Drywall Butt Joint ‘Humps’: Prevention With Back-Blocking and Repair Options

A drywall butt joint hump is a raised bump where two boards meet at a butt joint, not a smooth seam. You’ll see a ridge under the paint and you can feel a hard lump when you run your hand over it. It differs from a simple tape curl or surface crack because it sits up and catches light, not just a line in the mud.

The main risk is that a visible defect can telegraph through paint and finish, making the room look crossed-up. To diagnose, look for a persistent ridge and a stiff feel when you press with a finger, and compare to nearby joints; if you need specifics, check the label or datasheet for guidance.

Visual signs and tactile checks

A ‘hump’ at a drywall butt joint is easy to spot. Look for:

Raised ridge: You’ll see a noticeable bump along the seam, usually wider than other joints.

Shadow lines: Light hitting the wall casts distinct shadows on either side of the hump, making it stand out.

To confirm, run your hand over the seam. A hump feels like a small, firm bump under your fingers.

Common root causes

Humps usually happen due to:

Unsupported seams: If the seam isn’t properly supported with back-blocking or tape, it can bow out.

Differential board thickness: Boards of varying thickness cause uneven pressure on the seam.

Improper fastening: Inadequate screws or nails allow the boards to shift and create a hump.

Substrate movement: The wall itself moving (due to settling, for example) can push the drywall out at the seam.

How humps differ from cracks, nail pops, and tape ridges

Humps aren’t the only issue you’ll face with drywall seams. Here’s how to tell them apart:

Cracks: Unlike humps, cracks are narrow gaps that run along the seam. They don’t bulge out.

Nail pops: These appear as small, round bumps where a nail head has pushed through the drywall paper. Humps are longer and wider.

Tape ridges: Tape ridges are narrow, raised lines caused by excess joint compound or improper application of tape. They’re narrower than humps and usually run the entire length of the seam.

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Start with the basics: a wide taping knife, sanding block, and cutting tools for precise back-blocks. Include joint compound, tape, and a proper backing material if you plan preventive work. Have a level, square, and measuring tools ready to verify alignment.

Optional items can speed progress, such as a pole sander, automatic mud mixer, and a stud finder. Always verify product labels and compatibility with your substrate and climate by checking manufacturer instructions or local rules before use.

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Back-blocking adds continuous support behind the butt joint to resist movement and reduce differential settling. With solid backing, mud and tape can sit evenly without creating a raised edge. It acts as a structural bridge that keeps the joint flat as the wall dries.

Back-blocking is most effective when the wall is new or when retrofits can be applied without compromising framing. In retrofit work, plan for access and cut blocks to fit behind the joint without creating additional gaps. Assess the wall condition and follow the backing method that suits your space.

Material specs and standards explained

The right materials ensure your back-blocking job lasts, looks good, and doesn’t crack. Here’s what you need:

• Type N mortar: Used for most masonry work, including back-blocking. Look for ASTM C270 compliance on the bag. It’s strong enough for most jobs and won’t cost you an arm and a leg.
• Concrete blocks (CMU): Standard 8″ x 16″ x 32″ size is fine. Check they’re ASTM C90 compliant. Avoid cheap, low-quality blocks that can crumble or crack over time.
• Reinforcement (rebar): #4 rebar is common for back-blocking. Look for ASTM A615 grade 60. It’s strong and affordable. Don’t skimp on reinforcement, it’s what keeps your wall from leaning or collapsing.
• Grout: Use a sanded grout mix for filling joints. Check the bag for ASTM C138 compliance. Avoid unsanded grouts that can crack easily.
• Expansion joint material: Neoprene or closed-cell foam strips are common. They prevent cracking due to temperature changes and settling. Don’t forget these, they’re crucial for long-term durability.
• Waterproofing membrane (optional): For below-grade walls, use a self-adhesive waterproofing membrane. It prevents moisture from seeping into your wall. Don’t skip this if you’re back-blocking a basement or retaining wall.

Pro tip: When buying materials, check the bag for expiration dates and storage instructions. Keep mortar and grout mixes dry until use to prevent premature setting. Always follow manufacturer mixing guidelines for best results.

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Begin with clean, dry surfaces and mark the joint location. Measure and cut a back-block that fits behind the butt joint without binding the studs. Secure the block firmly to the framing to provide solid support.

Apply tape and mud with steady, consistent passes, then set the joint flush and check fit before mud dries. Use light, careful pressure and avoid over-mudding to keep the seam level and ready for finishing touches.

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For minor humps, sanding and a light skim coat can smooth the surface without major disassembly. This approach is suitable when the backing remains solid and the edge is already well seated. Consider timing and access to determine if a quick fix will hold long term.

More involved options include removing and re-backing or replacing affected boards if the hump is pronounced. Choose the path based on how severe the bump is and how easily you can reach the joint without causing new movement.

Quick Cosmetic Fixes and When They Work

For minor humps, quick fixes can save you time and effort. Here’s what you can do:

Spot-sanding: Lightly sand the hump with a fine-grit sandpaper to flatten it out. Be careful not to sand too hard or you’ll create a depression.

Thin skim coats: Apply a thin layer of joint compound over the hump, feathering it out on either side. Let it dry, then lightly sand and apply another coat if needed.

These fixes work best when the humps are small (less than 1/8 inch) and not too wide. They won’t solve major issues or prevent future problems, but they can tide you over until you can do a proper repair.

Rebacking and Re-mudding a Joint

If the hump is larger but not too wide, you might need to remove the finish layers and reinstall a back-block. Here’s how:

First, remove the finish compound from both sides of the joint using a utility knife or drywall saw. Be careful not to cut into the board.

Next, install a new back-block as explained earlier in this guide. Then, apply joint compound over the back-block and hump, feathering it out on either side of the joint.

Let it dry, then sand lightly and apply another coat if needed. This method works well for humps up to about 1/4 inch wide.

Board Removal and Full Replacement

For large, wide humps or those caused by damaged boards, you might need to remove the board entirely. Here’s how:

First, cut along both sides of the hump with a drywall saw. Then, use a utility knife to score and snap off the excess drywall.

Next, remove the damaged board and install a new one. Make sure it’s properly back-blocked to prevent future humps. Then, apply joint compound over the new joint as usual.

This method is more involved but necessary for severe cases. It ensures a solid, flat wall surface but may require some touch-up work on surrounding walls or ceilings.

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Plan a sanding strategy that follows a logical grit progression so you don’t gouge the surface. Sand evenly across the seam and avoid thinning the mud near the joint. Pause to reassess the texture as you go.

Check the seam under different lighting and angles to confirm flushness. Ensure the surface remains consistent before applying primer and paint, watching for remaining reflections that indicate a hump.

Avoid over-sanding, which can expose tape or create new ridges. Use the right joint compound for the task and ensure proper backing to prevent future movement. If movement recurs, address the underlying framing or substrate issues first.

Document movement patterns and check for flex or settlement that may cause recurrence. Establish a routine inspection and proper maintenance plan to slow or stop humps from returning.

Troubleshooting persistent or recurring humps

If you’re still seeing ‘humps’ despite your best efforts, it’s time to troubleshoot the problem. Start by checking for any underlying issues that might be causing movement in your walls.

Framing movement: Check if there are any gaps between the drywall and the studs. If so, you may need to reattach or reinforce the drywall. Also, look for any signs of structural movement in your home’s framing.

Moisture can also cause problems. Check for water damage: Look for water stains, warping, or soft spots on the drywall. If you find any, address the source of the moisture and repair the damaged area accordingly.

Misaligned studs: Sometimes, humps can be caused by studs that are not perfectly straight or aligned. To check this, use a level to ensure your studs are plumb and properly spaced. If necessary, shim or adjust the drywall to fit the studs correctly.

Long-term strategies to reduce future movement

To minimize ‘humps’ in your drywall joints over time, consider these long-term strategies:

Proper fastening patterns: Ensure that drywall screws or nails are driven into the center of each stud and spaced appropriately. This provides better support and reduces movement.

Humidity control is crucial. Maintain proper humidity levels: Keep your home’s indoor humidity between 30-50% to prevent excessive expansion and contraction of the drywall.

Use flexible finishes where applicable: In areas prone to movement, consider using flexible joint tape or adding a flexible additive to your joint compound. This can help absorb minor movements without cracking or humping.

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If DIY efforts don’t produce a flat, consistent surface, it is reasonable to seek professional help. A pro will assess backing, framing, and mudding technique and may rework the joint as needed. They will also address any hidden issues before finishing.

Prepare by gathering notes, photos, and measurements of the affected area and any past repairs. When you contact a pro, verify licensing, references, and local expectations rather than quoting prices or schedules.

Signs you should hire a pro

If your drywall humps are causing concern, here’s when to call in the pros.

Structural movement: If walls or ceilings are cracking or showing signs of movement, it could be a structural issue. That’s beyond DIY and needs pro attention.

Large-area humps: Small humps can often be fixed by the homeowner. But if you’re seeing large areas affected, it might be time to call in help.

Unseen substrate problems: If you suspect there are issues with the underlying structure or framing that are causing the humps, leave it to the pros to diagnose and fix.

Lack of access: If the areas with humps are hard to reach or involve removing other materials (like tile or cabinets), it’s safer and easier to hire a pro.

What a pro assessment and repair typically includes

A professional drywall contractor will start by assessing the damage. Here’s what you can expect:

Diagnostics: They’ll inspect your walls, check for moisture, and look at the framing. They might use tools like infrared cameras or moisture meters.

Repair scope: Based on their findings, they’ll outline a repair plan. This could range from simple mudding and sanding to full board replacement or even structural repairs.

Documentation: A good contractor will provide you with a detailed report outlining the problem, the proposed solution, and an estimate for the work.

Questions to ask and how to verify credentials

Before hiring, here are some questions to ask and checks to make:

Bids: Get at least three bids. Ask about their process, timeline, and what’s included in the price.

References: Always check references. Ask for before-and-after photos if possible. A good contractor will have happy customers.

Credentials: Make sure they’re licensed and insured. Insurance protects you if something goes wrong on the job. Local licensing ensures they meet your area’s standards.

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