Mansard Roof

[s.houghtling]

My attempts at a mansard roof have failed. My issue seems to be that the steeply pitched aspect of the roof is extending beyond my choosen cut-off level. Is there a sure-fire way of creating a mansard roof?

[bwbarch]

Two possible ways come to mind--both with a certain amount of manual work to accomplish the task. But first, is your "chosen cut-off level" the transition between the steep and shallow pitches?

One way to accomplish a mansard is to place two roofs on top of each other, but the lower steeper pitched roof needs to be sketched as a "doughnut", i.e., don't just sketch a rectangle, or just pick walls as the case may be. In the same sketch you need to draw an inner rectangle at a consistent offset from the outter edge of the roof. Make sure you uncheck the "define slope" button for these inner lines.Then, finish the sketch and draw another roof aligned to the inner edge of the lower roof. You will need to adjust the second roof's height in section to align its edges to the break point of the lower roof.

The other method is to draw a roof by extrusion in multiple pieces that in section would include the steeper and shallower slopes as part of the extrusion profile.Then you would have to use the "Cut Plan Profile" button to carve up the pieces to fit at the hips. Again, a level of manual calculation of roof plane intersections and, especially with this method, no simple way to adjust slope except by editing the angle of the lines in the sketch of the extrusion profile.

[twiceroadsfool]

Roof by face may prove to be easier for a mansard roof. Use an in place extruion or a sweep (if it terminates at an upper flat roof... think mcdonalds), then you can roof by face on the different planes of the mass. keep the mass in place for editing, and you wont have to fuss with the joinery...

[bwbarch]

Since there is not a specific OOTB tool for mansards, there are pluses and minuses to any method used. The two separate roofs method gives you more direct control over traditional slopes, overhangs, etc. by using the normal roof tools, including "pick walls", etc. But, you must manage separate roof pieces. The mass and extrusion methods are great for just that--massing, but still require that you use manual methods to manage the slopes and overhangs, and in some cases, roof intersections.

I would give each method a try and see which one is the most manageable for you based on your project's needs ans staff skills. Don't forget to take advantage of some well-placed reference planes either.

[twiceroadsfool]

Using pick walls is only going to be benficial for the lower half of the mansard, unless the OP is going to build the structure underneath as a wall, which is more than likely going to be handled by detialing in wall sections anyway. Having said that, with the way roofs behave correlating to their Host level, sketching two seperate roofs for the top and bottom piece is a pain in the rear, especially when dimensions get altered through the design process. the roofs will shift up and down slightly as you adjust their meeting point, and youll certainly be on the constant adjustment plan to make sure theyre all tweaked just right. Plus, you have to constantly make sure the "depths" of the sketches on ALL sides of BOTH donuts are contiguous, or the eaves will get out of alignment and further frustrate you.

Massing is massing, but ROOF by mass will put actual roof system families ON the mass, which (of thought through properly) can be much more efficiently managed for a Mansard roof. A profile family can be loaded that has some minor parameterics in it to use a "traditional roof slope," and that profile can then be used within the in-place Mass to manage the mansard. Related Hosts > remake will keep everything current. Align the path lines to the walls, and youve got a pretty manageable system for not letting the roof get out of hand.

You are correct, both methods WILL work... But having spent a large amount of time messing with mansard roofs, i would want the geometry of the ROOFS taken care of, and if i had to pick between tweaking the sketch of a sweep path and one profile, or constantly fidgeting with two roof sketches / heights / slopes, and all the subsequent joinery issues, Roof by Face is winning, in my opinion...

[bwbarch]

So I guess we much each pick our battles... until Revit gives us a true "multi-slope roof tool". :) Individual experiences do vary.

Actually, "pick walls" can be easily used for the upper roof as well when you set up a level to control the break between the two roofs. The upper roof can then be associated to this upper level, and a short wall hanging from this level provides the "pick wall" alignments for the upper roof. Also, the upper level enables you to use the "Cutoff Level" and "Cutoff Level Offset" parameters of the lower roof object to adjust its upper intersection, precluding the need for the "doughnut" sketch method I mentioned in a previous post. (And maybe this goes back to address s.houghtling's original comment.)

IMO these methods take alot of the hassle out of manipulating two roofs, but I certainly understand your preference for the roof-by-face method. I normally prefer to use the "pick walls" and rafter-vs-truss methodologies of the standard roof objects whenever possible since I have found these to be a huge benefit for accurately modeling wood frame construction. To each his own I guess.

BTW, I do find that these discussions are good motivation to fine-tune our own methods and to always be willing to consider other ones as they are presented to us. I do like your parametric profile suggestion to control the geometry of the mass.

[patricks]

I suspect the OP's problem is that when you specify a cutoff level for a roof, that level tells the BOTTOM SURFACE of the roof where to stop, and the rest of the roof extends up vertically from that level. Then when you create your upper roof, the 2 don't join correctly due to the different slopes.

You can also create a mansard roof by using the point/ridge/valley editing tools you get when you first draw a flat roof and then select it. The drawback here is that the thickness of the roof layers does not stay consistent. The thickness stays the same in the vertical dimension, which makes the actual thickness less when you start to slope the roof.