Editorial & Photography By: Dr. Christopher Cooper
All concrete slab-on-grade construction has a common problem. If water management is not strictly observed, the timber sills can easily be destroyed when they come in contact with the concrete itself through capillary action*. Our “Old Home Living House One Garden Oasis Project” has a severe case of water mis-management! On the north side of the building the grade or soil level is a full 12-15 inches above the concrete slab, therefore the wooden structure including stud ends and sill are subject to rot (see Image 1). It amazes me every time when the sill is uncovered in situations such as this how the building is able to remain straight and plumb!
On the interior of the building I built a quick 2 x 4 wall tightly under the roof rafters to support the building while the existing sill was being replaced (see Graphic 2) I built it using screws rather than nails so the timber could be easily recycled and reused. If the building is in such a condition that it is out of level and plumb this would be the time to consider sympathetic jacking to set things right. As this building is perfectly plumb there was no need for jacking or straightening. Remember before jacking or supporting any building retain a qualified, professional structural engineer to give his/her opinion on the works and how best to proceed or leave this project to a qualified contractor familiar with this type of work!
After the structure had been supported, I set to the task of hand-digging a trench approximately 10 inches below the concrete slab (see Image 3). After the excavation was complete, one could easily see that there was a severe rot issue with respects to the decayed sheathing boards (see Image 4, opposite page top).
With the aid of a circular saw (set to a ¾” depth) I cut the tongue and groove off the existing sheathing and removed the two lower boards (see Image 5). The stud ends and sill were a mass of rot, in some areas the sill had been reduced to soil, again how this building has remained standing is beyond me (see Image 6)!
After measuring the areas of rot vertically, I have determined that a triple sill (4 ½” thick) will have to be installed after the stud ends are cut down to sound timber. With the aid of a jig (three short 2 x 4’s nailed together) I scribed a line on all three exposed sides of the stud with the aid of the jig to mark the cut line required for each stud end (see Image 7). Using a reciprocating saw I carefully cut the stud ends, making sure to cut on the line scribed on all three sides of the exposed stud to ensure a straight plumb cut (see Image 8).
After the stud ends are cut, an extra layer of EPDM pond liner is placed on-top of the concrete slab (see Image 9) and a 2” x 4” (new sill plate 1 of 3) is fastened to the concrete slab using Ramset concrete nails. The subsequent plates are built-up and nailed into the first sill plate fixed to the concrete. The stud ends are checked for vertical plumb and toe-nailed into the built-up timber sill (see Image 10).
After the entire sill is installed, the EPDM pond liner is flipped-up and stapled to the new sill with stainless steel staples (see Image 11). The area below the existing wooden sheathing is filled in with new timber sheathing (see Image 12).
You will note a large piece of EPDM pond liner attached above the work area (see Image 13). This rubber membrane (EPDM) is draped vertically over the new works and extends the full 10” below the top of the concrete slab to ensure protection of the timber structure will not rot again, the rubber membrane (EPDM) is fixed to the wall using stainless steel staples! As the area will be properly swailed (a small ditch), the EPDM pond liner is just added security against rot.
Image 14 above: The east side gable sill was a different size. Therefore, three 2” x 6” were employed, the material used was sound recycled timber (note weathered look), note the EPDM pond liner material.
This project has saved this building from demolition and will have many more years, possibly hundreds of years of use left in it. This type of work is definitely very high on the scale of difficulty for the average DIY’er and may be best left to a professional! Not withstanding this, the entire project including replacing the sill on the east gable end of the building as well (see Image 14) took me (by myself) approximately 8.5 man hours to complete, with a cost of under $500.00. Well worth it I would say.
In hydrology, capillary action is responsible for moving groundwater from wet areas of the soil to dry areas (i.e. concrete or timber). As concrete acts somewhat like a sponge holding copious amounts of water, a vehicle such as EPDM (ethylene propylene diene M-class rubber) pond liner material is recommended to separate the timber construction from the damp concrete to not allow the creation of a wet rot situation.