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Question DetailsAsked on 10/27/2017

What are the benefits and potential consequences of not putting up particle board before I install Hardie bk siding

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Don't know what Hardie bk siding is - I presume that was a typo and was supposed to say Hardie Plank, so I am presuming plank lap siding, because underlayment would not normally be required for a sheet-type siding product like HardiePanel.

My personal opinion - you never get a net benefit using particle board or OSB - use exterior rated plywood rated as sheathing and avoid the cracking and swelling issues that occur with particle board which gets damp - which under most cement sidings, especially plank, is almost certain to happen at some time. I have seen WAYYY too many siding jobs go bad in a few years to a decade from particle board or OSB or LDF (even fiberboard in some cases) soaking up water and swelling and bulging and deforming and rotting behind the siding.

In some areas you are required to have a sheet underlayment under lap siding - in fact, it is a presumption in the structural building code but not enshrined in words because that was how it was always done on houses when the code provisions were written so they never anticipated people would start putting up plank siding without underlayment.

Also - because of the presumption in the code that there is exterior underlayment or a pattern-nailed siding (which plank siding does not meet the strength requirements for), at least if in a high seismic or wind area I would require even it if only for lateral load resistance.

If it does not have underlayment now, adding a layer will introduce issues at windows and doors - because the face of them will now lie about 1/2" further in, relative to the siding face, than before. Should not be a problem if new windows and doors are going in because they go in "over" the sheathing, but if doors and windows are staying in then could be issues both with matching up any built-in flashing in them, and with them being partly recessed relative to the siding. Can be remedied with new flashing and brickmold trim and such, but can increase the difficulty of the job and therefore the cost.

On the underlyment thing - if leaving the old doors/windows in place (not replacing or resetting them) then that means the water barrier at the doors and windows would be incomplete - because they should underlie the outer frame trim/flashing, not just meet against the side of it, so it does increase the risk of water penetration when compared to windows/doors being installed after the sheathing - but still better than no sheathing at all, because the ends of plank siding are a major leakage point.

Not having rigid sheathing under the concrete board can result in a wavier result on the siding, because using sheathing tends to even out any out-of-alignment studs, if your walls are crooked or the studs were well wetted during initial construction and bowed out.

Lap siding is pretty much wide-open to airflow, so not having sheathing promotes airflow through the wall - which the water barrier (felt or housewrap) will not totally stop. I have also seen cases where the water barrier tore in the walls in very high winds, even though under the lap siding - giving a wide-open pathway to cold and moist airflow into the wall. And of course, if you lose any siding in a storm, not having sheathing under it means your weather protection is down to the underlayment/housewrap until repaired.

Putting on siding without sheathing means any misses on nails (missing the studs) are true misses - not even holding in sheathing, and believe me - there are a LOT of nailing misses in normal construction, both due to worker inaccurary plus due to uneven or odd stud spacings. With sheathing the nails not only are guaranteed to bite into wood, but they can also be spaced closer than stud spacing - giving a more secure fastening. Also,, nailing just into studs means cracked corners on the siding many times - because they recommend 1 inch inset on the nails from the edge to prevent splitting, but that is reduced to 3/8 or 1/2" from the end so both abutting planks can get nails into the studs - which is just way too close to the end to avoid cracking many of them, which then means loose plank ends. Typical installation instructions here FYI -

Note the manufacturer instruction requires, if applying to studs without sheathing, that the studs are required to be braced - fire blocking or fire bracing in the wall (which is becoming less and less common these days, even if still generally required by code if you do not have exterior sheathing or pattern-nailed sheet siding) - meaning additional work will likely be needed on the studs unless the contractor just ignores that part of the instructions - which could void your warranty.

Bottom line - while door and window interfaces can be more work when adding sheathing, (whcih is why door and window replacements are commnonly done at the same time as residing), I can't see justification for going on the cheap and eliminating the sheathing, again assuming you are talking a strip or plank product - not full sheet siding.

Answered 2 years ago by LCD

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