 3" cross section x 10" deep.
That's a tough one. Even if you DID have a chain mortiser, are they only 1/4" thick?
I'm imagining some sort of plunge router scenario, but the 10" depth in conjunction with the 1/4" diameter limitation seems to pose a real problem.
Maybe a thicker "knife" would allow a more conventional solution.
Edited 9/14/2007 1:09 am ET by Ragnar17
chain saw...
Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming
WOW!!! What a Ride!
Forget the primal scream, just ROAR!!!
I've done it with both an electric chainsaw and a gas chainsaw (the electrics have a narrower bar profile).
You can drill through the bar and bolt a jig made of plywood or UHMW, etc., to provide a steady rest for the bar and a "hilt" to limit the plunge depth of the bar.
You're a smart guy, so I'm hoping you get my vague drift, but if not, I'll do a sketch and post it for you in the AM.
When doing these, I fab the "knife" so that there is a steel plate between the bottom of the log and the concrete with a piece of Ice and Water or heavy felt between the bottom of the log and the plate. I always treat the bottom of the log and the slot with a good dose of that wolmanizing stuff or a couple of those anti-fungal rods that you drive in and plug.
Edited 9/14/2007 1:26 am by Notchman
As others said, chainsaw. This is simply a mortise & tennon joint and in woodworking, you generally make your tennon to fit your mortise, as opposed to the other way around. That applies here as well. Make a sample mortise and fab the "knife" tennons to fit the mortise you can make.
Mike Hennessy
Pittsburgh, PA
Edited 9/14/2007 8:53 am ET by MikeHennessy
Gene asked the same question a year or two ago, IIRC....
Can you use a round solid steel rod instead of plate steel? 7/8 inch diameter.
Then just drill a 1 inch round hole.
A single rod won't give resistance to torque the way the "knife" tenon will. However, I suppose he could use two or more rods to achieve a similar result.
Where David is, like where I am, Seismic considerations will probably require the base of the post to be not merely held in lateral position, but anchored.
This is pretty simple to do and, if the locatio9n of the horizontal bolt is counterbored and plugged, does not detract from the wood structure.
But a round pin won't work; it can be done with, say 2" steel pipe as an alternative to a steel "blade."
I've checked out some similar installs and there are generally two carriage bolts thru the post (and thru the knife) within a foot or so of the ground. That's where the 10" figure comes from. I agree that rod would not work since there would be no hold-down capacity at all.
As far as the thickness of the plate goes, it could be anything. I had considered trying to plunge the tip of a chainsaw in but the bars on mine are pretty wide. I assume that log builders have a way of prepping their posts but I have never seen it.
Gotcha.
How about a square tube inside a round hole? Or form the flat stock into a C channel with a wider mortice.
Just thinking out loud, so to speak.
Check out the arborists or carvers bars at that big chainsaw supply place that I have a catalogue that I can't find and the name escapes me...that place.
I'll get back to you after the brain fart clears..........
Aha! Baileys..dats it. I think.
Spheramid Enterprises Architectural Woodworks
"If you want something you've never had, do something you've never done"
Edited 9/14/2007 4:59 pm ET by Sphere
plunge cut with a chain saw...
common practice...
hand chisel for a better fit...
use the CS for the rough in cut...
Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming
WOW!!! What a Ride!
Forget the primal scream, just ROAR!!!
Does the 3x10 "blade" of 1/4" steel have a bored or punched hole, through which you want to get a bolt?
We did a detail such as that, although with a blade length of only about 5 inches. The bolts went into counterbored holes, and the heads and ends and nuts were below flush. The recesses were filled with Bondo.
This, all for a bunch of 4x4 cedar porch roof supports, all primed and painted.
If you are not cross-bolting as we did, but are relying on gravity and the load to keep things down, and want the blade as an anti-rotation feature, just do a pair of dowels.
A pair of #4 rebar pieces, spaced on 4" centers, ought to do just fine, and you can bore the sockets with a drillbit. Do the holes at 3/4 dia., and jamb some high viscosity epoxy in them before setting the posts on the studs.
Gene,
Did you do anything to keep the ends of those 4x4s off the concrete (i.e. a standoff)?
I have a similar situation with an exterior newel post and am concerned about water constantly wicking up into the end grain.
Besides sealing the endgrain with epoxy before seating posts, here is what we did.
View Image
Thanks for that sketch, Gene. It looks like a good system for what you needed.
You mentioned that this hardware was to anchor posts that supported a porch roof. Have you ever designed anything (or used a commercially available product) that would be good for a newel post configuration? A newel post is slightly different in that it does not receive as much horizontal support at the top end, of course.
I would drill a series of 5/16" to 3/8" holes with an auger bit. Then carefully clear out the remaining wood with a chainsaw. I have a Makita chain morterer...but it wouldn't go that deep anyways.
Yeah, the Makitas would never work. Mafell makes one with a 5/16" wide bar for just this application though. I suspect $3500 is a little high for the OP, though.Like others have said, chainsaw. You can buy or fabricate a plunge fitting that bolts to the bar if you don't have the skills to freehand it.