Building confusion

[Anonymous]

hello,

this is not really a question on technical issues just rather so help in general.

i have been taking 2d cad drawings that have been drawn using point cloud data and replicating the drawings in 3d in revit. i have come to a bit of an issue with dormers on the buildings i am replicating.

as can be seen from the below images, i have set the levels of the peak of the roof and base of the roof correctly (almost, I am unsure how to get precise accuracy with roofs it something else i have been struggling with if anyone can give pointers). 

2D cad drawing (shown for reference to levels)

levels set in revit:

then to get the walls of the dormer in the right place i have used a floor plan which was also drawn in 2d using point cloud data. 

my issue is that if i have used the floor plan as a stencil to get the wall in the right place and also matched the peak and base of the roof levels correctly, why is the dormer coming through the roof at 21.7289 instead of 21.66? any help would be much appreciated. 

[barthbradley]

The DWG is incorrect.  

Does your DWG have side elevations drawn? If so, I'll bet they show it differently.  Pull your dims from the DWG's side elevations. 

[SteveKStafford]

Between the point cloud source and a 2D drawing sketched over a point cloud...which do you trust more? There is an inherent margin of error in point clouds which can only be magnified by sketching over it. 0.7 and 0.6 feet is a difference of 1/10 of a foot, slightly over an inch difference. That's not too shocking and the pitch of the roof alone could account for it. A roof built over trusses will be fairly uniform pitch, assuming they were assemble to a conventional value like 6:12 or 8:12 for example.

If the walls are in the correct X/Y position based on the plan orientation of the point cloud and sketching I'd hazard a guess that the pitch is slightly different than what you're relying on to make the roof. Revit's roof are planar and at a fixed pitch (assuming you've used By footprint and set a pitch). The real roof might flex (age) or have a patch and/or multiple layers of roofing material (bump/bubble) so a point cloud would catch that variance from the true design pitch intended.

All said, the discrepancy you're seeing may not really matter much. Even if you're going to change the size of the dormer the contractor will deal with what is actually there versus what your model "says is there".

[barthbradley]

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@SteveKStafford wrote:

The real roof might flex (age) or have a patch and/or multiple layers of roofing material (bump/bubble)

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You thinks that's a "bubble"?  I'm inclined to think not.  But to your point, the drafter needs to make some assumptions about the construct.  Like assuming the underlying structural is level and plumb.  However in this case, I think it was just misdrawn.  

[SteveKStafford]

Not an actual bubble. When I replaced the roofing on my mother-in-law’s house there were definite measurable bulges in several places because of re-roofing and roof repairs over the years. The house was built in early 1920’s. There was at least a couple of rafters that sagged too. A scan would/could detect those subtleties. 

[Anonymous]

Hi Brad,

Attached is an image of the dwg side elevation. The same measurement on the side elevation is not marked out, but if I measure and calculate the level from the drawing it does show the front elevation to be about true.

[Anonymous]

I did adjust the roof levels to match that of the side elevation and the results seem more accurate although not perfect. 

I am confused as to why the levels used from a side elevation provide more accurate results than levels used from a front elevation? 

[Anonymous]

a lot of money was put into the surveys, in which the surveyor operates on a 4mm tolerance. As it stands the originally posted results are showing a 68mm difference to what the sketched point cloud results are showing, meaning it could be anywhere between 64-72mm out which is a lot.

In regards to your second point I understand how this could be true. but how would i know if this would be safe to assume? What tolerance of measurement do you allow for in your drawings?

Also I've never presented a set of plans to a contractor so don't know if a discrepancy like this will or will not matter

[barthbradley]

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@Anonymous wrote:

I am confused as to why the levels used from a side elevation provide more accurate results than levels used from a front elevation? 

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[Anonymous]

Hey Brad, thanks for the response.

Are you able to give some annotation to the image you just shared?

Also, are you 100% sure the images are aligned correctly? I would have though they align as I have demonstrated in my attached image,

[barthbradley]

I'm not going for dead on ballz accuracy here. I'm just showing you what I see. There is an obvious mismatch between the side elevation and the front elevation.  

[ToanDN]

Unless I were re-creating a 3d model of a masterpiece jewelry from a laser 3d scan, I wouldn't be worried about the discrepancy as you described one bit. As-built conditions, especially for a residential construction, are not expected to be absolute plumb and flat. Overlapping layers of materials such as waterproofing can give "bulges"; slate tiles are not meant to be absolute flat; the house was built by human from pieces, not from 3d printer; everything gets sagged over time...

[barthbradley]

It doesn't have anything to do with the scan at this point.  It has to do with the drafter's CAD translation of the Point Cloud.  

[ToanDN]

It may. The point is even if the drafter translate the scan to DWG correctly, the Revit modeler still needs to factor in all the imperfection of the actual construction, thus use the DWG as a 'guide', not the 'ruler'.

[SteveKStafford]

As the others have written. The contractor will deal with what is actually there. The changes you're making to the building are what is important. Explaining how you're changing it to arrive at the new design. If you're removing the bay then all of the detail about it is nearly irrelevant. They'll fill in the hole (how big is it?) it leaves behind. If you're expanding it's size then they'll want to know what sill height you want and what pitch the bay roof should be. The existing conditions should be documented accurately enough to alert the contractor to things that will/may affect cost (their price) as well as getting the owner to understand how the changes will affect it.

[Anonymous]

Thank you all for the replies.

Whilst I understand what you are saying in regards to the roof not being plumb irl, where do you draw the line in assuming a bump or dip in the structure?

I have 3 dormers in this drawing and the furthest one out is 74.8mm which feels quite significant amount out 

[Anonymous]

how can I be confident that it's a CAD draftsman error? There was a lot of money invested into the DWG's and the draftsman is a highly trusted and has been previously used