How to Prevent Lap Siding from Buckling by Using a Butt Joint

It’s a silly name, but a “butt joint” is an application technique where two pieces of material are “butted” up against each other. It is the simplest joint to make, and a butt joint can be either end to end or end to face.

Depending on the width of the wall, butt joints will occur where two pieces of lap siding come together, creating a vertical seam. LP® SmartSide® lap siding products are available in 16’ lengths, and can help reduce the amount of seams where a butt joint would normally occur when using shorter pieces. 

Installing Butt Joints Using LP SmartSide Lap Siding

Whether you are installing your siding over code-approved nailable sheathing or directly to the studs, you will want to make sure you correctly space your butt joints. Properly spaced butt joints will allow for the siding to expand without putting pressure on the adjacent piece of siding.

Here are some Frequently Asked Questions for creating butt joints in new installation when using LP SmartSide siding. For full installation guidelines, visit LPCorp.com.

• Where do joints go? Joints must occur over studs. They will occur within horizontal courses when using lap siding.

• How much space do you need between lap siding pieces? LP requires a minimum 3/16” spacing at all butt joints.

• Do I have to caulk at butt joints? It depends on the option you choose, but if joint caulking option is selected, seal all gaps at butt joints with a high-quality exterior sealant meeting the ASTM C920, minimum Class 25 sealant. The alternative is to use a joint molding in place of caulk.

• What is the moulding option for butt joints? If joint moulding option is selected, add the thickness of the web to the gap allowing a net 3/16 in. space for expansion.

• Do I need to seal edges at a butt joint? If you are using primed siding, you must seal all exposed cuts of siding, including those at butt joints. Butt joints that are covered with joint moulding, sealant or factory prefinished end, are considered sealed from the weather. If siding is prefinished, gap does not require sealant or joint moulding when backed with minimum 4 in. wide flashing and the ends of the siding are factory finished.

Repairing Butt Joints Using LP SmartSide Lap Siding

In the event of buckling due to improper installation, repair to the butt joints may be needed.

The remedy process is as follows. You can also view the process in this video.

• Separate the affected piece of lap. Use two wooden wedges in between the affected piece and the one above.

• Protect the housewrap. Slide a piece of metal sheeting behind the butt joint.

• Create a proper 3/16” space (at minimum) between the two pieces of siding. Use a keyhole or reciprocating saw with a fine-tooth blade to remove enough material from one of the pieces of lap siding. Remove any sawdust, dirt or debris from the gap to prep it for finishing.

• Address siding deflection. If any piece is deflecting against the side of the wall, face nail only that area to flatten the piece back against the wall. Properly caulk and paint all exposed nail heads.

• Prime and seal the gap. Using a brush, prime all exposed wood fibers in accordance with installation instructions. Seal the gap with an ASTM C290, minimum Class 25 Sealant, in accordance with the manufacturer’s instructions.

• Finish with paint. Following the manufacturer’s instructions, finish the repaired area using an exterior paint consistent with LP installation instructions.

If you have questions about installation or corrections to improperly installed LP SmartSide products, please contact LP by email or phone (1-888-820-0325) for more information and support.