I was wondering if someone knowledgeable might be able to help me a bit with the dimensions for the floor joists for our upcoming bathroom in a log house (single-story) on a plinth foundation.
In the floor joist plan below, I've drawn in CC300, the distance between the existing plinths, and some more measurements. Where it says CC600, a walk-in closet is planned.
The thick beams represent interior walls in standing timber with a lying log at the bottom and top. The interior wall running along the house previously rested on a beam between the two middle plinths on the gable; this had rotted away, and we have currently solved the problem by attaching 2 screw-glued 45x195 between the two plinths on which the lowest log rests. They are fastened with 2 Gunnebo MQL Fasadplugg 160mm in each plinth. Is this acceptable?
For the floor joists, I mainly wonder what dimension the studs should be? We also plan to screw-glue the flooring chipboard on this, as well as pour about 30mm of leveling compound with reinforcement mesh and Ardex K75, then tiles and grout. The idea is to have a bathtub in the upper left corner if possible. Since it's quite low to the top of the window, it would be desirable to keep the construction height as low as possible.
The floor joists should be 45x170 C 24. It is primarily the deflection requirement that determines this. It doesn't matter if they are even higher. Bathroom floors should be a bit rigid, in my opinion. Your screw-glued double 45x195 should be able to handle a point load in the middle of about 1500 kg. Whether that is sufficient here, I cannot determine with the available information.
Thank you Justus, I obviously agree, especially since we will have tiles on the floor and on the walls. We will build freestanding wooden walls from the timber frame so that the movements in it do not affect the walls.
Could one notch out the 45x170 beams resting on the pilings by 30mm at the bottom to lower the floor? We currently have 50cm of crawl space underneath, so losing a few cm doesn't matter right there. We have 45x195 beams so we would have to cut them down if we go with 45x170, because I suppose you can't notch out 55mm from these?
Regarding the double 45x195 - The inner wall resting on these is standing timber in the approximate dimension of 230x190x2300mm with a door opening of 90cm. We are considering possibly having a storage loft above this section later.
It's always best to avoid notching, but sometimes it becomes necessary. There are methods to calculate how large a notch can be made. Plus factors include the wood's strength class (C 24 is better than, for example, C 14) and the beam's remaining cross-sectional area (larger is obviously better). The minus factor is the shear force (the vertical load) on the support. Applying that reasoning, a 30 mm notch on a 45x170 is equivalent to a 55 mm notch on a 45x195. In your case, I see no issues with doing these. Due to deflection requirements, the beams are oversized regarding bending strength. It, therefore, has nothing to do with the notching's proportion of the height.
The floor joists should be 45x170 C 24. It is primarily the deflection requirement that determines the sizing. It doesn't matter if they are even taller. Bathroom floors should be a bit rigid, in my opinion. Your screw-glued double 45x195 should be able to handle a point load in the middle of about 1500 kg. Whether that's sufficient here I can't determine with the available information.
