Roofs

taharvey · Posts: 58

Andy,

Roof issues/questions:
1. If the overhang on a roof is set to 0, it still has an overhang. How do I get no overhang?

2. A previous post indicated the auto generated overhang doesn't actually provide shading in E+. True? Why not? So all roofs need to be manually created if overhangs are to function?

3. If the gable option is checked, and the roof is sqaure in area, the program still makes a hip roof.

Andy Tindale · Posts: 1205 Location: Stroud

1. Yes, you always get a small overhang with roofs created using the 'Pitched roof' block type because of the way these blocks are created using the perimeter control line. The small overhang is necessary in order to make the roof zone generated for EnergyPlus line up with zones created in the block below using the equivalent perimeter control line.

2. As you say, 'Pitched roof' block types do not create overhangs in EnergyPlus simulations at the moment. This is a limitation of using the 'Pitched roof' block type at the moment and we will be addressing this problem soon by generating additional shading devices to give the equivalent shading of the actual roof. If you need to model overhangs you could either draw your roof using horizontally extruded blocks or you could continue to use Pitched roof block types but add a component block just below the roof eaves to give you the shading you need.

3. Yes square roofs always generate hip roofs, because there is no unique gable solution for a square perimeter. If you need a gable roof you could make the roof a bit bigger in one direction or else draw the roof as a horizontal extrusion.

Regards

Andy

rankoromane · Posts: 66

Can you fool DB as follows?:
Create a construction line at the actual roof overhang position, then generate a roof with zero overhang using the construction line as the perimeter.

I have tried this and it appears to work.

Will the 'small overhang' mentioned in the previous reply compromise the geometry?
How large is the small overhang?
Presumably it doesn't affect the shading?

regards,

Andy Tindale · Posts: 1205 Location: Stroud

Yes this is a good solution - you will get exactly the overhang that you define with your roof perimeter in your EnergyPlus simulation and you will get a very small overestimate of the overhang in visualisation view.

The size of the small overhang is equal to the thickness of the roof construction multiplied by cosine of the roof slope angle - and it does not affect the shading in EnergyPlus.

Thanks for the idea!

Andy

Andy Tindale · Posts: 1205 Location: Stroud

The next version of DesignBuilder to be released (version 1.1.0) will have automatically generated overhangs for pitched roof block types. We are currently finalising testing on this new feature. It will use EnergyPlus shading devices positioned appropriately and so there will be no need for any special modelling tricks for pitched roof overhangs.

Andy

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