Auxiliary Products: Marble, Sill, Trim, and Sub-Frame
Guide · 2026-05-14 · PenSoft Team · 6 min read
A window or door often does not come alone. There is a series of auxiliary products that ensure a smooth, waterproof, and aesthetic transition between the rough construction opening and the joinery: sill, trim, sub-frame, and marble. When these are neglected, the installation remains incomplete, water issues arise, and the appearance deteriorates. In this article, we introduce these auxiliary products one by one.
Sill (Exterior)
The sill is an inclined profile that is placed at the outer bottom edge of the window and directs rainwater away from the joinery (usually aluminum). Its function is critical: water must be expelled outside without seeping under the joinery and into the wall. The sill should be considered with the correct slope, side closure pieces, and drip detail. A missing or incorrectly installed sill can lead to staining and moisture issues on the facade over time.
Marble (Interior Sill)
On the interior side, the horizontal surface placed under the window is commonly referred to as marble (or marble composite). It provides an aesthetic finish and transforms the area under the window into a usable surface. Marble is selected based on its thickness (such as 2 cm, 3 cm) and depth, and it usually sits a few centimeters additional to the wall on the sides. Accurate measurements ensure that the marble sits cleanly against both the joinery and the wall.
Trim
Trim is the finishing profile that surrounds the joinery and covers the gap between the rough construction and the joinery (foam, installation marks). It can be used on the interior and/or exterior. Its function is both aesthetic (providing a clean frame appearance) and practical (hiding gaps and installations). Trim comes in various types such as flat, clip-on, and T-shaped, and is chosen to match the color of the joinery. It is calculated by linear metre, and when used on both sides, the measurement doubles.
Sub-Frame (Blind Frame)
The sub-frame is a front frame that is fixed to the rough construction opening before the actual joinery is installed. The joinery is then mounted onto this sub-frame. Its advantages include:
- It facilitates and speeds up installation; the joinery sits on a ready-made frame.
- It maintains the dimensions of the opening during the construction process.
- It makes it easier to remove and replace the joinery in the future.
The sub-frame is selected based on its cross-section (such as 20×40, 30×40, 40×40) and which edges it will be used on (top/bottom/left/right) is determined by the project.
Connection (Joint) Profile
When multiple joinery units are combined side by side or stacked (for example, between two windows in a wide facade), the connection (joint) profile provides the joining element. It takes on both structural joining and aesthetic finishing; it is calculated along the joint line by linear metre. It is indispensable in multi-chamber, composite positions.
Threshold Profile
In balcony and terrace doors, the bottom profile can be removed and replaced with a special threshold profile (aluminum or PVC). Threshold-less or low-threshold transitions are preferred for both aesthetic and accessibility reasons; however, they require careful water insulation.
All Are Part of the Quotation
The common feature of these auxiliary products is that they all affect the price but can easily be forgotten in manual calculations. Omitting to include a trim or sill in the quotation directly results in loss. Moreover, the measurement for each is derived using different logic: trim is calculated by perimeter, sill by bottom edge, sub-frame by edge, and marble by depth and additional.
For Fabricators: Transferring Auxiliaries from Drawing to Quotation
At PenSoft, all these auxiliary products are treated as part of the position. Marble, sub-frame, trim, sill, connection, and threshold are reflected in the drawing (for example, trim edge bands and sill steps are visible) and added to the quotation with accurate measurements. If the trim is double-sided, the measurement automatically doubles; marble is priced according to depth; sub-frame is calculated based on edges. When the color changes, the price remains synchronized. Thus, the quotation includes not only the joinery but also every part that completes it.
Summary
- Sill directs water away outside; marble provides aesthetic finishing inside.
- Trim covers the installation gap; if double-sided, the measurement doubles.
- Sub-frame is the front frame that facilitates installation.
- Connection profile joins multi-chamber positions.
- Threshold profile manages water and accessibility in balcony/terrace transitions.
- All affect the price and must be included in the quotation.
Interior Sill Material Options
There is no single material for the interior sill (under the window); the choice affects both aesthetics and budget:
- Natural marble: Provides a classic, high-quality appearance. Each piece has unique veining; however, it is costly and requires maintenance against staining.
- Marble composite: Made from marble dust and resin, it is a more homogeneous and generally more economical option. It has high color consistency.
- Quartz surfaces: A durable, stain-resistant, modern alternative.
- Laminated/PVC sill: The most economical solution; can have wood or stone patterns, and is lightweight.
When presenting these options to the customer, explaining both the appearance and practical features (stain, scratch, maintenance) sets the right expectations.
Installation Order and Water Detail
The quality of auxiliary products depends not only on the right material but also on the correct installation order. Especially exterior sills and water management can cause issues if the order is incorrect:
- The exterior sill should be installed under the joinery with a slope that directs water away from the joinery and with side closures.
- If the joints under and on the sides of the sill are not well insulated, water will seep into the wall; this means staining and moisture on the facade over time.
- The interior marble should be placed after the exterior sill water detail is completed; the two sides should not interfere with each other.
Planning this order from the beginning — which product goes first, which goes next — increases both the speed of installation and the reliability of water insulation. When auxiliary products are applied in the correct order and with the right details, they truly complete the window.
Frequently Asked Questions
Is the sill the same as marble? No. The sill is an inclined profile that is placed at the outer bottom edge of the window and directs rainwater away (usually aluminum). Marble, on the other hand, is the horizontal surface that comes under the window on the interior side; it provides an aesthetic finish and transforms the area under the window into a usable surface. One manages water outside, while the other completes the space inside.
Is a sub-frame necessary? It is not mandatory, but it facilitates installation. The sub-frame is a front frame that is fixed to the opening before the actual joinery is installed; the joinery then sits on this frame. It maintains the dimensions of the opening during construction and makes it easier to change the joinery in the future. It is especially a practical solution in new constructions.
How does the cost change if trim is used double-sided? Trim is the finishing profile that covers the installation gap and is calculated by linear metre. When used double-sided, the measurement doubles. Therefore, whether the trim will be single or double-sided should be clarified during the quotation phase; otherwise, the price may be under or overestimated.
Why are these auxiliary products often forgotten? Because the "main product" is assumed to be the window, and the auxiliaries come to mind later. However, they all affect the price, and each has a different logic for measurement: trim is calculated by perimeter, sill by bottom edge, sub-frame by edge, and marble by depth and additional. Omitting to include a trim or sill in the quotation directly results in loss.
Planning from the Start: The Way to Not Forget Auxiliaries
The biggest issue with auxiliary products is not technical but related to planning: they are often remembered later, resulting in either incomplete quotations or surprises during installation. The solution to this problem is to think of them not as "extras" but as an integral part of the position from the beginning.
Good planning starts with the drawing of the position. While drawing the window, answering the following questions simultaneously prevents future gaps: Will there be an interior marble under this window, and if so, at what depth? How will the exterior sill and water detail be resolved? Will trim be placed around it, and will it be single or double-sided? Will a sub-frame be used to facilitate installation, and on which edges? Will this window connect with the adjacent one, and if so, will a connection profile be needed?
Answering these questions during the drawing phase provides two significant benefits. First, complete quotation: no auxiliary product is forgotten, so the price does not come out short, and there is no later loss of saying "we did not account for this." Second, installation compatibility: roller shutter boxes, sill steps, marble depth, and trim are planned in the correct order without clashing with each other.
Here, a visual tool provides great convenience. When auxiliary products are reflected in the drawing — when trim edge bands, sill steps, and sub-frame edges become visible — both you and the customer can see the complete state of the window along with its surroundings. The measurements are also automatically derived from this entirety: trim from the perimeter, sill from the bottom edge, marble from depth and additional. Thus, the "main product" is not just the window but a complete system along with its surroundings. This holistic view secures both the quality of the work and the accuracy of the quotation.
Auxiliary Products and Perceived Value
Auxiliary products are not just a technical necessity; they are also elements that ensure the job looks "finished"; therefore, they directly affect the value perceived by the customer. A well-installed sill, a clean trim finish, and an aesthetic interior marble are just as noticeable to the customer, if not more so, than the window itself. Because these are the points where the window meets the wall, meaning they are the most visible aspects of workmanship. A poorly left installation gap or a crooked sill can make even a perfect window look "incomplete"; whereas a clean and meticulous finish can make even an ordinary window appear valuable. Therefore, auxiliary products should not be seen as "extra costs" but as investments that complete the perceived quality of the work. Presenting these to the customer from the start, as part of a whole — "we will have interior marble under your window, an exterior sill, and trim around it; thus, we will achieve a completely finished and clean appearance" — sets the right expectations and elevates the value of the work you provide. A holistic finish ensures that the customer thinks, "this has been done professionally."
A window is completed with the auxiliary products around it. Planning these from the start and correctly incorporating them into the quotation preserves both the quality of the work and your profit.