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Roof framer does not work properly


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To my great surprise, I may need to do some roof framing diagrams and even (shudder!) a hip roof. "No worries, mate!" I said. But, alas: I should have known that it won't work. In the test run (repeated 3 times) some (!) hip members jump above the roof.


Why did I think that purlins are above rafters and support the roofing? Obviously I was wrong: NNA puts purlins to the undersides of rafters. Surely they must know.

Edited by Petri
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It's not the pitch but their entire z-level. I have now moved the offending hips downwards and purlins on top of rafters, but this is not fun!

Fortunately one can select all purlins:

SelectObj(INSYMBOL & INVIEWPORT & ('FramingMember'.'type'='Purlin'));

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Well, every time so far which is every time I have ever used this framing thing.

Created now four brand new files. Drawn a rectangle. Said "Let there be a roof". Said "Let there be framing". Always the same result - identical, mind you. The left hand side ridges every time at least approximately the same distance displaced.

I guess I should change the settings more. Maybe if I leave out rafters, purlins and ridges...

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The problem of different terminologies in different countries. What it calls purlins we call underpurlins here in Australia. Similarly beams are collar ties to us. The dialog box is clear enough for this not to be a problem though.

The Roof Framer doesn't draw the elements that the roof covering is fixed to, and I wouldn't expect it to. Our terminology gets confusing here:

- Profiled metal roofs are fixed to purlins.

- Tiled roofs and the like are fixed to battens.

The Roof framer gets you most of the way there. Any errors that the it produces are easily fixed. Just select that member and correct it via the OIP.

Edited by mike m oz
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The problem of different terminologies in different countries. What it calls purlins we call underpurlins here in Australia.

I guess someone at NNA should correct Wikipedia:

"In architecture or structural engineering, a purlin (or purline) is a longitudinal structural member in a roof. Purlins support the loads from the roof deck or sheathing and are supported by the principal rafters and/or the building walls. The use of purlins, as opposed to closely spaced rafters, is common in pre-engineered metal building systems and some timber frame construction."

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The Glossary of Australian Building Terms defines a purlin as follows:


A horizontal beam in a roof, at right angles to the rafters or trusses, to provide:

(a) intermediate support for rafters, itself supported at intervals by struts or cross walls; or

(b) support for the the roof covering, bearing on the principal rafters of roof trusses and spanning between; or

© support for common rafters between trussus.

Our common term useage for timber roofs is as follows:

- Underpurlins are horizontal timber members under and supporting rafters.

- Purlins are the members at right angles to the rafters or trusses which the roof sheeting is fixed to.

- Battens are small versions of these at relatively close centres such as those used for tiled, slated and shingled roofs.

What they are called in different parts of the world is not that critical in my view. In our increasingly global world there will be an emerging consensus and understanding of what they are.

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The role and significance of the American purlin escapes me...

I know next to nothing about timber roofs, but in a steel structure members bolted to the undersides of beams, without any independent support (ie. they are hanging from the beams) would not serve any useful purpose.

However, getting the real purlins (or battens) into the framing diagram would, in my opinion, be essential. If we raise our sights ever so slightly, we may realise that steel roofs are occasionally also framed. In fact, in my projects all steel roof structures have had some sort of frames. The secondary or tertiary members called "purlins" in English can be quite a significant component.

Let's say I have trusses at 12000 c/c. The "purlins" could be 6000 c/c HEA250 or HEB320. Or something like that. I agree that no-one is interested in the battens of a house roof, but there are other kinds of projects, too.

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The Roof Framer produces traditional framed house type timber roofs. (Maybe in the future we will get options for roof trusses and steel as well.)

If I need to show the purlins or battens I do them manually. With purlins the end spans are usually less than the intermediate spans because of the higher wind loads that exist in these areas, combined with the end span fixing condition. On big roofs several end spans may be less than the intermediate spans. For the Roof Framer to provide correct purlins it would need to be able to have nominated end spans for 'x' purlin spacings both top and bottom. It would also need to be able to add the trimmer purlins either side of the hips and valleys.

If we take it a bit further then it would also need to be able to automatically model:

- the valley boards on valleys.

- the outrigger framing on gables and dutch gables.

- the barge boards on gables.

Edited by mike m oz
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If I create, say, 230x240 "purlins" in class "Steel", that is good enough for the HEA240s. If they would just be rotated and placed on top of the (non-existing) rafters.

Of course it would be nice to have the profile, too. Not to mention other aspects relevant to other than domestic timber construction.

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Any CAD framing program is limited by this.....ON and OFF. That's it!

If you gave a seasoned professional framing carpenter a CAD program and said frame a house I'm afraid he would find a real tall bridge or cliff to jump from with the clear expectation of not surviving the trip for fear of relying on a set of switches to tell him how to do his job.

The VW framing tool has tremdous potential but currently is limited to the same complexity as the roof object tool. So trying to make it frame properly depends on the users ability to construct addittional 3D objects for support.

What I have found, however is if you take a real simple roof and put it through all the variables offered in the roof framing dialog you can begin to understand its power and at the same time the limitations. It does work when you know how to manipulate it. It's kind of like a universal remote...you know there are buttons on there that you have no idea what they do but vow to someday figure it out, however you seem to be too busy watching TV to figure it out.

Oh and by the way McMansionism has slowed to a trickle...but the mini-mac is back and going strong.

Pete A.

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I would not imagine that a CAD-program could tell a pro how to do things, but it should tell everyone what and how much/many.

If in the process it does simple routine calculations of details (notchings etc) and rough measurements of member lengths, even better. This is all I expect.

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While we're at it, there are quite a number of improvements and bug fixes that would make the roof framer a great tool instead of a so-so utility that limps along and is barely worth the time it takes to learn how to use it. Starting with the ability to separately specify how the rafter ends are cut - this can be adjusted after creation, but why not at creation?

I could go on for a couple of pages enumerating the bugs and the ways this tool departs from actual construction practice. Like the way you often get rafters only at corners if you select the option to have them align at hips, and the way hips are not end-cut to align with the wall or fascia plane.

And on a related issue, the roof object's fascias have their outside corners higher than the roof plane. If I built something this way, I'd be sued. NNA: please, please have practicing architects go over these tools carefully before issuing them! With just a little more effort, they could be practically perfect.

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