Roof Designing and Roof Drafting
If you can't think easily and every which way in three dimensions, swear off roof design. You'd better lay off cornice work, too. Come to think of it, plumbing isometrics won't be your friends, either. AG 2002
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This is about roof designing and roof drafting. When necessary, computer software control and tool references will be to AutoCAD, because that’s the drafting device I use and with which I have become the most familiar. Whether you're working on your first rooflines or roof design is a way of life for you, chances are excellent that you'll find something eminently useful in the tutorial.
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What is it about roofs that attracts our attention?
 | Roofs shelter. |
| Roofs tell us about house size, function and fortune, lifestyle, valued spaces. |
| Roofs preside over facades, or our public faces, either in complement or conflict to site and structure. |
| Roofs own most of the visual third dimension – depth – of house design. |
Moreover, roofs offer challenge to conception and custom home building. Many client prospects come to us with long-considered custom floor plans, maybe a sketchy elevation, and almost always no roof design. Why is that? It’s because roof design and drafting can be really tough. Not just tough to do artfully, but tough to do overall. Mathematical and custom home building imperatives rule. Roofs operate demonstrably in three dimensions; our trick is to engage roof design on its own terms: we draw roof designs in 3D (with tips and tricks to follow).
If you’re working with a single pitch of a single roof type, especially gable roofs, roof design and roof drafting need not be difficult for full-storied structure, commonly, single-story or two-story. But when it comes to certain classes of residential design, roof structures can challenge –
| Multiple pitches, the more wide-ranging the tougher it can get by addressing key issues of holding eave lines level and soffit depths equal but not both. (Herein, some styles enable and some don't.) |
| Multiple roof planes, for judgment and options of fit and appearance. |
| Story-and-a-half structures, where your work is only some done when you’ve designed and drawn the roofline. Thereafter, you must arrange L2 underneath those lines, and then very carefully consider load paths from roof on down to foundation since the L2 walls are mostly if not entirely interior to L1’s perimeter walls. |
| Ragged and asymmetrical perimeter wall lines, especially in hip roof applications simply for the rigorous nature of hip roof layout. |
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This is our methodology for drafting roof designs – any designs, anytime – a/k/a/ roof design and roof drafting tips & tricks.
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Start working on roof design once the custom floor plan view is done for the level the top plates of which will support the roof itself. Not before. In one-story and story-and-a-half structures, that means finishing up L1’s plan view. In two-story structures, it’s L2’s plan view (with an eye to what of L1 is outside L2’s perimeter). Therewith, you’ll know about function and value of sheltered spaces, plus exterior perimeters.
Roof design has one general imperative: roof structure at its perimeters runs to top plates. It doesn’t matter whether it’s truss roof structure or hand-framed roof structure. Interior bearings are matters of functional layouts or formative engineering; perimeter bearings are almost always exterior wall dependent (at their top plates).
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Draw the rooflines in 3D. Why?
| Because we can. We and AutoCAD are that good. |
| Experience sheer joy in the intellectual challenge. |
| Set dead-reckoning bases for elevations and roof plan views. |
| Establish determinates of below-roof space allocation in, say, story-and-a-half layouts of occupiable space. |
| Specifically site and associate roof elements such as dormers, eaves, valleys, rakes, chimneys, skylights, etc. |
| Acquire visualizations of roof appearances often impossible to observe in 2D elevations, e.g., views of intersections, ridges, rakes, and eaves in perspective. |
| Enable watershed and runoff calculations and specifications. |
| Enable calculating and specifying roof venting, especially passive venting. |
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Draw wireframe rooflines from the interior, top edge of top plates. The purpose of basing rooflines on these interior top edges is to surrogate structural vectors with line vectors, that is, to track rafters’ bottoms of face (assuming fully-seated bird’s mouths) or certain truss bottom chord bottoms of face, i.e., closely approximate interior space for attic or occupation. Roof structure will eventually bulk up these rooflines drawn this way once member height is factored in; to achieve roof look, roof function, roof form, single lines off roof plate interior top edges demonstrate with simplicity much of what interested parties need to know about roof design front-end.
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Be sure each line you draw starts where it should and ends where it should, notably a sensitive point with AutoCAD drawing on the one hand for the potential failure to connect objects and on the other hand the awesome controls and tools to detect and correct.
Roof design can generate lines all over the place, many or most of which get wiped out in the endgame, but are necessary to setting angles and parallels and meeting points, etc. Especially in more complex roof structures it’s tempting to stand off the structure to keep an eye on form. But it pays bigtime to zoom in and out very frequently, in order to assure the substance of lines run where they’re supposed to. There are no excuses for missing a specific point acquisition or not meeting peak lines. Such inaccuracies can ruin a draw if left unaddressed or, worse, undetected.
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Shift views to check on the reality of a roof design in progress. We do, all the time – both planar and perspective views. Lines in one view can appear right on, and in another can present differently. Misread the UPC, revolve in the wrong direction, mistake one acquired point for another, and you can go wrong and stay wrong for a long time while troubles mount in fillets, extensions, and trims.
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Dead reckon a slope and then copy it. Aside from time consumption in drawing each slope in turn, we’ve found that sooner or later, we can be a little off in defining a given slope over and over from scratch. And that misdefinition can wreak havoc with putting together a roof design. So, we carefully layout a given slope of line, and stay with it by copy selection, mirroring, and revolution. Occasional run-rise checks, and more frequent changing views and working controls and tools can verify the integrity of slopes.
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Draw entire roof system segments one at a time. There’s a siren song to hear in putting up all the basic rooflines at the outset. It is our experience that all the rooflines are basic, whether they’re the ones you use to set others and then trim or delete or they’re the real deal leftovers when the draw is done. This prescription is all the more potent in multiple hip roof plane designs where so many lines need laying out before telling which few whole lines and segments survive intersections and trimming.
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Apply color at every opportunity. As a practice, we identify separately by color primary, secondary, tertiary, etc. rooflines during our draws. Later, we can color control to a layer or white or whatever; during a draw, color perceptions help us with bearings, imagined depths, unexpected conflicts, and the like. And, of course, we’ll use colors as needed to pick out lines in one view when it’s tough to distinguish one in a bundle when views shift, for example, in ridge intersection cut-offs.
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Design and draft roofs in the same file with the other sheets. This notion is not good for everyone. Sometimes, it seems that it’s not good for us. We have come to design and draw a given project in a given file; effectively, we keep all sheets before us in origin view. Practically, this one-file-fits-all methodology greatly benefits efficiency in not only first-draws, but also subsequent modifications. We can reposition custom floor plans or elevations in-line between levels or other planes quickly and cleanly. Particularly in roof design and overlays, this one-file methodology can be very powerful. Admittedly, this approach is not for everyone – we operate a muscled standalone (with a muscled backup and a transferable data base in an external drive for duplicating our work often during a day’s drawing); speed is always a concern – file retrieval, view access, object acquisition, can be troublesome in larger file management. All things considered, our single-file system has performed superior to alternatives we’ve tried and had to abandon in self-defense.
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(If this is your first visit to Before The Architect, please consider spending a few moments looking over the Site Map, in order to get a feel for the architecture of the site itself. Our E-mail: jrp2h2000@yahoo.com.)