residential roofs

[thomasf]

I've looked all over the internet and the forums here, but have not found an answer to this question.

Is there a tutorial anywhere that explains how to create a residential roof with varying pitches?

Everything I have come across, even examples of complex roofs, use the same pitch all the way around, but in the real world most houses have one pitch side to side and a different pitch front to back.

I have a roof that has a 12:12 side to side pitch and a 7:12 front to back pitch. I know from years of experience that using the standard construction methods here in Texas that a 12:12 pitch has a 1'-0" overhang and that a 7:12 pitch has a +/- 1'-8" overhang.

However, even using the align eaves command, Revit gives me an overhang in excess of 2'-1" for the 7:12 pitch and, even though I had the "Extend into Wall" box checked the roof did not intersect the inside face of the stud as it should.

Is there a tutorial out there for this type of roof? Any tips/tricks I should be aware of?

Any assistance would be greatly appreciated!!

Tom

[johnf.77896]

Tom could you post an example of what you are attempting?

[thomasf]

All I have is a pdf file of the entire project, but it is over 7Mb. I'm not sure if I can post something that big here.

[thomasf]

OK, I used my snipping tool to grab just the roof plan.

As you can see, in one direction the pitch is 12:12 and in the perpendicular direction it is 7:12.

85375

[sbrown]

You can use slope arrows and make multiple roof slopes, for complex roofs like this don't fell like you have to do it all in one sketch.

[thomasf]

If I use slope arrows don't I need to know the height at the tail of the arrow or at the head? I've only done simple roofs with the same pitch on both sides, anything beyond that is a bit over my head. That's why I was hoping to find a tutorial or some step by step instructions. I've got a lot of the pieces done, although the over hangs are close they are not correct, but I still can't get some of the roofs to join together.

[patricks]

Yeah I would do a couple of main roof forms and then add various projections as necessary using the Join Roof tool, and possibly some vertical or dormer openings if needed.

As for the slopes and overhangs, it depends on how it will be built, i.e. placement of rafters on wall top plate, eave condition (aligned, not aligned), etc.

If you build a roof with 12/12 slope on two sides, rafters extending out 12" past the brick/siding, and the bottom edge of the rafters (bird's mouth cut) aligned with the top inside edge of the wall top plate, then the 7/12 rafters could end up extending out more or less depending on how the rafter ends are detailed. The 7/12 rafters' bird's mouth cut would also have to be different than the 12/12, or the 12/12 rafters may need additional blocking above the wall top plate (or taller walls).

I just made a roof as described, with 12" overhangs on the 12/12 slope and 20" overhangs on the 7/12 slope. The roof is set to Rafter and Plumb Cut. After aligning eaves (using Adjust Plate Height) and adjusting the roof height so that the 7/12 rafter bottom edges aligns with the inside wall top plate, I see that the 12/12 rafters would need 3 1/4" additional height to get to the bottom of those rafters. Also the 12/12 rafters would need more trimming on the bottom to get an equal soffit height all around.

Examine the attached file to see what I mean.

*edit* You could make it so that the bottom of the rafters sits on the wall top plate all around, but then the eaves would not align. The eaves of the 7/12 roof would slope down at each end to mate up with the 12/12 eave end, which I have seen some houses built that way. Though I think that detail looks pretty weird. It makes me think they just couldn't work out the roof right and just "fudged" it to make it work.

[Alfredo Medina]

There is another way to do complex roofs, that is overlooked. In brief words is this: In the conceptual design environment, draw the floor plan of the roof, as shown in your image, using reference lines. Then create reference planes in elevation. Of course you need to know the elevation of the ridges in advance. Then, create faces, then lift the points to the correspondent reference plane and elevation. Then, load into project and apply roof by face.

[thomasf]

The only problem is, if you look at your roof in section, one of the pitches is plating correctly, from the inside face of the stud wall (see section 2), but the other pitch is not. But, I just remembered the math and it is very simple actually. The overhang is the run (12") divided by the rise (x) times 12 = overhang in inches.

so for a 12:12 pitch it's 12/12 = 1 x 12 = 12"

7:12 would be 12/7 = 1.714285714285714 x 12 = 20.57142857142857"

this is from outside face of stud, not face of finish material. So, i'm going to try using this overhang measurement & see if it works.

[jsteinhauer]

Thomas,

Set your overhang from your walls, and go from there. Or, does it matter where the ridge of the roof is? If it gets to complex for a single roof, then break your roof into several sections. I did a roof a few years back and just used the thickness of the sheathing and the roofing material. For my truss profiles, I went into section and drew truss elevations from there. I gave the truss manufacturer my rough dimensions, and they gave me a truss design that worked with the truss elevations.

I hope that helps,

[patricks]

The only problem is, if you look at your roof in section, one of the pitches is plating correctly, from the inside face of the stud wall (see section 2), but the other pitch is not. But, I just remembered the math and it is very simple actually. The overhang is the run (12") divided by the rise (x) times 12 = overhang in inches.

so for a 12:12 pitch it's 12/12 = 1 x 12 = 12"

7:12 would be 12/7 = 1.714285714285714 x 12 = 20.57142857142857"

this is from outside face of stud, not face of finish material. So, i'm going to try using this overhang measurement & see if it works.

That's what I'm saying, if you want aligned eaves, either one set of rafters will have to have larger bird's mouth cuts, or the other set of walls will have to be slightly taller or have additional blocking above the wall top plate. That's the only way, geometrically, to get aligned roof eaves.

When I say aligned roof eaves, I'm referring to the UPPER point of the eave, i.e. where the gutter attaches.

[thomasf]

Unfortunately, that doesn't help since I am not using trusses. Thanks anyway though.

Using my math above I've gotten it pretty close, but still not correct. I hope this is something they keep working on cause this is a big obstacle to designers like myself who work on both commercial & residential projects from completely embracing Revit!

[thomasf]

That's what I'm saying, if you want aligned eaves, either one set of rafters will have to have larger bird's mouth cuts, or the other set of walls will have to be slightly taller or have additional blocking above the wall top plate. That's the only way, geometrically, to get aligned roof eaves.

When I say aligned roof eaves, I'm referring to the UPPER point of the eave, i.e. where the gutter attaches.

I understand what you're saying, in order for it to be geometrically correct in Revit, you have to do one of the two things you mentioned. But in the field that's not how it works. The pivot point is where the roof rafter intersects the inside face of the stud, the bird's mouth cut is always 3 1/2" deep so the roof rafter will fully seat on a typical 2x4 wall, the horizontal cut at the rafter end is a fixed distance of 1'-0" from the top of plate, the vertical cut is a variable distance (based on the pitch) from the outside face of the stud. Using this method, the eaves will always align!

I first looked at Revit in 2002 and the inability to draw a residential roof, the way it's built in the field, is what put me off from totally ditching AutoCAD, and 10 years later it hasn't gotten any better :(

[patricks]

I sketched out the two conditions you described, put dimensions on them, and made the roof by manually specifying the overhangs for each side and then aligning eaves by adjusting plate height.

I noticed when I created the roof first by picking all walls at 12/12 slope, changing 2 sides to 7/12, and then doing Align Eaves by adjusting the overhang (adjusting the 7/12 eave height to match the 12/12 eave height), the 7/12 overhangs went longer than they should, and the original 12/12 sides no longer aligned with the inside top plate. IMHO that shouldn't happen. Seems like a bug.

Check out my revised file and take a look at the drafting view sketch.

[thomasf]

Okay, I'm glad somebody understood what I was saying and was able to re-create the problem. I performed the exact same steps as you and got very similar results.

I looked at your file and made some adjustments to the drafting view, as we use 2x6's for roof rafters. I also made a small adjustment to the wall type, again based on local building methods, the 1/2" sheathing on the exterior of the stud is included in the 5 1/2" brick ledge. I then attempted to re-create the roof, manually using the distances determined in the drafting view, and was unsuccessful. Even though each roof edge is set to define slope, the overhangs are correct and extend into core is checked, none of the rafters align with the inside top plate!

Perhaps this is just something Revit isn't capable of doing, or as you said, a bug in the portion of the program that calculates overhangs, when you select the Align Eaves - Adjust Overhangs option.

[patricks]

I got your file to work. I went to Section 1 and moved the bottom edge of the roof up to the inside edge of top plate. Then go to the drafting view and measure the vertical height of the rafter end (below that 7 xxx/256 dimension). Transfer that number to the Fascia Depth on your roof. That will give you the 12 inches from top plate to bottom corner of rafters, and the overhang dimensions will match the sketch.

So you can make it work, but I think there's a bug when initially sketching the roof and adjusting eave overhangs and plate heights.

[thomasf]

OK, I was able to get it to work to with that method and following the same the technique was able to get it to work after adding the roof decking & asphalt shingles. That is a lot of steps to get the roof to work though!

[patricks]

Yes it is. If I get a chance I'll be submitting a support request about it.

However, I don't recall seeing houses with soffit depths that vary based on roof slope. I always see (and would rather have on our commercial jobs) soffits that are equal all around, equal height, etc. which would necessitate having the bearing point for the steeper slope higher than that of the shallower sloping roof, to get all eaves and soffits to align and be equal all around.

Is the varied soffit/overhang depth a common thing in your area?

[thomasf]

Actually, my experience is the opposite of yours. I have never seen a builder change the bearing the point to create an equal depth soffit. Every homebuilder/framer, I've ever worked with, that frames with traditional stick framing as opposed to trusses, builds them as I've described, creating deeper overhangs to obtain alignment, and I've done houses from Virginia to Vegas.

The varying pitches usually don't come into play until I get into move-up homes, most starter homes have equal pitches all the way around.

[patricks]

I've seen it plenty of times in upscale homes around here in the Southeast. There are home shows with 3500 - 5000+ SF homes where I'll go in the attic and see *very* steep rafters of up to 14/12 or even 16/12 sitting on a ~12" high framed knee wall above the wall top plate. Or in many cases I've seen a roof with 2 pitches, where the steeper pitch rafters are cut off right at or just past the outside face of framing, and then separate little rafter pieces are scabbed onto each main rafter at the lower roof pitch specifically to make the eaves and overhangs equal all around.

For the life of me I can't think of a name for that condition and I can't seem to find a stinking image or diagram anywhere, but I see it a LOT around here.

*edit* here's an image, though I think this one shows equal pitches on both sides, with equal "flare-out" pitches on both eaves. But I've seen this done with the flare out on a steeper side to match the eave to the adjacent shallower-sloped roof face. The portion of the hip just above the eave ends up as a 45° angle in plan view, but the rest of the hip above the interior of the house would not be at a 45 in plan view.

http://www.tebroc.com/documents/32_installations_metalvalley2_1.jpg

[thomasf]

We just call that a kick-out. We do that here on high-end stuff as well, because people like the look of those very steep roofs, but you need some overhang to put in the soffit vents so we scab on a shallower pitch as you describe. I thinks it's really interesting to hear about different framing techniques used in other parts of the country. I think ti all depends on how much the builder is willing to pay and how fast they want it done.

The method used here is easily laid out with a carpenter's square and since most builder's pay a bonus for speed, production homes can get framed in less than 3 weeks!! I wouldn't want to live in one, but that's how face they get thrown up around here. Even though they're my bread & butter right now, we have always referred to them as future tear-downs.