Curing All Sills

Curing all Sills

Old House Journal
December 1999
by Gordon Bock – photographs by Jon Crispin
Reprinted with Permission

Curing all Sills

Curing all Sills

Sills may be the lowliest parts of a wood frame house but, like the keel of a boat, they form the base that supports all other structural members. When old house sills succumb to rot and insect damage, it takes technique and experience to replace them.

GEORGE YONNONE RESTORATIONS has specialized in this heavy repair work for over 25 years. Since George welcomes any opportunity to share his knowledge, we visited him on the job at an 17905 timber frame house to see the process he uses to successfully jack and repair all kinds of buildings.

Like the best restoration projects, sill repairs start with assessing the present conditions, Sills tend to decay from the outside in, and while they may look good from the basement side, 1/3 to 2/3 of the wood can be gone under the exterior siding. Along with probing with an awl for deteriorated wood fiber. George relies on “sounding” to assess sill integrity. Tapping the wood with a hammer from the basement side produces a telltale tone. A good sill rings with a high pitch; if there’s a little rot on the outside, the pitch is lower. If the sill is really suspect, and the owner is comfortable with a little exploratory surgery, George pops some siding off to survey the sills from the outside.

When inspection reveals a bad sill, the first repair step is to remove siding at the second-story line above and below windows to expose the timber spanners for their equivalent in Later buildings. Next, George screw’s a hook into the top of the building at the center of the wa1l, to which he hangs a “come-a1ong” cable winch. Then after carefully measuring the positions of studs and structural members along the second story line, he transfers these dimensions to two 3″x 10″ oak planks that will be the plates for jacking the building. It is important to use two planks for this purpose so they will flex Like a hinge as jacks lift the building, After double-checking measurements, George hoists the plates into position with the come-along, and lag screws them to the side of the building.

Setting up the jacks is often the most time-consuming phase of the jacking process. For the average house, 38′ to 40′ on a side, George will use four jacks to a wall. He positions each jack 24″ off the sill-an angle that is effective for lifting while providing clearance to work at the foundation line. After excavating down to undisturbed earth, George sets a jacking pad of laminated lumber in the angled soil. The 24″- square pad not only spreads the jacking load through a large area of soil, it provides ample room to finesse the jack’s position. On top of the pad comes a block, then the jack and a steel plate.

George selects 20-ton hydraulic jacks for most applications. Even though 12-ton jacks are more than strong enough to carry an average frame house, the larger jacks have a longer cylinder throw-on the order of 9″-that allows for more lift without reworking the setup. Also, the bases are larger, distributing the load across a wider area of the pad and increasing support.

The 4″x4″ oak posts running between the jacks and the plates on the house have to be measured and cut for each position, and this dimension is subject to how the jack “loads-up” in the soil. When the jacks are tested, loamy soil often compacts for some time before the building starts to rise. Sandy soil, on the other band, tends to load quickly. It takes patience and the experience to “read” the soil around a house and position all jacks optimally.

In simple terms, the object of jacking is to raise all the house’s low areas so they are level with a benchmark. Typically, this benchmark is a high point in the foundation or central chimney that represents the most stable base in the structure. Jacking helps remove the old sill but, more importantly, it re-establishes the correct height of the floor and wall framing. Originally, the house may have been built on an 8″ square timber -Common practice as late as the 1890s. Over decades, this timber could have shrunk and deteriorated to as little as 6-1/2″ in height. Simply replacing the sill with a new 6″ timber does not regain the building’s original dimensions. Plus, the new material will also settle and shrink in time, bringing sill height down to, say, 5″ -a loss of 3″!

Most houses tend to fall or sag towards their centers, so there is usually more distance to jack in the middle of a wall than at the corners. In such a job, this means the jacks will run out of travel or lift capability before the middle is level, and the setup will have to be reworked slightly. Typically, George will extend the middle jacks 9″ or so, then place blocks and shims or cribbing under the studs at the middle of the wa1l. With the house temporarily supported this way, he can collapse the jacks and build up the pad ( or extend the jacking post) to resume jacking.
Skeletal Secrets

In timber framing, as well as balloon and platform framing, studs, joists, and posts converge along the second-storey line-a structurally stable point to jack. This method also lifts the first-storey walls without putting them under any load, the ideal way to avoid cracking historic plaster finishes. In contrast,
jacking at the foundation level risks pushing in the walls. Where bolting a long plate may
interfere with mortise joints, George substitutes a jacking pad.

While the whole point of George’s thorough jacking and leveling of the structure is to avoid piece-meal repairs with short lengths of material, the typical sill job does require splicing sills. No single timber available these days will span the 35′ to 40′ length of most walls, so George makes a 40′ repair with two 22′ timbers. Sill joints have to be as strong as the timber itself, and for this reason George always makes his lap joints no less than 24″ long. He also makes sure the joint is in full contact, inserting a reciprocating saw and trimming both faces at the same time, if necessary.

When the fit is correct, 1/2″ lag screws fasten the timbers together. George positions the screws in a staggered pattern, offset by about 1″ so no two fasteners are in line to potentially split the wood, George prefers white oak for many sills, especially for the extra weight of timber frame houses. Since oak is one of the densest woods, he bores 3/8″ pilot holes for the lag screws, as well as 1/2″ holes for the shafts. (Lagging into softwoods requires only a small-diameter pilot hole or none at all.) George’s experience has also taught him to lubricate long lags screw with soap or wax. Otherwise, as these hefty bits of hardware go into the oak, they generate enough heat to seize in the wood halfway down.

Before the sill will mate with all the members in the house, it must be cut for the housings and mortises that hold floor joists and studs. Rather than attempt to measure all these pockets to fit the settled, often slightly eccentric dimensions of a 100 or 200 year old building, George lays out the cuts to some standard spacings apparent in the construction, then deals with irregularities as he goes. Though the goal is to have joists rest in pockets as tightly as possible, practicality may call for trimming a joist slightly to get the proper fit.

Even with the most perfect of fits, however, maneuvering 22′ of massive oak so it meshes with floating framing takes equal parts skill and experience. George and his crew call on a variety of bars and levers to “shoehorn” the new sill under wall studs and sub flooring. A favorite tool is a flat bar, a tapered piece high-carbon steel that is actually nothing more than a foot or so of truck or car leaf spring. George has several of these tools on hand for inserting under wall studs to make clearance for the sill. Once the new sill is “home,” floor joists and lapped corners secure it in place, ready for the next phases of restoration carpentry. In fact, having the load off the foundation presents “the ideal opportunity for masonry work on the foundation, such as resetting the capstones visible just above grade-the finishing touches on so many 18th- and 19th-century old houses.