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I am helping a friend install a bathroom in his basement. The basement floor is partially rock which has been covered in plastic vapour barrier and then covered in a thin layer of concrete. (The builders obviously didn't want to spend the money on excavating the rock to build a full basement with a level floor) That area of the basement floor which is rock slopes basically in two main directions (but the slopes aren't uniform - picture rolling hills!) towards the corner of the basement where he wants to place his bathroom.

In order to create a level floor surface, I think tapered joists will have to be laid from the deepest part of the basement and will have to be extended to where they horizontally intersect the concreted rock. To maximize finished floor space, the joists would have to be tapered considerably and over different lengths depending upon where they contact the concreted rock.

Height is also a problem. The wider we make the level floor, the higher up we have to secure the deep end of the joists. We can probably go up about 10 to 12 inches at the deepest corner before the ceiling clearance will be too low.

So my questions are: a. How do I anchor and support the tapered end of the joists where they meet the concrete covered rock,

b. How skinny can I make the tapered end of the joist (bearing in mind that the longer I can make the joists, the wider the bathroom floor can be)

c. How can I find the "level" line on the concreted rock when the rock is not uniformly sloped. (Is there something like a spinning laser level that would project a level line on the concreted rock that I could then mark with chalk?)

Thanks for any help or light you can shed on this project.

Dave Nanaimo, British Columbia Canada

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Use some self-leveling mortar or concrete.

It's a mortar/concrete that is liquid. You pour it out, and help it a bit and it levels itself automatically.

If the slope is extreme (more than an inch or two), then get it semi-close (but lower) with some regular concrete, then finish with the self leveling stuff.

You say joists, but are these joists actually suspended above the floor? You will have a very hard time cutting them to match a bumpy floor.

To find level on the floor get a bucket of water and a thin, flexible, transparent hose. Fill the hose with water and submerge in the bucket, then attach it to the bucket so it doesn't fall out and take the end of the hose to your wall, then mark the water level all over the place.

Subtract a fixed amount from the water level (cut a piece of wood the right length) to get the actual height you need.

  • Ariel, Thanks for your help. My friend was hesitant to use levelling concrete, so we tapered the joists and trimmed them to fit the uneven ground (the other end was supported by joist hangers). It was a bit of a job, but they ended up matching the ground contour fairly well. We used wood to concrete glue to secure them at the tapered end to the concrete. The joists are 12" apart and we have used no more nails along with flooring screws to fasten the sub-floor plywood to the joists. All in all, it seems very solid. Thanks again, Dave – Dave Embemor Jan 27 '15 at 15:14
  • Even though I couldn't help, I'm glad it worked out, since that's the important part. – Ariel Jan 28 '15 at 5:00

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