12 December 2013

Skirt-roofs & Overhangs

Marquesinas  Each downstairs window, except for the ones opening onto the front porch, has a protective flat awning-style skirt-roof over it, sticking out about two feet from the wall, to direct rain water away from the window opening.  Above, the main roof overhangs the walls all around by about three feet.  The original clay barrel tiles stuck out about 2.5" past the roof slabs, not enough to prevent rain washing down to all fall freely off the roof.  (The new concrete tiles are set out with 4" of exposure to reduce the chance of this occurring in the future.)

Years of moss build-up, occasional cracks and accumulated dirt meant that in any rain surface tension would cause some of the water to back up the undersides of the roof tiles and then run down the slab edges and onto the roof tablas and vigas (sheathing boards and beams).  While there was a layer of tarpaper laid on top of the boards before pouring the reinforced roof slabs, there was no effort made to continue protection over the outboard tablas. The constantly re-wetted lowest-down board edges, and the beam ends, demonstrated serious rot, especially on the west edge of the upper roof.  Here are some "before" pictures to illustrate.  We used fotos like these to bid down the purchase price of the house, since we knew we couldn't leave things in this state and it would take serious effort to correct the situation.

It should be said here that the 4" wood machimbre (tongue & groove) boards and heavy beams that were used as a base for pouring the roof slabs, and which form the very attractive open-beam cathedral ceilings in the upstairs bedrooms are, for the most part, now, non-structural.  This is in the sense that we could remove them and the reinforced slab roof would still be in place.  The only small area which would have to be worked over bit by bit (should all the wood be removed) would be the perimeter joint where the walls meet the roof slab.  Here work would have to be done replacing (with concrete grout and metal ties) the ¾" thickness of wood that is sandwiched between the top of the wall and the underside of the slab.

It was obvious that there would have be some method of flashing the concrete-wood seam along all the exposed roof edges.  But, it seems like no one down here has heard of flashing, and it is not available in any of the construction supply houses, and I visited virtually all of them in the city.  The words I used to convey the very idea of "flashing" were tapajuntas de escurrimiento (something like "run-off joint cover") -- all I got was blank stares and directions to another supplier who just might have heard of such a strange thing.  We were about to order some made up at the one specialty sheet metal shop in Córdoba, when Home Depot opened it's new store there. We actually found 8' lengths of galvanized sheet-metal "90º el"-shaped stock (about 3cm x 3cm) there, which could do the job with a little pounding out to open up the angle a bit.  To see if we were going to do this to all our windows, and all along the roof edges, I worked out the kinks by using this material on the marquesina over one window.

In the skirt-roof I chose, only the very lowest exposed board had moisture-caused rot in it, and the protruding beam ends were in good shape. The first task was to insert a hacksaw blade between tabla and viga to cut off the nails holding the board in place.It was then a simple matter to insert a good board in it's place, toenailing up thru the corner of the beam to affix it.  Very fortunately, there was enough good machimbre material left from the demolition of the skirt-roofs from the laundry building and over the double doorway into the dining room space, that I didn't have to locate a sawmill which could work up new matching t&g boards. All the wood that replaced the bad in the roofs was treated on all sides with festermicide, a preservative and termite & fungus preventative, before being put into place.

The flashing technique which I used involved inserting the slightly opened up angle between the tarpaper layer and the wood, lapping any ends in the angle stock by several inches, and caulking the metal in place along the upper angle fold and the concrete slab.  I located a good quality, gray slate-colored acrylic latex/silicone calafateo (caulk), which worked as an adhesive as well as a sealant.  Since the joint shouldn't "work" any, other than some expansion of the metal in the hottest weather, and no freeze-thaw cycles to worry about, this should be a long lasting joint.  If there are any problems, some additional caulk squeezed into any cracks that develop should take care of things.  Due to the pounding out of the angle, the metal hangs down outside the edge of the tablas, providing a sharp drip edge for water to fall free of the wood.   I worked around all the roof edges reachable by stepladder (the porch and living room window skirt-roofs), leaving the upper story for the younger, nimbler guys to complete.

The only other issue to be worked out was how to replace the rotten beam ends that were under the various roof overhangs.  One beam over a living room window obviously had problems, and when I started working on it is was clear it had to be totally replaced, due to end rot and then termite damage farther up.  This meant, for this beam end, actually cutting it off almost flush using my sierra reciprocante (reciprocating saw), then digging out the remainder well into the wall surface with various large brocas (drill bits) and cincels (chisels) and wrestling the remaining wood out of the hole. The guys used a a good beam end salvaged from the laundry room roof for a visually perfect replacement, which they cemented into the hole in the wall and nailed into the t&g boards which were cantilvered above it.

Fortunately, the remaining beam ends were not this bad, with the damaged ones mostly on the west and high north sides of the house.  (Natch, the easiest ones to replace would be the ones on the east, where one could stand on the porch roof to get to them.  These however, were in the best shape and didn't need much attention.)  The vigas supporting the tablas, on 1 meter centers are full-dimension 4"x6" in cross section, and the two under the roof hips were 6"x8").  José Luís, my foreman, ordered up milled ocote (hard pine) from a friend with a mill in Coscomatepec.  They arrived, rough-planed, after a couple of weeks, and they were treated with festermicide and sticker-stacked on the front porch to cure a bit while the crew devoted themselves to building the new building addition.  The damaged beam ends and rotted tablas were removed pending replacement

Later in the year, after all the masonry work was done but before the house painting was to start, the guys moved into rough carpentry mode. Each viga was trimmed back to good wood, and the bad tablas removed.  Then, a replacement beam end was cut to length, with the recurved end fashioned to match the existing ones.  After all the trimming was done, the pieces were sanded, treated again with festermicide and then stained to match the existing beams.  The technique developed to affix the beam ends involved drilling two deep aligned ⅜" holes in both the old beam where it was cut off and the newly fashioned outboard end.  Lengths of rebar were driven into the holes and the new beam end pounded into place.  Heavy galvanized mending straps were then lag-screwed onto both pieces, bridging the joint on both sides.  Replacement t&g boards were slipped into place and tacked in. For the two heavy hip ridge beams (upon which the smaller vigas rest), an additional piece of rebar was used. Fortunately, I brought along some 18" and 24" long wood boring bits in amongst my tools, as I haven't seen any of these here in hardware stores. including the new Home Depot.

We did  fully appreciate the new selection of hardware carried by the HD store, as large mending starps and big lag screws are other things that  seem to be hard-to-impossible to find otherwise.  When all the beam work was done, the guys completed the installation of the rest of the flashing. Festermicide was applied to all the unpainted wood surfaces inside and outside of the house, and injected by syringe into cracks, perforations and crevices wherever possible.

Next:  Foundation & Sewer Lines

No comments:

Post a Comment