ASCEND panels can bow, lift, or “fish-mouth” when the plank is compressed, stretched during lock engagement, hard-nailed, cut without proper clearance, installed over an uneven wall, or overlapped incorrectly. Learn how to diagnose the cause and correct the installation before the problem spreads across the wall.

ASCEND Contractor Resource Center

What Causes ASCEND Panels to Bow or Fish-Mouth?

ASCEND panels can bow, lift, or develop a condition contractors sometimes call fish-mouthing when the cladding is placed under installation stress. Common causes include forcing or compressing the first course, stretching panels while engaging the stack lock, hard-nailing the nail hem, cutting panels too tightly into receiving channels, creating an incorrect plank overlap, fastening over an uneven wall, or failing to support a qualifying rip-cut section with finish trim.¹

“Fish-mouth” is a field description rather than a formal performance classification in the current ASCEND installation guide. Contractors commonly use the term when an edge, overlap, butt, or locking area opens outward instead of lying flat against the wall.

The visible distortion is usually a symptom—not the root cause. Pushing the raised edge back into place without finding the source of the stress may temporarily improve its appearance, but the problem can return after temperature changes or wind movement.

Common Causes at a Glance

The first course was forced downward or compressed into the starter strip.
The plank was stretched tightly while engaging the stack lock.
The panel lock was not fully engaged.
Fasteners were driven tightly against the nail hem.
Fasteners were placed incorrectly in the slots.
The plank was cut tightly between fixed receiving channels.
The under-plank was not flush-cut correctly at an overlap.
The required 1/2-inch plank overlap was not maintained.
The insulated foam edges were butted together.
The factory overlap edge was bent, cracked, or forced.
The substrate is bowed, uneven, soft, or inadequately prepared.
A rip-cut section is unsupported because required finish trim was omitted.
A panel overlap above or below an opening lacks required shimming.
Panels or accessories cannot move because clearances were eliminated.

What Does “Fish-Mouthing” Look Like?

Fish-mouthing usually appears as a localized opening where one portion of the cladding lifts away from the wall. It may occur at a plank overlap, along a butt edge, near a receiving channel, around an opening, or where a cut panel lacks proper support.

Visible Warning Signs

A plank edge curls or opens outward.
An overlap does not sit flush.
The butt edge projects farther than adjacent panels.
A horizontal wave appears across one or more courses.
The wall looks flat in cool weather but distorted in direct sun.
A panel can be pushed flat but springs outward again.
The stack lock appears partially engaged.
A cut panel collapses inward or bows outward near a window or soffit.

Can Compressing the First Course Cause Bowing?

Yes. ASCEND instructs installers to seat the first course fully into the starter strip while specifically warning them not to force the plank downward or create compression during fastening. The plank should lie flat against the wall.²

When the first course is pushed downward after it has already engaged, stored compression can remain in the plank. That pressure may cause the butt edge or face to lift, especially after additional courses transfer weight and movement into the first row.

Correct First-Course Engagement

Install the starter strip perfectly level.
Engage the first plank completely.
Do not push it farther after it locks.
Confirm that it lies flat against the wall.
Fasten without pulling or compressing the panel.
Inspect the entire course before installing the second row.

Can Stretching the Stack Lock Distort a Panel?

Yes. The ASCEND quick-start guide distinguishes between a lock that is not engaged, one that is stretched tightly, and one that is fully locked without being stretched. The correct condition is complete engagement without tension.³

Stretching a plank while engaging the stack lock can preload the panel. After it is fastened, the plank may pull against the fasteners, retract from a receiving channel, distort at an overlap, or lift along the butt edge.

Not Fully Locked

The connection is incomplete and may separate or leave the face uneven.

Stretched Tight

The lock is engaged, but the plank is under unnecessary tension.

Fully Locked

The stack lock is completely engaged without stretching or compressing the panel.

How Does Hard-Nailing Cause Bowing?

ASCEND requires approximately 1/32 inch between the fastener head and nail hem. Fasteners are generally centered in their slots so the plank can expand and contract.⁴

Hard-nailing pins the panel to the wall. When the cladding attempts to move, the trapped expansion pressure may force the face outward, create a wave between fasteners, stress the nail slots, or lift the plank near a fixed end.

Signs of Hard-Nailing

The nail hem cannot slide beneath the fastener.
Fastener heads are pressed tightly against the slots.
The nail hem is crushed or distorted.
Bowing appears between fastening points.
Distortion increases during warm weather.
The panel ends press tightly against corners or channels.

Can Missing Expansion Clearance Cause Fish-Mouthing?

Yes. ASCEND 7-inch Plank requires a 3/8-inch clearance between the cladding and receiving channels, corner posts, and fixed obstacles. Without that space, the plank may press against the accessory as it expands and be forced outward.⁵

The raised section may appear several feet away from the tight end because the compressive force seeks the weakest point in the course. The visible problem is not always located beside the installation mistake.

Check Clearance At:

Inside and outside corner posts
Window and door receiving channels
J-channel and lineals
Mounting blocks
Gable and rake channels
Soffit and eave channels
H-trim and cladding transitions
Vents, faucets, pipes, lights, and utility penetrations

Why Do Plank Overlaps Sometimes Fish-Mouth?

ASCEND 7-inch Plank uses a specific over-plank and under-plank lap procedure. The under-plank must receive a straight flush cut through the entire panel, including the nail hem and foam, before it slides behind the factory edge of the over-plank.⁶

The manufacturer requires a 1/2-inch overlap measured from the cut of the under-plank and warns installers not to butt the foam edges together. Maintaining that overlap creates approximately a 1/2-inch foam gap between the planks.

Overlap Errors That Can Create Distortion

The under-plank cut is crooked.
The cut does not pass flush through the nail hem and foam.
The under-plank is forced behind the factory edge.
The overlap is less than the published 1/2 inch.
The panels overlap too far.
The foam edges are butted tightly together.
The factory over-plank edge is bent or cracked.
The lower-course fastener near the overlap is missing.
The overlap lacks support over a soft wall or opening.

Why Should Foam Not Be Butted Together?

The installation guide specifically states, “Do not butt foam to foam.” When the two insulated backing sections contact each other, they can prevent the lap from settling into its intended position and remove movement space from the joint.⁶

Foam-to-foam contact can place outward pressure on the factory edge and create a raised, open-mouthed appearance. It can also make the installer believe the panels are fully seated when the overlap is actually being held apart behind the visible face.

Can the Factory Overlap Edge Be Damaged?

Yes. ASCEND warns that failure to follow the published overlap procedure can result in bending or cracking of the factory edge. The guide states that damage caused by an improper lap method can void warranty coverage.⁷

Once the factory edge is bent, it may no longer hold the under-plank flat. The joint can open outward even if the surrounding panels appear properly installed.

Do Not:

Pry the under-plank aggressively behind the factory edge.
Hammer or strike the overlap closed.
Force a crooked field cut into the lap.
Bend the over-plank edge outward to gain access.
Install a plank with a visibly cracked factory edge.
Attempt to conceal damaged product within the wall.

Does Fastening Near the Overlap Matter?

Yes. ASCEND directs installers to place a fastener in the lower course nail slot closest to the plank overlap. It also shows staggered fastening through the overlap area.⁸

Missing this attachment point may allow the lower course or surrounding wall surface to move excessively near the lap. The fastener still must be installed with the correct head clearance; driving it tightly is not the solution.

Can an Uneven Wall Cause Panels to Bow?

Yes. ASCEND has a forgiving profile that can help conceal minor wall imperfections, but it cannot correct severe bows, humps, depressions, loose siding, rotten sheathing, or unstable framing. The manufacturer requires a smooth, flat installation surface and directs remodelers to use fanfold, foam board, or rigid sheathing over qualifying existing siding when needed to create that surface.⁹

If the wall changes plane sharply, the panel may bridge over a depression or be pushed outward by a high point. That can look like product bowing even though the cladding is following the substrate beneath it.

Substrate Conditions to Inspect

Bowed sheathing
Protruding framing members
Uneven existing siding
Loose wall materials
Thick flashing transitions
Poorly aligned furring strips
Fasteners or debris trapped behind the panel
Rot, swelling, or moisture damage

What Is a “Soft Wall” and Why Does It Matter?

ASCEND uses the term “soft wall” for wall conditions that do not provide a conventional nailable substrate like OSB at the plank overlap. In those applications, the guide requires a No. 8 truss-head or pan-head screw at the lap that penetrates through the soft wall sheathing.¹⁰

If the overlap is attached only to compressible foam or another weak surface, the joint may lack enough support to remain flat.

Can Missing Finish Trim Cause a Cut Panel to Bow?

Yes. ASCEND finish trim is used when a horizontal rip cut removes the top nail hem or bottom stack lock and the remaining section needs support. The trim allows the cut panel to maintain its projection while securing it inside the receiving channel.¹¹

Without the required finish trim, the cut section may fall inward, lift outward, or sit at a different projection than the surrounding wall.

Inspect Finish Trim Around:

Window sills
Window headers
Door headers
Wide mounting blocks and obstacles
The final course beneath a soffit
Horizontal transitions where the nail hem or stack lock was removed

Why Can Panels Bow Above or Below Windows?

ASCEND recommends avoiding the first plank overlap immediately above or below windows, doors, and other obstacles. For later overlaps in those locations, the guide may require shimming behind the overlap to preserve proper plank projection and a secure fit within the J-channel.¹²

Without support, the overlap may bridge the opening detail or fall into the space behind the plank. The resulting projection difference can resemble bowing or fish-mouthing.

Can Incorrect Starter-Strip Installation Cause Bowing?

Yes. The starter strip must be installed level, securely fastened, and positioned so the first course can fully engage without being compressed.

A starter strip that is bowed, uneven, pushed against flashing, or installed at inconsistent depth can transfer that problem into the first course and every course above it.

Starter-Strip Problems Include:

The strip is not level.
Fastener spacing is excessive.
The strip is hard-nailed or distorted.
Adjoining strips are butted tightly together.
The strip contacts flashing and blocks engagement.
The wall behind the strip is uneven.
The first plank is only partially engaged.

Can Accessories Push the Panels Outward?

Yes. Receiving channels must have enough pocket depth to accommodate ASCEND’s approximately 3/4-inch projection. A shallow, twisted, improperly overlapped, or tightly fastened accessory can interfere with the panel and force it out of plane.

The installation guide also warns against overlapping and fastening through the nail flanges of adjoining accessories. Stacked flanges can create a raised area behind the cladding and prevent components from moving independently.¹³

Could the Product Have Been Damaged Before Installation?

Yes. Panels must be stored flat, supported along their full length, and protected from excessive heat, impact, bending, and construction damage. The manufacturer instructs installers to inspect cladding for breakage, foreign objects, surface defects, color consistency, and correctness before installation.¹⁴

A panel with a bent end, damaged nail hem, compressed foam backing, or cracked factory edge should not be installed with the expectation that fastening will straighten it permanently.

Is Every Bowed Panel an Installation Error?

Not necessarily. Product damage or a manufacturing issue is possible, which is why the manufacturer requires inspection before installation. However, the first diagnostic step should be a disciplined review of the substrate, clearances, fasteners, stack-lock engagement, overlap preparation, storage, and accessory details.

Do not declare a material defect based solely on the visible symptom. Determine whether the panel was installed according to the controlling guide and preserve the evidence before removing or altering it.

How Do You Diagnose a Bowed ASCEND Panel?

Identify the exact location and pattern of the distortion.
Determine whether it changes with temperature or sunlight.
Check the nearest receiving channels for proper clearance.
Test whether the nail hem can move beneath the fasteners.
Inspect the stack lock for complete engagement.
Inspect nearby overlaps for correct cut, spacing, and foam separation.
Check for the required fastener near the overlap.
Determine whether finish trim or shimming was required.
Inspect the substrate for a hump, depression, or trapped object.
Review storage and handling photographs.
Document the condition before removing the panel.

Should You Simply Add More Fasteners?

No. Adding fasteners without identifying the cause can make the problem worse. Extra attachment points may pin the plank more tightly, prevent movement, crack the nail hem, or conceal an improperly prepared overlap.

Correct the source of the stress. That may require releasing overdriven fasteners, restoring clearance, rebuilding an overlap, supporting a rip cut, correcting the substrate, or replacing a damaged panel.

Can You Repair Fish-Mouthing Without Removing the Panel?

Sometimes a minor issue can be corrected by carefully releasing an overdriven fastener or restoring movement at a receiving channel. More serious overlap, lock, substrate, or panel damage may require removal and replacement.

Contractors should follow ASCEND’s written replacement procedures rather than prying, screwing, face-nailing, gluing, or heating the visible edge until it lies flat.

Do Not Attempt to Repair Bowing By:

Driving a visible face nail through the panel
Adding adhesive behind the cladding
Filling movement pockets with caulk or foam
Heating the panel with a torch or concentrated heater
Hammering the overlap closed
Forcing the panel lock with a pry bar
Adding fasteners through another accessory’s nail flange

Common Installation Combinations That Create Bowing

Visible Condition	Possible Cause	What to Inspect
Raised plank overlap	Incorrect cut, foam-to-foam contact, damaged factory edge, or insufficient overlap	Flush cut, 1/2-inch overlap, foam gap, nearby fastener, and factory edge
Wave between fasteners	Hard-nailing, missing clearance, or excessive substrate variation	Fastener-head clearance, nail-slot position, panel ends, and wall plane
Lifted butt edge	Compressed first course, incomplete lock, stretched plank, or uneven starter	Starter strip, stack lock, first-course position, and panel tension
Distortion near a window	Tight channel clearance, unsupported rip cut, or unshimmed overlap	Finish trim, J-channel, 3/8-inch clearance, flashing buildup, and overlap support
Broad wall bow	Uneven substrate, damaged material, or multiple hard-nailed panels	Sheathing plane, storage records, fastening pattern, and panel condition

How Can Contractors Prevent Bowing and Fish-Mouthing?

Inspect and flatten the wall before installation.
Store and transport panels fully supported.
Reject damaged or questionable material.
Install the starter strip level.
Seat the first course without forcing it downward.
Fully engage each stack lock without stretching the plank.
Maintain 3/8-inch clearance at receiving channels.
Center fasteners and leave approximately 1/32 inch beneath the heads.
Make straight, complete flush cuts at every under-plank overlap.
Maintain the required 1/2-inch overlap and foam gap.
Install the required fastener near the overlap.
Use screws where required over soft-wall assemblies.
Use finish trim and shims where the installation guide requires them.
Inspect each course before the next row conceals its fasteners and locks.

What Should Contractors Document?

Once the wall is complete, many fastening, overlap, clearance, and substrate details are concealed. Progress photographs provide a record of how the installation was completed.

Quality-Control Documentation

Wall condition before cladding
Level starter-strip installation
First-course engagement
Fastener position and head clearance
Panel-end clearances
Under-plank flush cuts
Plank overlap and foam gap
Fastener placement near each lap
Finish-trim and shim locations
Completed elevations in different lighting conditions
Product labels, color, profile, and manufacturing information
Installation-guide version used by the crew

Can Improper Installation Affect Warranty Coverage?

Yes. The ASCEND quick-start guides warn that failure to follow the full written installation instructions may affect product performance and warranty coverage. The full guide also specifically warns that an incorrect plank-lap method can bend or crack the factory edge and void warranty coverage for that damage.¹⁵

Bowing caused by tight panels, hard-nailing, improper overlaps, inadequate substrate preparation, missing finish trim, or damaged installation components may be treated as a workmanship issue rather than a manufacturing defect.

How Should Contractors Explain the Problem?

A Clear Contractor Explanation

“ASCEND panels usually bow or fish-mouth when something prevents them from lying flat and moving normally. We inspect the panel lock, fasteners, end clearances, plank overlap, insulated backing, finish trim, and wall surface before deciding whether the panel needs an adjustment or complete replacement.”

The Bottom Line

ASCEND panels may bow or fish-mouth when they are compressed into the starter strip, stretched during stack-lock engagement, hard-nailed, cut too tightly, installed over an uneven wall, or unsupported after a rip cut.

Plank overlaps are especially sensitive. The under-plank requires a straight flush cut, a 1/2-inch overlap, separation between the foam edges, proper fastening, and careful engagement behind the factory edge. Forcing the joint can bend or crack the edge and leave the overlap visibly open.

Do not hide the symptom with extra nails, adhesive, caulk, heat, or force. Find the source of the stress and correct the installation detail. A panel that is fighting the wall will eventually show it.

Your Local ASCEND Composite Cladding Distributor

Suburban Wholesale & Supply supports contractors, builders, and remodelers throughout Eastern Iowa and Western Illinois with ASCEND Composite Cladding, installation literature, compatible accessories, finish trim, technical resources, contractor education, estimating assistance, and jobsite delivery.

Our knowledgeable team can help contractors review plank overlaps, fastening requirements, wall preparation, expansion clearances, trim details, and current manufacturer instructions before installation begins.

Why Contractors Choose Suburban Wholesale & Supply

ASCEND Composite Cladding product expertise
Plank-overlap and installation guidance
Current installation guides and technical resources
Fastener, clearance, finish-trim, and accessory support
Contractor-focused training and product education
Material takeoff and estimating assistance
Competitive contractor pricing
Jobsite delivery across Eastern Iowa and Western Illinois

What Causes ASCEND Panels to Bow or Fish-Mouth? | Suburban Wholesale