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Walls: Applying Functions to Compound Layers

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In this article we will introduce the concept of Compound Layers within Revit Wall structures. Specifically, we will look at the 5 hard-wired Layer Functions (6 if you count the “Membrane” layer which typically has a thickness of 0). And to round off we will take a look at how the “Priority” system works, with regards the Layer Functions. If you are very new to Revit, I would strongly suggest you take a quick look at this article first, which gives a basic overview of Walls within Revit Architecture.


In real life walls are very rarely built-up from a single layer of material. Normally, they consist of many layers, each of a different material and performing a different function. Some layers are there to form a structural support for floors or roofs, other layers serve to form an insulation or moisture barrier function.


As you would expect Revit has the ability to model complex wall structures, representing the various constituent layers that make up a wall.

Let’s just dive in and take a look at how this all works. I’m going to open a new Revit Project file and select the wall command. As soon as I do this, the Type Selector gives me a choice of all the wall types defined within this template. I am going to choose “Exterior- Brickwork on Mtl. Stud”.

Now instead of placing a wall into the model, I’m simply going to hit the “Edit Type” buttin on the “Type Selector”….

Upon doing so, we are presented with the “Type Properties” control panel……

You will notice that on the left hand side of the panel is a Preview of a section of wall. If this preview is not immediately visible, just hit the “Preview” button at the bottom of the panel and it will pop out to the left.

If we place our cursor in the preview pane and zoom in, we can see in detail the make up of this particular wall type…

Now you can clearly see the various layers that make up the total thickness of the wall. But “how” do we get to control these layers? Easy, just hit the “Edit” button next to “Structure” on the right hand side of the panel….

Upon doing so, the panel changes to show the “Edit Assembly” control panel….

This is more like it! As you see from the variety of controls and parameters, there’s lots of fun to be had in here! In fact there’s too much to cover in a single article, so we’ll save some of it for another day. The main area we want to focus on today is the large panel on the right, labelled “Layers”….

As you can see, there’s lots going on here. So let’s dissect it a bit at a time.

Each row in the table represents a different layer in the wall. Some layers represent “real world” layers (such as the masonry brickwork, whilst other layers are notional and have no thickness (such as the two “Core Boundary” layers).

Go ahead and click on each row in turn- it’s easiest to click on the row number on the left hand side of the row. As you click on each row (form 1 to 9) notice how the corresponding layer is highlighted in blue in the preview pane. In the image below you will see that I’ve highlighted row 6, and the corresponding layer has lit up in the preview…..

An important thing to note is that the order of the rows matches the order of the layers in the wall- 1 being on the exterior of the wall and 9 being on the interior of the wall. When you create your own wall assemblies, keep the order of the layers in the same format (ie the first layer being on the exterior and the last one being on the interior)- this will make your walls behave correctly in the model.

If we now look at the columns in the table, you will note that there are 4 different parameters:-


This parameter stores the function of each layer. The function is inherently linked to a “priority system” which we will discuss in a short while


From here you can choose a material for your layer


Self-explanatory really. This controls the thickness of the layer. PLEASE NOTE: The two “Core Boundary” layers (rows 5 and 7 in our example) have a thickness of 0. Additionally, any “Membrane” layers should also have a thickness of 0 (take a look at row 8 in our example).


This is a simple on/off parameter for each layer. It controls whether the layer tries to wrap back to the core, at the end of each length of wall. Wall wrapping is a whole topic in itself and we’ll cover it in a separate article.

OK. Let’s get back to “Function”- after all, that is the topic of the article! Go ahead and choose any row and click on the function entry itself. This will cause a drop-down list to appear- allowing you to choose a Function for this layer….

The important things to note are:-

There is a total of 6 different functions to choose from. The first 5 each have a number in parenthesis, whilst the last one (“Membrane Layer”) is unnumbered

These functions are hard-wired into Revit’s compound structure assemblies- ie walls, roof and floors. You cannot create new Functions, rename the existing ones or delete any. You are stuck with the ones here!

The most important thing to know is that the numbers represent the “priority” of the layers that you assign the functions to. “Priority” is used when joining walls together- to ensure the various layers “clean-up” correctly.

There are some basic rules of Priority, that all compound structures follow, when joining together.

1) The higher priority layers always take precedent. For example, a Priority 1 layer will barge it’s way through lower priority layers in order to join up to another Priority 1 layer.

2) Lower priority layers cannot cut through higher priority layers, during the “clean up process”- they are just stopped by them.

3) The exception to both of the above are layers that fall within the Core boundaries. A priority 2 layer “within the core boundaries” will override a priority 1 later” that is situated “outside of the core boundary”.

Let’s take a look at this is practice. Let’s put a small section of wall into our model…

Now let’s add a second piece (of the same wall Type) and get it to join the first, forming a “T-junction”….

You will note that the “Structure” function / layers have joined together. “Structure” is a Priority 1 function- so they just pushed through everything else in their way and got together!

The plasterboard layers (“Finish 2” / Priority 5) both met up and joined because there was nothing at all in their way.

But all three of the other layers coming in from the right are stopped in their tracks by the big grey layer (Priority 1). They are not allowed to get past this and hence cannot join up with their corresponding layers on the left.

Obviously in this example, we have joined two section of wall that are of the same Type. But the Priority rules still apply when you are joining 2 different compound structure types.

So the whole moral of this story is that when you are creating your own compound structure assemblies (be it walls, floors or roofs) think carefully about they will interact with other assemblies when they meet.

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