Adobe Laying in Winter: Some Cautionary Notes
It's winter in northern New Mexico and a lot of my colleagues are laying adobes like mad. Can't blame them -- it's their job, but should they really be laying adobes in freezing weather?
Had I asked this question of my father's old hand, Tony Barela, he would have said "ni modo." But back in 1954 when Tony was making adobes for my dad at a penny apiece things were different; we weren't in a hurry.
Years later, during the mid-70's in El Rito, about an hour north of here, the consequences of rushing the job along became poignantly clear. During the restoration of the San Juan Nepomuceno church, the entire west wall collapsed and had to be completely rebuilt. Local knowledge concerning adobe informs us never to lay more than three or four courses of earthen blocks in mud without allowing adequate time for the moisture to evaporate. The reason is that mud is a non-hydraulic binder. That means that the water in the mix actually leaves the material -- in other words the mortar "dries" out. In a hydraulic binder, like a Portland cement-based mortar, the water is incorporated into the crystal of the final material -- concrete. This process, referred to as "setting," takes place over a few hours (as any do-it-yourselfer who has taken too much time to pour a porch slab knows very well).
Back to mud. The contractor in El Rito, for whatever reason, was in a hurry. He laid too many courses of adobe in cool weather when the drying process is considerably slower than during warm, dry summer months. Some of the courses in the thick walls were laid in freezing weather: double jeopardy since moisture in a multiple-wythe (several blocks wide) wall needs even more time to dry. Come springtime the piper came a'calling. Warmer weather released the frozen moisture from the mortar saturating the adobes. Spring and summer were spent rebuilding the wall a second time.
The first time I saw adobes laid in Portland cement mortar was sometime in the late fifties, ironically also in El Rito. It was a house belonging to a friend of ours who wanted to be in by spring; using Portland, which unlike mud "sets" up quickly in cold weather, allowed the contractor to move along more rapidly. It seemed like a sort of creative way to get a job done, but I remember thinking that it just didn't look or feel right. Aside from the lack of purity of using mud to bond mud, and then using mud to plaster it with, it just felt intuitively wrong to put something as hard as concrete between layers of something as soft a adobe.
A good example of what is wrong with the conjunction of hard and soft materials can be seen on the north side of Paseo de Peralta, just across from Allsup's. There you will find an unplastered adobe wall on a concrete footing. The wall resembles a honeycomb; the earth in the blocks is eroding away, while the hard Portland-based mortar stands out in high relief.
The reason is that each adobe is sitting in an impermeable bowl of concrete that collects water. Damage occurs during all seasons; in the warm months the moisture accumulates to the point where it can actually puddle and overflow the edge of the mortar carrying particles of earth with it. During freeze-thaw cycles (particularly in a more moist winter than this), chucks of adobe spall away from the blocks. Were this wall hard plastered (with Portland, for example) the damage would be even worse, not least because it would be unseen. Plaster will crack; even the hideous, new elastomerics, despite claims to the contrary. Cracks admit moisture, but the more-or-less intact sheathing prevents evaporation. The result is often catastrophic. When the moisture in the adobe reaches twelve to fourteen percent, the block slumps. The wall on Paseo at least has the advantage of being able of disperse water out of the exposed edges of the adobe.
So, what's a person to do? We're not going to revert to the good old days when time wasn't an issue, but using fast-setting hydraulic mortars is the kiss of death for adobe. Hand-wringing time. Nothing I can advise will provide a better result than an adobe wall built using mud mortar during warm weather. But for the hasty there are other possibilities.
I dislike stabilized adobes (I'll explain why another time), but they do offer one advantage to the foul-weather builder -- the stabilizing agents retard the absorption of water. If you build with stabilized blocks, use mud mortar, but keep a watchful eye. The two caveats to this reluctant recommendation are (1) that the wall still needs time to dry out before it gets plastered and (2), "retard" does not mean "prevent." Build stiff, using as lean (higher percentage of aggregate), and dry a mortar mix as possible.
If you're using non-stabilized adobes there are two options. First is a very lean (in this case low in Portland) soil-cement. You will have to experiment with local soils, but a 1:7 to 1:10 mix of Portland with your mud mortar usually will provide just enough hydraulic effect to avoid the moisture retention problem, and will still allow for a reasonably vapor-permeable mortar.
Finally, try a lime mortar. A lean (this time high in aggregate) mix, say 1:4 bag lime to no. 8 masonry sand, will dry quickly and remain permeable. Though lime, like mud, is a non-hydraulic binder, it releases moisture more rapidly than mud, a dynamic of particle size and shape.
And what if it's too late? If you used Portland-based mortar to lay adobe your mantra should be "maintenance, maintenance, maintenance." Prevent the invasion of moisture -- in other words, keep the caulking gun handy.