Bringing Back the Beauty
The MacKenzie House
3501 Bryant Ave. S.
Minneapolis, Minnesota
Hidden Support
The installation of the new porch foundation ensured that there was solid support for the existing porch posts all of the way down below the frost line. However, since the new upper posts would not align with the existing posts below,
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| Exact position of the new roof was determined by trial and error. Large Image | | |
structure was necessary inside the porch roof to support the new posts and transfer the loads all the way down to the foundation. In spite of the fact that the new posts and roof were recreating what once existed, it was impossible to know if the framing inside the porch roof was adequate. It was also unclear if the necessary structure was present within the existing house walls to support the other ends of these beams and their concentrated loads. Only further investigation could answer these questions.
Determining if adequate structure existed within the house walls was relatively easy. After first trying a stud finder without success, the trial and error approach of probing with finishing nails was used. The nails were driven into the plaster walls at a spacing of approximately 1/2 inch in the general areas were support would be required. With this approach it was easy to determine that no structural support existed in the vicinity where it would be needed. Removal of the plaster and lath between the wall studs in these two areas confirmed that the needed structure wasn't present and also provided access to the wall cavity so new posts consisting of double 2x4s could be inserted.
Determining the support, if any, present inside the existing porch roof was a bit trickier. Since the existing porch rubber membrane roof would need to be patched anyway after the new porch and newel posts were secured into the existing framing, it made some sense to remove a bit more of the membrane and sheathing to provide the access needed for further investigation.
 | | New LVL beam rests on blocking supported by double 2x6 beam (hidden). Blocking on top of beam will align with new porch post above. Large Image |
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Once again this exploration revealed the inadequacy of the existing framing. It wasn't a surprise that beams did not exist to support the load from the upper porch posts. It was a surprise that there wasn't a perimeter beam above the existing porch posts. The box "beam" seen from the outside was hollow, nothing more than three boards assembled into a "U" shape without an actual structural beam in the center.
This discovery meant that short beams made up of two 2x6s had to be inserted within this hollow box beam spanning between each pair of columns flanking the front door. These short beams would in turn support one end of each of the 2 new longer beams positioned under the new porch posts with the other end supported by the new 2x4 posts in the house walls. A hole in the existing wall sheathing allowed these longer beams of doubled 7 1/4" Laminated Veneer Lumber (LVL), to be inserted into the existing porch roof from inside the house.
 | | Temporary posts support finished roof framing. Large Image |
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It was clear from the one historic photograph available that the new south porch post would align with the south and west edges of the roof below so it was easy to determine the exact position of the supporting beam inside the roof. However the exact position of the north post wasn't as clear. Efforts to determine the proper location before construction from the physical and photographic evidence was not only inconclusive but actually contradictory. The only way to address the problem was to build a full scale mock-up that could be repositioned against the house as necessary to find the correct alignment through trial and error.
First the existing roof returns that would interfere with the new roof framing were removed according to physical evidence that indicated where the trim had been patched at the time the original roof had been removed. After verifying the existing roof slope a full-size temporary truss was built to match and this was moved and adjusted during the course of 3 to 4 hours until all of the critical relationships indicated proper placement had been determined. Once this was accomplished the LVL beams could be installed within the existing porch roof and the new roof was framed and supported on temporary porch posts.
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