Rebuilding an Historic Sill

Whenever we approach a project on an historic house, nearly universally we have to deal with rot. Rot in window sills, rot in columns, rot in framing, and rot in siding and trim are among the worst enemies to aging houses. For the last five years Red House carpenters have been using a product specified by the U.S. government for official restorations. And it is perfect for a situation like this window sill from 1840.

Rotted Sill 

This sill is made of ‘nearly’ bomb proof exterior grade wood which has successfully kept the building envelope of this house dry for 171 years. Not a bad track record. Unfortunately it has undergone several prolonged periods of paint neglect. Without paint, the wood absorbs moisture from the surface water, the wood expands, and then as the weather clears, the wood contracts. When this moisture cycle is repeated over and over again the wood starts to check, which means denser and softer parts of the wood begin to separate because of differing expansion rates of different parts of the wood. Think of the winter and summer growth rings on a tree trunk. These absorb and release moisture at different rates, and they expand as the absorb moisture. Once these checks open to the surface they create crevasses that hold surface water, which both exasperates the checking process and also allows microorganisms a perfect breeding ground, even for a high quality exterior wood. This is where rot begins.

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Fast forward to where we are now. This historic sill is integrated into two side by side windows, there is no off-the-shelf option for replacement of the 2-3/4” thick face on this sill, and the wood options available for the replacement sill are limited and expensive, if we are trying to achieve the same durability and longevity as the original. Facing these obstacles to replacement, and being aware of the intrinsic value of leaving as much of the original fabric of a house as possible, we have been using this epoxy system to salvage and re-use high quality historic wood when it makes sense economically and structurally.

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For this sill, we dug out as much of the loose material as possible. This may involve cutting through the soft surface of the wood to access rotted material underneath. What remains must be dry before initiation of the restoration process. This can be achieved through time and shelter, or various solvents can be employed to speed the process. Once the wood is dry on the interior of the sill, we mix and apply a liquid consolidant. This consolidant penetrates through any soft wood remaining in the sill, binding it to the solid wood still remaining underneath. It remains slightly flexible so it can move with future seasonal expansion and contraction cycles. And the consolidant acts as a primer for the next step. We apply as much of the consolidant as the wood will absorb, then we apply a paste like epoxy compound to build up and re-create the missing part of the sill. This re-creation has to cure until completely hard, a process which can be sped up with the application of heat.

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After the epoxy is cured, multiple types of sanders (always with a Hepa vacuum attached) are employed to “sculpt” the fill material back to the original dimensions. Once the shape is finished, the material can be primed and painted the same as any other wood on the house.

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Our finished product now touts heritage. It retains the originally selected antique heart pine from 1840, with all its rot resistant qualities. This original wood is now combined with an epoxy product that is itself unaffected by moisture and impervious to rot. And, as with this example, we often tool the new product with eased edges to match the worn edges of the other existing sills on the house.