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Question DetailsAsked on 1/21/2018

Hardie backer crack

I installed hardie backer board on new construction sub-floor before the heat was turned on so they could staple the tubing up,heat is now on and I noticed shear cracks in the hardie this a concern? Did the sub-floor just expand with the heat being turned on?

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1 Answer


Do you mean shear cracks (splitting diagonally across the board) or just joint cracking ?

Sounds like the subflooring was damp and dried out after the heat was turned on - causing shrinkage in the framing and subfloor. Normally this would not crack hardiboard, but if it had been sitting out in the rain or snow the subflooring could well have been wet enough to. This is why a house should be "closed in" and the HVAC run to dry it out for a couple of weeks before starting interior finishes.

I admit I am a bit confused why the hardiboard - unless you mean this is prepping for a tile or stone floor and they wanted it nailed (or better yet screwed) down before putting in underfloor heating loops so you did not penetrate the tubing with the backer board fasteners. Normally one wouldnot optherwise put down hardiboard on subflooring - normal hardwood, laminate, carpet etc goes right over the subflooring in most cases. Sheet flooring like sheet vinyl or linoleum may get a 1/4" overlay for smoothness, depending on the quality of the subfloor material.

Depending on what is going over this board, and on whether the cracks bother the flooring installer, might or might not be a problem. At a minimum it should be fastened down along the new edges also to keep them from moving (without penetrating the heating tubing), and them filled in with proper cement-based concrete backer board joint compound (NOT drywall compound) - and of course let it dry out the rest of the way before doing that orit will just recrack.

Since shrinkage would be the normal action of the subfloor wood - unless you live in a really dry area - I am at a bit of a loss why the "shear" cracking unless you reallyo meant tension cracking - unless the backer board actually sheared in "thrust faulting" - sheared through in compression of the board, in which case it would have raised edges and likely is lifted up of the subfloor in areas (at least along the cracksw) so that would be a problem, if that is the case.

Answered 2 years ago by LCD

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