Half-lap madness - Woodworking Talk - Woodworkers Forum

Old 09-25-2015, 08:14 PM Thread Starter
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Building a small shed and want half-lap rafters with 4x4s at 3 in 12. This gives cuts about 15 degrees, which if butted would give me a low roof of 3 in 12. But everytime I do the half-lap, I wind up with a finished 75 degree angle whichi is about 9 in 12. Only if I butt the cuts, i.e. 15 degree at the end of each 4x4 do I get what I want. But doing a half lap just doesn't work. And BTW, I'm dyslexic. I can't retain any visual memory. The Blind Carpenter at work. Any help would be greatly appreciated. How to do it would be wonderful. My guess is that you can't do a half-lap with dimensional wood larger than a 90 degree. Thanks in advance...English44
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Old 09-26-2015, 02:03 PM
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I think you're trying to cut two half laps at the ridge of the roof to join two rafters. Is that right?

If so then maybe this drawing will help.

You're going to need to use 28° to cut a half lap at a 3/12 roof pitch.

Jim Rogers

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Old 09-26-2015, 02:19 PM Thread Starter
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Clarification?

I'm not seeing it. I really don't know what your annotated angles mean.

It looks like you've got a 28 degree cut on the inner and outer edges? Is that correct? A 3 in 12 angle is ~15 degrees. How did you come up with 28 degrees? Lastly, I planned on using 4x4s for the rafters.

It would help me if you draw an arrow to each cut and the corresponding angle for the cut. I'm visually dysfunctional. On my 4x4s I had an end cut on each of 15 degrees and an inner cut of 15 degrees on the half-lap. The pieces didn't mate properly. When the two pieces met, the angle created by that was somewhere around 165 degees (I think), which mimicked the roof line (3/12).

Last edited by ENGLISH44; 09-26-2015 at 02:28 PM.
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Old 09-26-2015, 03:22 PM
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I'm not sure how you are cutting or measuring your angles but it sounds like you are ending up with the complimentary angle to what you are trying for. (90-15=75). We would need more info to find out why exactly.
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Old 09-26-2015, 04:30 PM Thread Starter
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What do you need from me? Am I correct that each of your interior cuts at the half-lap is 28 degrees? You're right about the complementary angle. I used a jig from Fine Homebuilding's Heirloom Shed. He had a 9 in 12. I figured is your used the complementary of that (~15) I would get what I wanted. Didn't work! The 15 degree angle worked in the half-lap, but the 4x4 ends protruded past the edge of each overlaying board. Hope that makes sense. I concluded that you can't do an obtuse angle on a 4x4 or perhaps on dimensional lumber. Thanks for your help.
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Old 09-27-2015, 10:54 AM
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Quote:
Originally Posted by ENGLISH44 View Post
What do you need from me? .
I need to know: are you trying to do the half lap at the ridge where your two rafters meet? Because this is what I thought you wanted.

I opened my timber frame designing program, and I entered in two 4x4 rafters at the 3/12 pitch you said you were using. That angle is 14.0362° from level. Not 15 degrees.
Then I told the program to cut away the necessary wood to make it a half lap joint. As shown in the 3d view.
Then I rotated that view to a side view in order to measure the angles to show you what angle you need to cut your 4x4 rafters to in order to create two half laps that joint at the ridge. This is the 28° angle.

If your half lap joint extents beyond the roof surface then cut off the excess or cut your angles only on the ends creating a 4" half lap.

Jim Rogers

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Old 09-27-2015, 12:31 PM Thread Starter
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Being fairly visually retarded, cut 28 degree inside angles for the half lap. Put the boards together and not even close. So i must have misunderstood you. Tried 52 degrees: a little better and finally cut the half-lap angles at 72 degrees. That was much closer to what I wanted. I might just go the easy way as those half laps are going to be a lot of work. I can just cut the timbers at 15 degrees, butt them and use Simpson construction strong ties and be done with it.

I appreciate your help and feel a little completed as I now know what I would have to do to get the low roof line. Thanks.
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Old 09-27-2015, 03:31 PM
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Man, you need to learn Sketchup. It would really help you. Especially with the angles, determining the actual lengths,cuts, everything.

It may not be feasible now, but for future projects, it could be very beneficial.

I can visualize stuff pretty well, but please don't tell me to make a mirror image!!!
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Old 09-27-2015, 04:13 PM Thread Starter
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Tried Sketchup a number times. Too much work for me. Strangely I build some very find furniture. Much like the Blind Samurai Zatoichi, I guess. I just have to work through my own way of doing things: making models, work-ups, observing for a long time and then putting it all together. I'm retired and in no hurry, so I muddle through. Thanks, and by the way, I don't know what you are referring to as a "mirror image".
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Old 09-27-2015, 05:28 PM
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Quote:
Originally Posted by ENGLISH44 View Post
Tried Sketchup a number times. Too much work for me. Thanks, and by the way, I don't know what you are referring to as a "mirror image".
Mirror image. The right side looks like the left side, just reversed, thus the mirror image.

I have had to make several pieces over because I made two just alike instead of reversed!

I don't know if I am dyslexic or just not paying attention to details. What ever it is, I am good at it!
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Old 09-28-2015, 03:57 AM
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roof pitch in degrees

Quote:
Originally Posted by Jim_Rogers View Post

I opened my timber frame designing program, and I entered in two 4x4 rafters at the 3/12 pitch you said you were using. That angle is 14.0362° from level. Not 15 degrees.
Then I told the program to cut away the necessary wood to make it a half lap joint. As shown in the 3d view.
Then I rotated that view to a side view in order to measure the angles to show you what angle you need to cut your 4x4 rafters to in order to create two half laps that joint at the ridge. This is the 28° angle.

Jim Rogers
This chart shows a 3/12 pitch to be a 14.4 degree angle:

If you are having trouble visualizing this, mock it up with some 1 x 4's. Layout your shed's width on the flat ground, find the center, angle the boards to intersect and overlap at the center at 3/12 or 14.4 degrees from horizontal, then scribe both angles on the bottom board. Now you have a pattern to make your other cuts. Cut the bird's mouth as well, if needed.

This will take about 10 minutes or less, not much math or calculating. I don't know why you want to use 4 x 4's for rafters, that being really a doubled 2 x 4 as far as strength.... not much, in my opinion. I would use 2 x 6's and have done so in all my 3 sheds. You will also want a collar tie for a gable roof, again just my opinion.

The answer to your question will only be as detailed and specific as the question is detailed and specfic. Good questions also include a sketch or a photo

Last edited by woodnthings; 09-28-2015 at 04:02 AM.
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Old 09-28-2015, 07:51 AM
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Read your chart correctly it says 14.04 not 14.4. If you round up my angle to that decimal place you get 14.04.

Jim Rogers

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Old 09-28-2015, 08:10 AM
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If you need a visual, find a large open paved space, and snap some chalk lines for the layout. Then lay your boards out and mark the angles...
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Old 10-06-2015, 08:55 AM Thread Starter
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I had thought that the angle for the 3/12 would be calculated at the ridge. NO! The 14.04 angle is from the lower edge of the roof. That makes all the difference. I wonder why no one said that?
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Old 10-07-2015, 09:02 AM
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The above chart clearly shows it.

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Old 10-08-2015, 07:30 AM Thread Starter
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Solution

Some people have to do it their own way. Charts, math, illustrations are very little help to me. Set up a dado set for a 3/4" high cut. Used the miter guage set at 50 degrees and ran 2x4 for the rafters angles. The pieces mated perfectly. Got the half laps and the low angle roof. Thanks guys...
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Old 11-30-2015, 01:01 PM
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I faced a similar issue making mitered half laps in 6x6s for an octagonal gazebo base. I have a pad full of sketches, note and drawings. I looked at Sketchup.

Being old-school, my solution was an easel pad from Staples with 1" grid lines on it, (Think of 25" x 30" graph paper.) I taped together 3 sheets and made a full size drawing of one side of the octagon base, including all the lap cut lines, post base outlines and mortise/tenon outlines. It really helped with my visualization, and got all of my dimensions nailed.
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