Calling All Roofing Specialists

[Bill Tillman]

I have a project I'm doing for a friend. He wants to build an addition onto a house which is basically a lean-to on two sides at the back corner of the house. The roof consists of two planes. One will be on a 2/12 pitch and extend about 13 ft from the edge of the house. The other will be on a pitch which I don't know yet but it will be steeper as the owner does not want to extend in the short direction anymore than 2 ft because of some newly planted trees.

 

My problem is how to make the transition between the two roof surfaces. I had this working pretty closely but no matter what I tried I ended up with the plywood decking always meeting at the corner in a not so accurate and resovable way. And further, the rafters on the short side do not rest on the header beam as they want. My friend who is a really skilled carpenter keeps saying, no problem, we do this all the time. But try as I might I just can't seem to get the hip rafter and the plywood layer on top of the rafters to meet up symetrically. This may well be the case where the field carpenters just shim or rip saw things into place, but I would think that I could get something working closer in AutoCAD than I've been able to do thus far.

 

I had to remove many things from this file and zip it to make it small enough to upload. Hopefully there is enough left to get the idea across.

ROOF.zip

[Cad Monkey 2]

I don't know if I would say that I'm a roofing expert, but...

If possible I would recommend having the two roof planes both at 2:12 and have the hip come right out of the corner (where the two headers meet) which would solve the problem of the hip & rafters being unsupported. You could use an angled "U" hanger at the hip to (what I assume is a) ledger at the existing.

I'm not sure if this will work out for sure without seeing the project in more detail - it's too bad larger file uploads aren't possible.

[Cad Monkey 2]

If that won't work, you could build a short stud wall on top of the header to support the hip & rafters, but it would have a sloping top plate and varying stud heights on the side of the hip closer to the corner where the headers meet. Not sure if this helps at all with the roof plywood issue though.

[Dana W]

I'd figure a way to canteliver that long beam far enough past the post on the left corner and return a stub beam lagged solidly into the corner of the house from the post on that far corner, then hang another beam between them to support the hip end rafters. The stub beam should overhang the post the same distance as the long front beam will run past the front left post. Geeeesh, that sounds nonsensical. I can't describe it and I can't draw it. I don't have 3D or I would show you what I mean. Anyhow, the support beam should run ALL the way around until it hits the house. Hanging the beams out past the left end of the posts may affect your thinking on where the posts need to be so the spacing looks balanced on both ends.

 

There can be quite a gap allowed in the plywood. It doesn't have to meet like flooring. I would be surprised if the carpenters get it to within a 1/2" up there. They'll lay in the ply on the short side first leaving it a bit low. They may double up along that hip rafter so they have plenty of surface to work with the joint. then they'll lay in the ply on the long side so it just comes to it, or even overlap it a bit. The waterproof sheathing and shingles will cover the ugly parts. They might even nail some metal flashing over the joint before putting down the roofing membrane that goes under the shingles.

 

Hope you are going to finish the inside with a flat ceiling. You haven't lived until you've tried to fit an upside down corner in at two different angles.

 

It's a good thing you are in Florida. They won't let us build a roof flatter than 4/12 up here unless it's engineered as a flat roof because of the snow load.

Edited May 1, 2011 by Dana W

[Bill Tillman]

Thanks for the replies. This is actually being built in Texas. I think we're at one of those places where the field guys say, "Oh yeah, we've done that before." When in reality they have only done something similar. I don't have the luxury of making the pitches the same as I have to make the short section pitch the same vertical distance in a much shorter horizontal dimension, otherwise the fascia boards will not line up. Building a small pony wall on top of the header beam on the short side seems to be the only solution, though my carpenter friend says he's done it in the past without having to do that. Once he sees the 3D model I think he will finally see that what the client want's is only possible with the pony wall framing.

 

The other problem I seem to have is the hip rafter itself. When I was using a 5 ft width for the smaller section, it worked out a little easier. The roof pitches were different but I was still able to get things working, with the exception of the plywood. I can easily see how the plywood won't seam together perfectly along the hip, but I was having trouble with the plywood on the short side dipping lower than the long section along the bottom surface. I think I may have had an error in my layout causing this but it wasn't showing itself even after multiple attempts at rebuilding it. Now that the client has set the max horizontal dimension for this section of the roof at 24" I am having real troubles getting it like I think it should be.

 

The other alternative I'm going to propose it to simply run the length of the long roof section all the way through and eliminate the hip all together. The client has said they don't like this as the roof will have to run higher along the short side to covel the patio below and it will leave that area where they want to put the BBQ grill open to the elements. So I'm still searching for a solution before giving up.

[Cad Monkey 2]

So your eave on the short side has to be shorter than on the long side, and that's causing the challenge with the fascia lining up?

[JD Mather]

I think it all can be fixed - I'll try to take a look tomorrow.

[tmld]

Move or replace the trees.

Seriously, nothing looks worse than funky design to save some trees that were planted in what turns out to be the wrong location. As a landscrape designer w/ 20 years in the business, and is just the state test away from getting my landscape architecture license, good [structure] design should not be sacrificed for anything that was “recently” planted.

Tell your friend to consider the entire project budget, consider the original cost of the recently planted trees (not the current emotional value), and decide if a poor design solution is more acceptable than good design and trees in a better location. Plants and trees are comparatively cheap while poor design solutions are an eyesore that linger.

[Bill Tillman]

JD, thanks, I'm sure if anyone can find the fix, you're the man.

 

Cad Monkey 2, yes, the design parameters are quite clear. They do not want to let the roof line stick out any further than 24" on the short side. The trees are right there and will be in the way. I originally drew this with the same width on the short side as the long side and of course everything worked fine. Heck I was even able to mirror along the centerline of the hip rafter and viola....then of course the client's wife stepped in and started making massive changes. Oh, well, if it weren't for people like her spending money, then builders would be out of work.

 

tmkl: I hear you bro. The worst thing is that these trees....are your ready for this....are mango trees. I have mango trees in my backyard and they bear beautiful fruit each June and July....but it hardly ever freezes here in South Florida. North Texas on the other hand gets several hard freezes every winter and these tropical trees will most certainly croak when the first one hits. We're still working the angle to get the client to change their mind about the trees. But I think that will be a hard one to sell.

 

Thanks again for all the replies.

[ska67can]

A hip roof must have equal pitches. See attached file.

ROOF II.zip

[JD Mather]

JD, thanks, I'm sure if anyone can find the fix, you're the man.....

 

Well I took a second look at it this morning and saw a couple of errors that were so obvious that I had second thoughts about getting involved in this one. Maybe I'll take a third look.

[Bill Tillman]

J.D. thanks but please do tell what you found. I'm self taught at 3D and try to do my best but cannot say that I always get it right.

 

I placed the tops of the rafters at the same height. And the top of the rafter at the overhangs are also at the same elevation. Those are my control lines so I can get the fascia boards to line up. I would appreciate very much if you would point out the errors you found.

 

ska67can - I beg to differ with you on that. I drew this the other day with a 60" width on the short side and it worked out. when the client demanded it be shortened to only 24" was when I ran into problems. I realize I'm trying to put 50 lbs of mud in a 5 lbs sack but my carpenter friend says he can do this, even if I can't draw it, which I find hard to believe. See attached PDF. The roof on the long side is 2/12 pitch and the roof on the short side is 4.875/12 pitch.

SKETCH(01).pdf

Edited May 3, 2011 by Bill Tillman

[ska67can]

My mistake, you are correct.

[Dana W]

J.D. thanks but please do tell what you found. I'm self taught at 3D and try to do my best but cannot say that I always get it right.

 

I placed the tops of the rafters at the same height. And the top of the rafter at the overhangs are also at the same elevation. Those are my control lines so I can get the fascia boards to line up. I would appreciate very much if you would point out the errors you found.

 

ska67can - I beg to differ with you on that. I drew this the other day with a 60" width on the short side and it worked out. when the client demanded it be shortened to only 24" was when I ran into problems. I realize I'm trying to put 50 lbs of mud in a 5 lbs sack but my carpenter friend says he can do this, even if I can't draw it, which I find hard to believe. See attached PDF. The roof on the long side is 2/12 pitch and the roof on the short side is 4.875/12 pitch.

 

I am measuring 3'-1" +- 1/16" for the horizontal run of the short side and 13'-2 1/2" +- 1/16" for the long side per your 3D drawing. I am looking at it with TrueView 2010. Am I doing something wrong trying to measure in the top, front, and end views of the 3d dwg? Is it to scale? Well, anyhow I measure 1 1/2" for rafter thickness and that's gotta be right? Right?.

Edited May 3, 2011 by Dana W

[SuperCAD]

I think I may be able to help you. I'll open this up tonight when I'm at home and figure it out (can't right now because I'm at work). Matching the fascia lines isn't a problem, you'll just need to have a deeper seat cut on one set than you would the other. If you're interested in learning more than you would ever hope to know about roof framing, there is an excellent book by Will Holladay called "Roof Cutter's Secrets" http://www.amazon.com/Roof-Cutters-Secrets-Framing-Custom/dp/1928580149. I own this book and it is a fantastic reference for roof framing.

[Bill Tillman]

I went back a carefully laid out the roof surfaces again. First off, you definitely can have a hip which joins two differently pitched roof surfaces. I've been able to get that to work even with the shorter 24" limit imposed by the client. The problem seems to be where the shorter rafters meet the hip rafter. I of course want to keep the top surfaces of these rafters in line with the other full length raftes along this run of the roof. But when I slice them off at the vertical surface of the hip rafter which is offset from the centerline of the hip by 3/4" the top limit of the short rafters end up 3/4" lower than the top of the hip rafter.

 

OK - So as of this moment, I seem to have the resolution. I set the UCS to orient along the length of the hip rafter and then sliced the top surface of it to match the 2 pitches. Of course this does seem to resolve my issue in autocad but I know my friend who will soon build this out in the field will never rip off the top surface of the hip rafter like this. Instead, he and his gorillas (ahem co-workers) will take their hatchets and chip away enough wood until the darn plywood sets down where they want it. So in the end it looks like we were all correct. It can be drawn but when they actually build this they won't use the method I have shown in my model. The will simply hack and saw away at the corner until they get the plywood close enough and then cover it up with 30# felt and composite shingles. The pony wall on the header beam may still be required based on where I can place the columns below. But that is a non-issue in this design. My friend will cover up any shim space with cedar sheathing and it will look great when he gets it done.

 

The file is too big to upload even after I zipped it.

[Dana W]

I went back a carefully laid out the roof surfaces again. First off, you definitely can have a hip which joins two differently pitched roof surfaces. I've been able to get that to work even with the shorter 24" limit imposed by the client. The problem seems to be where the shorter rafters meet the hip rafter. I of course want to keep the top surfaces of these rafters in line with the other full length raftes along this run of the roof. But when I slice them off at the vertical surface of the hip rafter which is offset from the centerline of the hip by 3/4" the top limit of the short rafters end up 3/4" lower than the top of the hip rafter.

 

OK - So as of this moment, I seem to have the resolution. I set the UCS to orient along the length of the hip rafter and then sliced the top surface of it to match the 2 pitches. Of course this does seem to resolve my issue in autocad but I know my friend who will soon build this out in the field will never rip off the top surface of the hip rafter like this. Instead, he and his gorillas (ahem co-workers) will take their hatchets and chip away enough wood until the darn plywood sets down where they want it. So in the end it looks like we were all correct. It can be drawn but when they actually build this they won't use the method I have shown in my model. The will simply hack and saw away at the corner until they get the plywood close enough and then cover it up with 30# felt and composite shingles. The pony wall on the header beam may still be required based on where I can place the columns below. But that is a non-issue in this design. My friend will cover up any shim space with cedar sheathing and it will look great when he gets it done.

 

The file is too big to upload even after I zipped it.

 

EDIT I should not read so fast. I missed where you almost say the same thing I just did below.

 

Yer probably still expecting too much from the carpenters. It's not that they don't care about quality. They do care a great deal. It's just that it's framing, not furniture. I seriously doubt if they will bother to cut anything but square edges on the plywood and then slide them together as close as they will go. Roofing ply doesn't have to fit tight at each joint. There are at least two more waterproof surface materials to go over it anyway. They will, of course, cut the long surface angles in the ply to fit the hip. The carpenter is not going to angle cut the top edge of the hip rafter either. They aren't going to make any really steep angle cuts across the ends of the rafters. The carpenters will make use of a number of "specialty" metal rafter hangers that will bend to the angle they need, similar to these made by Teco.

 

For the hip rafter, they probably will double up. The sister one will be just a tad lower to catch the underside of the short side plywood.

 

You are absolutely right that a hip roof doesn't have to be the same pitch.

 

Have you considered though, that you can make both sides the same pitch, and then just notch the short side around the house corner? Of course, part of the angled portion of the short side will run down the wall of the house. Personally, I think that may be less of an aesthetic compromise, but that is only my opinion.

Edited May 4, 2011 by Dana W

[SuperCAD]

You can get it to work with 2x8 rafters for the 2/12 pitch, 14" LVL rafters on the steeper pitch, and an 18" LVL hip rafter. It's a lot of work for such a minor detail.

ROOF.dwg

[Bill Tillman]

Thanks again everyone for your replies. No matter which way you slice this one it's alot of work for a small roof area and when the carpenter finally saw what I was talking about he agreed and got the client to 86 the hip roof design. We're already finished with this one and on to the next challenge.