Menu path
Cover > Underlay
🧾 What underlay does
The underlay command lets you apply and remove underlayment from the roof. Underlay is a moisture barrier material also referred to as 'sarking' in many markets, 'foil' or 'moisture barrier' in other places.
When you select a cover material (Cover > Metal or Cover > Tile), you can choose to apply underlay to the entire roof automatically. The underlay commands give you precise control over which areas receive underlay when you need partial coverage only.
⚠️ Important setup requirements
Before using underlay commands, you must:
Set up the underlay product under Set-Up > Insulation
Name the product exactly 'Sarking' or 'Underlay' (this naming requirement ensures compatibility across different markets)
Select the underlay material when choosing your primary roof cover material
Note: If your roof requires multiple layers of bulk insulation material, use Cover > Insulation instead.
🔧 Underlay command options
Command | Description |
Apply-Partial | Select specific roof planes to apply underlay to. Unselected planes will have no underlay. Defines the roof as having partial underlay coverage. |
Remove-Partial | Remove underlay from previously selected roof planes. If you remove underlay from all planes, the roof returns to full underlay coverage. |
Remove-Partial-All | Remove underlay tags from all roof planes, returning the roof to full underlay coverage. |
Show | Block fill areas where underlay is applied for visual reference. |
Dig-Part | Digitise custom shapes for partial underlay application on specific roof planes. Uses similar method to Construct Roof > Dig-Outline or Modify Roof > Cut-out commands.  |
Underlay Allowances | Set allowances for additional underlay to cover hips and valleys. |
🛠 How to apply partial underlay
Ensure underlay material is configured under Set-Up > Insulation with name 'Sarking' or 'Underlay'
Select your primary cover material and choose the underlay option
Go to Cover > Underlay > Apply-Partial
Click on each roof plane where you want underlay applied
The software will apply underlay only to selected planes
Important: Once you apply partial underlay, you must define all areas where you want it applied. Undefined areas will have no underlay.
💡 Understanding underlay coverage modes
Full underlay: Entire roof is covered (default when selecting underlay with cover material)
Partial underlay: Only specific areas you select are covered
To return from partial to full coverage, use Remove-Partial-All or remove underlay tags from all individual planes.
🛠 Setting underlay allowances
Use Underlay Allowances to add extra material for:
Hip coverage
Valley coverage
Overlap requirements
This ensures sufficient material for proper installation around roof intersections.