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Question DetailsAsked on 1/31/2018

Redoing side of house 8x4 cedar panels are being placed over old 8x4 cedar panels is this correct way to do it?

First I was told to take it all the cedar siding panels down then I was told can just cut out the areas of rotted panels replace and sheet over leaving up old boards with new panels. How will this affect the house?

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Not clear if you are talking 8DP - 8" wide panels which have two simulated 4" planks (grooved to look like 2 individual planks) each, so a "plank"siding where each plank looks like two but is put up on a bevel just like normal plank siding (just half as many boards in a given height so goes on faster and cheaper), or if you mean 4x8 foot sheets - like T1-11 siding maybe (aka T111). Both are addressed below - similar concerns with both:

1) on exterior sheathing, for structural strength and to minimize cupping and warping, use full sheets where possible - do not "cut out" bad parts of a whole sheet, which also runs the risk of leaving rotten core in the remnant which you just do not see.

2) generally, all exterior sheathings should have a moisture breathable water barrier under them - not a "vapor barrier" unless the house is designed for exterior vapor barrier (usually only in generally above-freexzing areas wtih a lot of blowing rain, like tropics and rainforest areas - and even then NEVER leave an underlying vapor barrier under a new one). This barrier should generally be isolated from any underlying solid sheathing (like the panels you are talking about leaving underneath) to prevent contact wetness, because they are not waterproof - just water shedding, so put them directly over a solid surface (as opposed to over just studs or firring strips) and you can get contact mold and rot.

3) if overlying two exterior layers, for above reason, generally you should remove down to (and including) any water barrier layer to avoid having two such layers, and to remove the old one which will be aged and have nail holes in it so is not going to be effective with a new layer over it.

4) generally, as per 1) and 2), to avoid direct contact of the back of the water barrier wtih solid material, in overlay, as opposed to replace jobs, firring strips should be used to separate the layers. These should be rot-resistant, and should separate the two sheathing or siding layers to provide an air gap between them, to allow for evaporation of the water which gets through the siding (which is significant in plank sidings, far less with panel but moisture still goes through the wet siding to the underside of it).

5) overlaying like that means having to redo the windows and doors, otherwise the water seal around them will lie between the two layers, not at the outside like it should be. Also, it means more thickness in the wall, so windows will have to be pulled forward (ideally), or at least the exterior trim and flashing and caulking and such redone to correctly stop infiltration around them. Can add a LOT of cost to the job in some cases.

6) if you just overlay the two layers, I have seen a LOT of cases of rot forming at the interface from moisture getting through the outer layer and condensing out at the interface. Ditto for interior wall moisture from inside the house coming through the wall and the first layer as it normally would, but thenn condensing out at the back of the outer layer because of the added insulation it provides and the small air gaps there - commonly you can get condensation promoting mold/rot, and even in colder conditions frost heaving pushing the outer panel off the wall. For instance, in my 35+ year old house, in sub-zero conditions (single T1-11 siding layer) interior moisture makes it through the interior vapor barrier and the wall, but condenses as frost on the inside surface of the very cold siding - and in a few places has accumulated enough to cause buckling out of the panel andpulling of siding nails when it thaws on that surface and causes expansion of the wood beyond what the expansion joints can handle - so it noticeably bowed out before I fixed the situation.

I have heard of and on maybe a couple of occasions seen this sort of bowing out and buckling, as well as more commonly delamination, from moisture caught at the interface o the two layers, in hot humid climates where the outside humid air and rainwater penetrate the panel as water vapor, and condense on the inside when it hits the colder wall interior created by air conditioning - generally seen in the deep South and humid Midwest areas.

So - bottom line - while you "can" put new siding over the old material (first replacing the rotten sheets) and new water barrier and new siding (ideally with firring strips to hold that away from the water barrier), generally it is cheaper and easier to just strip the old layer off and do it right from scratch - which also gives you the option, if desired, to install additional insulation (typically board insulation in this case) before putting on the new siding.

One possibility if a DIY job, using a bit of care with the prybars so you don't damage the unrotted panels - salvage the "good" panels without any mold or rot to use on the ends or back of house as replacements for the bad ones in those locations, and put all new panels on the front, being sure to use larger head nails or caulking the old nail holes so the used panels are not full of holes to let rainwater into the wood and wall. This will minimize the number of new sheets you need to buy. [Note - this assumes 3/4" sheathing - if the thin stuff, or cedar veneer on OSB or particle board or cedar sawdust board or such, those generally cannot be productively removed and reused - they break up too easy - though in my opinion that would just be good riddance of a bad product anyway.]

Answered 1 year ago by LCD

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