topic Roof Drain Calculations in Revit MEP Forum

Roof Drain Calculations

spakkala — Thu, 05 Jan 2023 19:16:26 GMT

Hello,

I am trying to make a schedule where I can calculate different pipe sizes based on roof areas and slopes. How do I make a "generic" schedule where I can just use a bunch of manual inputs and formulas to do this without creating a "roof" or some other schedule. Is there a way to do these types of calculations and also put them on the drawings without doing a specific schedule?

Thank you,

Re: Roof Drain Calculations

ToanDN — Thu, 05 Jan 2023 19:20:19 GMT

Yes you can create a generic annotation family with input parameters for roof area + other code required criterial, and a label from a calculated value parameter based with a formula using those input values.

Re: Roof Drain Calculations

HVAC-Novice — Thu, 05 Jan 2023 19:41:41 GMT

Many years ago I did that with an area plan. You should review your code what area you need to use, but it typically is projected area and for a sloped roof, the area of the roof, will be larger than the (horizontal) projected area.

I don't recall the details, but in my case there was a discrepancy between how IPC and our local code handles that. AFAIK, IPC used a gpm, and our code uses a roof area to read out the drain size. Both methods end up with about the same pipe size. But this is why I said you should look up what your local code requires so the JHA likes your plans.

Re: Roof Drain Calculations

spakkala — Thu, 05 Jan 2023 19:48:58 GMT

This is a flat roof so the area is the roof area. But I understand what you are saying for if it was a sloped roof.

Re: Roof Drain Calculations

spakkala — Thu, 05 Jan 2023 20:45:33 GMT

Unfortunately this doesn't work with a linked model. "Area is not enclosed".

Re: Roof Drain Calculations

spakkala — Thu, 05 Jan 2023 20:48:01 GMT

I will have to play around with this and see. I was essentially looking for something to just create a schedule that didn't have to reference something. Then I would input all the necessary data and calculations. Something I would typically do in Excel but don't want the Excel PDF on my drawings.

Re: Roof Drain Calculations

ToanDN — Thu, 05 Jan 2023 20:54:09 GMT

Okay then use area plans and Area schedule with calculated value parameters. Draw area separation lines manually to create the roof areas you want.

Re: Roof Drain Calculations

HVAC-Novice — Thu, 05 Jan 2023 21:50:16 GMT

IIRC, I created the areas manually per roof drain. You need to look at the sloping plan to see which area contributes to each drain. i don't think you could automate that easily or get from the linked model. I don't think areas really automatically place (like rooms or spaces do with room-bounding elements). So it's manual time.

Re: Roof Drain Calculations

iainsavage — Thu, 05 Jan 2023 23:56:37 GMT

I added a clearance zone to my roof outlets then used the area of the zone via parameters and formulas to calculate runoff rate.

I don't know how its done in other countries but in UK you measure the plan area covered by each outlet then adjust by a factor to take account of roof slope angle and you also need a factor for any runoff from walls which project above the outlet. If there are downpipes which dicharge onto lower roofs you then need to add that to any runoff from the lower roof itself.

My resulting schedule was like this:

I've attached a sample project with an outlet and schedule contained in it.

NB: for this to work the connector on the outlet has to be set as Domestic Cold Water - it won't work with sanitary category.

Re: Roof Drain Calculations

spakkala — Fri, 06 Jan 2023 12:11:21 GMT

That's pretty slick!

Re: Roof Drain Calculations

iainsavage — Fri, 06 Jan 2023 12:32:19 GMT

I can't take all the credit, it was based on an autodesk university tutorial.

Link to the tutorial contained in this forum link:

https://forums.autodesk.com/t5/revit-mep-forum/pipe-flow-calculation-by-creating-shared-project-parameters-in/m-p/10598244#M80966

Re: Roof Drain Calculations

HVAC-Novice — Fri, 06 Jan 2023 17:12:47 GMT

as iansavage said, you also may have to add stormwater from walls. And even if you have a flat roof here, I'd workout a scheme that works for all roofs. In the schedule you could create a parameter that you add area that you can add to the calculated area and then base the calculation of the combination of both (roof + wall). On roof portion without wall, you just leave that wall value zero. Read code what you should do for walls. Some recommendation is to use half the wall area. You want to get a somewhat conservative design.

Re: Roof Drain Calculations

spakkala — Fri, 06 Jan 2023 13:57:19 GMT

I am working on it exactly as you mentioned. A field for wall area that needs to be incorporated. So far its coming along

Re: Roof Drain Calculations

spakkala — Fri, 06 Jan 2023 19:38:02 GMT

I have this mostly working as I want, but I fear there is no solution to my last issue. I have multiple roof sections where I am getting all the correct information, but I also want the "grand totals" to be able to utilize a formula. Example of the pipe size. I don't simply want to add up all the pipe sizes for my overall pipe size. I would like to use a formula. Anyone know if there is a way to accomplish this?

Re: Roof Drain Calculations

fabiosato — Fri, 13 Jan 2023 00:54:12 GMT

Hello,

You can select the link, go to Edit Type and check Room Bounding option, then you will be able to create spaces.

Re: Roof Drain Calculations

iainsavage — Sat, 14 Jan 2023 21:37:45 GMT

@spakkala schrieb:
I have this mostly working as I want, but I fear there is no solution to my last issue. I have multiple roof sections where I am getting all the correct information, but I also want the "grand totals" to be able to utilize a formula. Example of the pipe size. I don't simply want to add up all the pipe sizes for my overall pipe size. I would like to use a formula. Anyone know if there is a way to accomplish this?

Can you clarify your question?

Do you want the totals in the schedule? You could add a text parameter to the roof outlets and use that to identify the appropriate roof area, then you can group your schedule by that parameter and get a total for each roof area.

Or do you want the information within the pipes? In which case just join them together and they should automatically totalise.

Re: Roof Drain Calculations

spakkala — Mon, 16 Jan 2023 12:12:30 GMT

I want to be able to enter a formula for the totals. When I do the calculations for each roof, it gives me what I want. But then when I want to see what size pipe that will handle the whole roof, I want the formula to transfer over to the total which would give me the size of pipe to handle the entire roof. I don't think its possible, so I may have to just create a separate schedule which has the total roof area.

Re: Roof Drain Calculations

iainsavage — Mon, 16 Jan 2023 16:29:35 GMT

If you join all the pipes from one roof together then the flows will transfer and automatically totalise down the line so you should then be able to select appropriate pipe sizes.

That would only work if all the pipes serve the same roof though.

If they serve different roofs with different slope aspects then there would be diversity factors to apply - to be honest I'm not an expert on that part because we would have passed that over to the Civil engineer at that point.