Timber framing on a slab?

[SwingOak]

Hi folks,

I have a shed on my property that is in really bad shape. It was probably built within the last 50-60 years, and was originally a chicken coop, I think. It's beyond fixing, and so it must come down. It sits on a concrete slab that is more or less level and in pretty good shape. The slab measures 26' x 12'.

I've been reading the Sobon book, and the shed plans sound like they would work. I would extend the length to 20', and keep the width at 12'. I really don't want a wood floor anyway, as I plan to move my smithy into the shed when I'm done with it. As it would go on a slab, instead of on a foundation, what special considerations should I give the sills? Do I need to elevate them off the slab to allow some air circulation?

Also, I plan on putting double Dutch doors in one end, for a total door opening width of 8'. Can anyone help with some layout suggestions for framing the end, considering there won't be a continuous cross sill?

Thanks!

[Jim_Rogers]

Welcome:
And good questions.

If you're going to do continuous long sills then you can place them directly on the concrete and or secure them to the concrete as long as you put a barrier between the concrete and the bottom of the sill. Some type of water proof membrane like roofing materials. Such as ice and water shield. This will prevent the moisture in the concrete to wick up into the sill timbers.

If the ends of these long sills, and cross sills, on the ends where the door opening will be are properly secured to the concrete then you shouldn't have a problem with the lack of cross sill where the door way is.

I'll modify a design I have and post a picture of what I mean.

Jim Rogers

[Jim_Rogers]

Here is a standard 12'x16' frame with one gable end modified for a centered 8' wide doorway with two door posts on the sides.



What I'd do is create two small or short cross sill ends and insert them into the standard mortise pocket in the long sill and then use these for holding the door posts in place.
The door posts are 4x5's with the five going inward. Then each door post has a two inch stub tenon on the bottom to hold it in place in the short cross sill. And a standard pegged 4" tenon on the top.

I also moved the standard brace back, making it very short. You might consider just not putting these in. Or putting them from the tie up to the post above the tie, so that you'd have some bracing on this end.

Also, we've built this standard frame here, and it's seems that the tie beams are just a little low for me.

If you considering this frame design you might move the tie beam up some to give you more room downstairs. Although without the floor system you're already gaining some height.

If you need detailed drawings of this modification or you'd like more help, drop me a private email and we can discuss it further.

Good luck with your project.

Jim Rogers

[SwingOak]

Thanks Jim! Those are excellent points to consider, and I appreciate the advice. That's a really good point about a barrier between the sills and the concrete. I'm also going to try & use locust for the sills, at least if I can get fairly straight pieces out of the logs I have (and will have) at my disposal.

I was thinking of raising the post height to accommodate a higher cross beam on the bents as well, as I also thought the braces are a bit too low for my liking. Sobon says his design is OK for a post height up to around 14'? At least I think that's what I read - I have to go back and read that part again.

It looks like you are using Sketchup? I'm in the process of designing my frame using Autodesk Inventor. I don't know how to use SU, and I use Inventor every day so it's easier for me.

Thanks again for the help!

Oh, and BTW, if there are any other Inventor users out there who are interested, I found an IPT template for wood materials that I downloaded from the AI discussion group, and I'll try to post a link to it (if I can find it again).

[Jim_Rogers]

Nope, not SU, it's Dietrich's timber framing program......

[SwingOak]

I remember reading something about the Dietrich timber frame program in another post. Sounds like a pretty slick application.

I'm thinking of using black locust for the sills, as I have a few decent trees at my disposal. Provided, of course, that any timbers I cut stay fairly straight and don't go "boing" after milling. 

I was wondering if it would be necessary to anchor the short sills on each side of the door opening, and I guess it is. How would you suggest I do this? I guess I could through-bolt with a sleeve or wedge anchor. Should I consider anchoring other sections of the sills as well? I would think the weight of the frame would be enough to keep it in place as far as that is concerned, but I can understand the need to anchor the short sills.

[kfhines]

Lurcherman,

I would like a link to that IPT template if it's not to much trouble. I used Inventor to model my addition. I thought it worked fairly well considering it is designed mainly for machine design. I would bet a program designed around Timber Framing would speed things up a bit.

It looks like Jim has taken good care of your questions.


kfhines

[Jim_Rogers]

I was wondering if it would be necessary to anchor the short sills on each side of the door opening, and I guess it is. How would you suggest I do this?

If you anchor the long sills and you make the connection between the short cross sills and the long sills correctly, that is tight not sloppy loose, you should be ok. But of course one bolt down into the concrete near the door post will secure the door post so that it doesn't move when opening and closing the door. Depending of course on how you hang the door. Will they be two 4' hinged doors? Will they swing out or in?

Should I consider anchoring other sections of the sills as well?

Although in my area we don't have to worry about earthquakes, most frames are secured to the foundations somehow.
Your profile doesn't say where you're from. So I can't comment on your needs for securing the frame to your concrete.
If you're doing this project with a building permit, I'll assume that your building inspector will want some type of anchorage.
I would secure it with at lease a couple, or three, bolts through the long sills into the concrete.
Wind could be a factor as well. Houses/building can be blown away......

Boring a small hole and then using an anchor bolt that self expands could be an easy method that will secure your sills to the existing concrete. Plan on bolts long enough to go into the concrete the distance the manufacturer recommends and up through the long sill and use large washers and nuts to hold the long sill down. Excess bolt can be cut off with a hack saw blade in a sawzall.

Good luck with your project, and planning it out right, is the first step in construction.

Jim Rogers

[SwingOak]

Here is the link to the thread in the Inventor discussion group: http://discussion.autodesk.com/adskcsp/thread.jspa?messageID=3362008

Jim,

The doors would swing out, and yes: 2 x 4' doors.

I'm in New Jersey (I thought that was in my profile, but I guess not. I'll have to add it). I wasn't really planning on getting a permit, but I guess I would have to. Maybe I could say I just repaired the shed that was there...

[SwingOak]

I'm closing in on finalizing the frame layout. I do have a question on the stub tenon for the door post.

My understanding is that a mortise should always be cut so that the longest edge of the mortise runs parallel with the grain. Seeing as there are going to be doors hung on this post, I expect side loading and torsion to have more of an effect on the post than vertical loading, as the corner post should be taking most of that stress. I was considering using a 4 x 8, or 6 x 8 door post.

I'm sure it's simple (it always seems like the best solutions are just that), but does anyone have any suggestions on the best way to handle this?

[Jim_Rogers]

If you're going to use a 4x8 or 6x8 which way is the 8? deep or left to right?

[SwingOak]

If you're going to use a 4x8 or 6x8 which way is the 8? deep or left to right?

Deep, so the door frame is the full width of the sill

[Jim_Rogers]

Ok, traditionally the stub tenon on the bottom of a post is only 2" deep into the sill. At least the mortise is cut 2" deep and the tenon is cut back from 2" to 1 7/8" so that it won't bottom out when everything shrinks when everything drys out.
So we now know the tenon length.
Next is the tenon width.
When a frame is designed the designer makes up a "general frame rule" and/or rules that apply to each joint (most of the time, there are exceptions to every rule). One such rule on this standard garden shed is that all joints are laid out 2" off the reference face and then 2" thick.
This tells us how thick the tenon should be and where it should be on the sill and on the end of the door post.
Another general frame rule is "all timbers are reduced to the next 1/2" in size." What this rule means is if your post is 6x8 then the join on the top has to be reduced to 7 1/2" as the reduction is always on the side opposite the reference face.
Tip (when inspecting old square rule barns, this is how you tell which side of the timber is the reference face).
Now some people also would reduce it on the face opposite the adjacent face of the timber.
The adjacent face is the one next to the reference face and the arris between these two faces is where you'll pull all your dimensions from, when using a tape measure.

So this tells us that the tenon is now 7 1/2" in length.

Here is a screen shot to show you the names of the dimensions of a tenon and mortise:



In the above shot you can see that one side of the tenon on the top of a post has been reduced by 1/2" on one side, in the above picture the left side. This reduction is usually about 1 1/2" down the timber, from the shoulder. And is only done on the tenon part of the joint, that means the housing where this tenon mates is the actual size of the end of the timber. And usually completed with a short 45° chamfer.
Also, reductions are usually NOT done on post bottoms, as most of the post's bottom surface is needed to help support the post, and thereby the frame above the post.

Next is the another standard timber framing rule that needs to be applied. This rule would not be noted on plans, as it is assumed that all timber framers know this.

When a tenoned post joins sill or plate and it at the very end of the sill or plate the mortise in the sill or plate is NOT cut right out to the end of the sill or plate. It is stepped back the offset distance or usually 2". So this also helps us to size the tenon as it has to mate with the mortise. The reason that the mortise is cut back on the end of a plate or sill is so that there is some wood there to hold the tenon on all four sides.

In the above picture this timber, that has the mortise in it, could be a plate and you'll see that the mortise doesn't go all the way to the end of this timber. (It is where the "C" label is placed on the mortise piece.)
So the tenon has to be the same size, and this shows on the tenon size on the top of what could be a post, in the above drawing.

Now that we understand all the standard rules and all the "general frame rules" that apply to this post joint we can now design the stub tenon to the sill. And the top tenon to the tie beam.

Here is a screen shot of the stub tenon and it's mortise:



Now you can see that the tenon is offset to one side of the bottom of the post. This complies with the standard rule for the end of sill location.

As this is a door post an additional rule applies, and that is that the center of the doorway is usually a reference plane as well as the standard reference planes, which are the outside of the building.
So in this case the reference face of the door post would be the face on the outside of the building and the adjacent face would be the side towards the center of the doorway.
Then the tenon would be reduced on the face opposite the adjacent side, as it can't be reduced from from to back as it has to maintain the 2" off and 2" wide rule.

Also post bottom never get a housing into the sill. This would weaken the sill.

I have lifted the post and rotated it 45° so that you can see the top of sill and bottom of post.

Next is the top tenon.

Now this top tenon is going to join to the tie beam, not a plate. And it is going to join it not near an end. So the 2" off the end of the timber rule does not apply. But the center of the doorway rule does.
So here is the tenon and tie beam joint:



Here we see the reduction on the opposite face of the adjacent face and the reduction is 1 1/2" down the side of the door post and ends with a nice little 45° cut. Then tenon is 5" length, reduced by 1/8" so as to not bottom out in the mortise.
The tie beam has been reduced down by 1/2" thereby creating the housing just the exact width of the reduced end of the door post, and it's mortise goes from edge to edge of the housing.
The peg hole has been bored at 1 1/2" off the shoulder of the housing through the tie beam and then same location on the tenon which centers it on the tenon from left to right.

Often, tenons are shaved a little more and heavy chamfers have been added to them to make it easier for them to enter the mortise and not bind up.

Here is a shot of some stub tenon ends on the bottoms of posts:



And some top tenons on posts:



You can see by just looking at the size of the tenons which posts are the corner posts that go at the ends of the sills, and plates.....

I hope this has helped you to understand all the sizing rules that apply to designing joints.

Jim Rogers

[SwingOak]

That's excellent information Jim. Thank you!

[Jim_Rogers]

Please show us your final design when done....
thanks

[SwingOak]

Please show us your final design when done....
thanks

Absolutely! I have the end gable rafters, braces, and girts to model, then I think I'm done. I've probably put in a few bits of joinery that are more complicated then they need to be. I based them off things I read about in the Sobon book.

[SwingOak]

I stopped off at the township municipal building earlier this week, as  I needed to turn in some stuff so they could find a reason to stick me even more on my taxes. While I was there, I stopped in at the building code department. A very pleasant and helpful person said that it looks like I need permits, an gave me all the form to fill out.

First, I need to get a permit from the Board of Health, as I have a well on the property. Then I need to get  permit from the zoning office. Then onces I get approvals on those permits, I can apply for a building permit.

They may not let me build the shed. The existing shed is 3' off the property line. As my proposed design is higher than 10' and greater than 180 sq. ft., I need a minimum of 15' of setback. That would put it in the middle of my driveway.

Maybe they'll let me squeak by on the setback, but I'll have to make it a little smaller and shorter.

Back to the drawing board...

Of course, they also said they only keep plans and permit records on file for ten years, and then they are destroyed. So if I built without a permit & they don't catch me at it, I only have to worry if I sell the property in the next ten years.

[Jim_Rogers]

You may have to see when these set back rules were adopted and that your existing shed which needs repair, was built before these set back rules when into effect. This would mean it's grand-fathered in . And may not need a complete "new" building permit. As you maybe only restoring or repairing an existing structure......
An avenue of using the rules to your favor....

[SwingOak]

The current shed has been there a long time. As I'd be putting the new shed on the same slab as the old one, I expect it's possible they might let the setback rules slide.

But there will be nothing of the old shed left, except the slab - so it might be difficult to call it restoration or repair.

I still have to modify my plans though, I think it's a bit too tall.

[Don P]

Some places have a one wall rule, leave one existing wall and it grandfathers. Neatest case I saw of that was a double wythe 3 story brick front wall sammiched between temporary I beams so it wouldn't collapse.

[Jim_Rogers]

But there will be nothing of the old shed left, except the slab -.

Do they know that?......

Show them an old picture......

[SwingOak]

I can probably do whatever I want. Unless someone complains, they won't have a clue I did anything. If it was going to be a shiny new stick-built shed, they might notice. But a timber frame? By the time I get it completely built, the wood will have long aged and weathered to gray.

It would be easy to say it's been there for a very long time, and if they only keep records for 10 years, how would they know it hadn't?

[moonhill]

You could tell them it was a concrete building and you used one wall as the floor. ;D  Tim B.

[SwingOak]

You could tell them it was a concrete building and you used one wall as the floor. ;D  Tim B.

Now that's funny ...   :D