The Roof Shapes of Spray Foam: Your Guide to Gable, Flat, Hip, Monitor, Gambrel, and Saltbox Roofs in Foambid
- Oct 13, 2025
- 7 min read
Spray foam jobs live and die by surface area. So when your app asks you for numbers — roof length, wall height, ridge slope — you need to know exactly what to measure and how to enter it. That’s why Foambid includes six roof configurations: Gable, Flat, Hip, Monitor, Gambrel, and Saltbox. Each one mirrors how spray foam actually behaves across real-world roof geometry.
This guide walks you through each of these roof types with complete clarity. If you're new to the app, you’ll learn what each field means, where to physically measure it on the building, and how Foambid processes those numbers behind the scenes. All instructions assume you’re measuring from the floor of the building using a standard laser distance measurer — no angles, no guesses, no tape.
Gable Roof
A gable roof has two equal sloped sides that meet at a central ridge. The short walls on either end form triangular gables. This is the most common residential roof type.
Inside Foambid, start by selecting "Gable Roof" under the roof type dropdown. You'll be prompted to enter the Long Wall Length, Short Wall Length, Wall Height, and either the Roof Slope or Peak Height.
The Long Wall Length refers to the full run of the building along the eave side. This should be measured at floor level, corner to corner.
The Short Wall Length is the gable-side width of the building, again measured from the floor.
Wall Height is the vertical measurement from the floor to the top plate — not to the peak or underside of the roof decking, just to the top of the structural wall where the slope begins.
For slope, you have two options. You can tap "Select" to choose a roof pitch like 4:12 or 8:12 if you know it. If you don't, or if it's easier to measure, you can tap "Use Peak Height Instead" and enter the vertical distance from the floor to the ridge.
If "Walls" is toggled on, gable triangles are automatically included in the wall surface area. If you're estimating roof-only, turning off the Walls toggle will reveal an "Include Gables?" switch. Use this to include or exclude the gable ends depending on whether you're spraying them.
Flat Roof
Flat roofs are common in commercial and modern residential construction. Though not perfectly flat, they typically have minimal slope that can be ignored for most spray foam purposes.
When you choose "Flat Roof" as the roof type, Foambid prompts for Long Wall Length, Short Wall Length, Tallest Wall Height, and Shortest Wall Height. These should all be measured directly from the floor.
The Long and Short Wall Lengths describe the footprint of the building. Use your laser to measure along the perimeter at floor level. For the two wall heights, measure from floor to top plate on the highest and lowest walls of the building. If the roof is completely flat and all walls are equal, simply enter the same height in both fields.
Foambid uses the difference between these two heights to infer a slope if needed, and then calculates total spray area accordingly. No roof pitch or peak height is required.
Hip Roof
Hip roofs slope inward from all four sides and do not have gable ends. They are common on ranch homes, low-profile barns, and some metal buildings.
After selecting "Hip Roof," the app will ask for Long Wall Length, Short Wall Length, Wall Height, Roof Slope or Peak Height, Ridge Length, and Hipped End Length.
Measure the Long and Short Wall Lengths at the floor as you would with any other structure. Wall Height refers to the height from floor to top plate, which should be uniform across all four walls.
For slope, you can either select a pitch from the provided list or choose "Use Peak Height Instead" and enter the full vertical distance from the floor to the ridge.
Ridge Length is the horizontal distance along the top where the two long roof planes meet.
Hipped End Length is the distance along the top edges of the short ends where the roof tapers down toward the corners. If these values are not directly measurable from inside the structure, they can be estimated based on plans or default framing proportions. Foambid uses these values to segment and compute each triangle and trapezoid in the roof model.
Monitor Roof
A monitor roof features a raised center section that runs down the ridge of the structure. It looks like a second, smaller roof sitting atop the main one, with short vertical walls in between. This style is common in barns, industrial buildings, and structures designed for passive ventilation.
Choose "Monitor Roof" and begin entering measurements. The required fields are Long Wall Length, Short Wall Length, Wall Height, Monitor Width, Monitor Side-Wall Height, Lower Roof Slope, and Monitor Roof Slope.
Start with the building footprint: measure the Long and Short Wall Lengths from floor to floor at the base. Wall Height is the height of the main exterior walls, from floor to top plate.
Monitor Width refers to the horizontal width of the raised center section — not just the roof, but the entire monitor unit. This should be measured at floor level by extending your laser upward or downward through the monitor zone if possible.
Monitor Side-Wall Height is the vertical distance from the top of the main roof deck to the top plate of the monitor section. If it’s not directly measurable, shoot from the floor to the monitor’s ridge and subtract the known wall height. That gets you close.
Both Lower Roof Slope and Monitor Roof Slope must be selected using the slope picker. These define the angle of the base roof and the mini roof on top of the monitor separately. Foambid handles the segmented geometry behind the scenes.
Gambrel Roof
Gambrel roofs feature a two-slope design per side: a steep lower pitch transitions into a shallower upper pitch. This configuration is most often seen on barns, agricultural shops, and post-frame structures where a full loft or attic space is desired without adding extra ridge height. The visual effect is a roof that curves out sharply at the bottom, then flattens toward the top.
When you select "Gambrel Roof" in Foambid, you’ll begin by entering Long Wall Length, Short Wall Length, Wall Height, and Roof Peak Height. These define the building footprint and the vertical rise from floor to ridge. As always, all height measurements should be taken from the floor using a laser — not from the top plate, and never from the roof deck itself.
Once these base measurements are entered, you’ll choose between two input paths: Slopes Mode or Knee Height Mode.
In Slopes Mode, you'll be prompted to enter both the Lower Slope and the Upper Slope. These are the two roof angles — one steep and one shallow — that form the compound profile of the gambrel. You’ll select each using the standard pitch picker (for example, 10:12 for the lower slope and 4:12 for the upper). This is the best choice when you have design documents or clear framing specs.
In Knee Height Mode, Foambid takes a different approach. Here, you’ll enter the Knee Height, which is the vertical distance from the floor to the transition point between the lower and upper slopes — also known as the “knee" or "curb" This can typically be measured from inside the structure by aiming your laser vertically up to the point where the rafters change angle. It’s especially useful in barns or shops where framing is exposed.
Once you've entered the Knee Height, Foambid only needs one of the following additional details:
You may enter the Lower Slope using the pitch selector.
Or you may enter the Upper Slope instead.
Or, if slope isn't easily measurable, you may enter the Knee-to-Knee span — a horizontal distance from one knee wall to the other.
The Knee-to-Knee entry is not a sloped measurement. It’s a straight-line, horizontal dimension taken across the building interior, from the start of the left lower slope to the end of the right lower slope. In practical terms, this is often the full loft ceiling span or the width between structural transitions where the roof flattens out. You should measure this with a laser or tape from interior framing or floor level, depending on what’s accessible. Foambid uses this horizontal span in combination with the Knee Height and other base geometry to solve the rest of the roof profile accurately — no additional slope info is required.
The key is that in Knee Height Mode, only one of these three options is needed. You don’t need to enter all three. Choose the one you can measure most reliably in the field. Foambid will use standardized assumptions to approximate the upper and lower slopes, ensuring you still get a valid board footage estimate even without full slope data.
This makes the gambrel the most versatile roof type in Foambid. Whether you're working from prints, walking a bare truss system, or eyeballing a standing structure with limited access, you can choose the input mode that matches your access and get reliable results every time.
Saltbox Roof
Saltbox roofs are asymmetrical versions of gable roofs. One slope is longer than the other, and the ridge is offset instead of centered.
Inside Foambid, selecting "Saltbox Roof" will display fields for Long Wall Length, Short Wall Length, Tallest Wall Height, Shortest Wall Height, Roof Peak Height, and Distance from Shorter Wall to Ridge.
Measure the Long and Short Wall Lengths from floor to floor as usual. Tallest and Shortest Wall Heights should be measured from the floor to the top plate on each respective side of the building.
Roof Peak Height is the vertical distance from the floor to the ridge beam — the same as in other roof types. The final field, Distance from Shorter Wall to Ridge, is a horizontal run measured across the building interior. It tells the app how far the ridge is offset from the short wall. Use a tape, laser, or plan dimension to capture this span.
By combining peak height with the distance from ridge to wall, Foambid models the slope of each side independently, even though the ridge is no longer centered.
Shared Fields and Final Notes
All of these roof types include a "Subtractable SqFt" field near the bottom of the screen. This allows you to remove foam area that won’t be sprayed — such as skylights, mechanical curbings, walk pads, or service platforms.
Most roof types also let you enter either a slope or a peak height. Always choose the method that aligns with what you can actually measure in the field with confidence. If you're unsure of pitch, just use your laser to shoot the peak.
Lastly, the “Include Gables in Estimate?” toggle only appears when “Walls” is disabled. When walls are active, gables are considered part of the wall system by default and will be included in the calculation automatically. Still have questions? Reach out to us! Find us on Facebook or send us an email at contact@foambid.co.
by Gage Jaeger, Owner and Founder of Foambid
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