Levelling a sloping garage floor

[BucksDIY]

Hi everyone,

I'm new here, but I have been browsing the threads for a while and have seen lots of helpful advice so far

I have a semi-integral garage in my detached house. The back 1/2 garage is within the house, under a bedroom and backing on to the utility room, with the hallway to one side and the external wall to the other. The front 1/2 extends beyond the front wall of the house. It has a flat roof and a concrete slab floor, with a rolling door on the front. There is an external door at the back side, but no internal doors into the house.

The internal size is around 5.1m front-to-back and 2.7m wide.

I hope that makes sense so far

The concrete floor slopes down quite a lot from the back to the door at the front (I know this is normal in garages to allow spills etc to drain). The fall is around 180mm over 5100mm (give or take a few). The external side wall is double-thickness brick (around 230mm). The external wall on the other side that extends beyond the front of the house is single-skin brick (105mm thick).

I would like to turn the garage into a gym. I am not interested (for now) in turning it into a proper habitable room (with building regs etc), and will be just leaving the garage door in place for convenience. I may at some point in future either replace it with a wall / window, or build a stud wall behind it and partition the garage into 1m of storage at the front and ~4m of room at the back.

The most urgent issue is the floor level. To use it as an unheated gym, the floor obviously needs to be flat... I'm not planning on trying to insulate it to building standards, as I think that would make it quite thick (there is probably 75mm of available space below the side door, but I don't want to construct more than I have to).

I have been doing a lot of reading, and I think there are basically 3 options for floors:

1. Concrete - I'd need to pour a lot of concrete to make up the level, which I do not like the thought of
2. Floating floor - place insulation boards on top of the concrete, then chipboard and then flooring. The issue with this is the slope - I think it would all be heavy enough to not move/slip on the slope, but it would be weird for a treadmill to sit on the slope, and it might roll if the flooring was too smooth
3. Suspended floor - hang level joists off the side walls and then chipboard over them. Insulation could be added between the joists (future job, since I assume there is little point in insulating the floor if there is a thin rolling door at the front?). The issue here is that I have read you need a ventilation gap of 150mm beneath the joists to prevent damp - that would make the new floor too high by the time I have 150mm gap, joists, chipboard, flooring.

A 4th option might be to place joists (more like battens I guess) across the concrete, trimming them or shimming them to make the tops level, attach the ends to the side walls, and then chipboard over the top. That would be a bit like a floating floor, but with wood.

I think any option requires a DPM over the existing concrete first, then the floor on top?

Questions:

1. Are there any other options?
2. Is #2 really a bad idea? I think floating floors need to be laid level
3. Would I need a 150mm ventilation gap for #3, given I am not going to heat the room, and there is no insulation in the floor?
4. Would #4 work? Or would it need ventilation under too (which would be blocked by the wood across the floor)?
5. Could I instead make firrings of the correct profile and lay them front-to-back on the floor - that would leave ventilation gaps at the front that run the full length?
6. What is the minimum depth of timber I should have at the back of the garage? Could I lay the floor so that at the very back, it is just chipboard resting on the concrete and then level from there, or should there be some depth of timber there (and raise the whole floor by that amount)?
7. If I decide in future to put some insulation between the joists (either real ones in #3 or the ones resting on the concrete in #4), does that make any difference to any of the questions above?

Phew! That turned out longer than I expected Thanks in advance

Trev

 

[vinn]

Very few, including myself, will bother to read such a long post.

The subject has been covered many times before - why not use the search box, and the similar threads, below at the bottom of this page?

 

[tomfe]

Floating floor level your joists with timber legs, if you use enough you could just use 4x2s.
Like a decking.

 

[cjard]

+1 for tomfe's suggestion, for my money I'd pick up a load of 3x2, 2.4m lengths, chop some of them up into varying length legs at 400 centres, glued to the side of a full length and set them out on the floor at 400 centres, then chipboard flooring or decking boards over the top. 2.4 won't reach all the way across your 2.7 garage so run another support perpendicular, with increasing length legs glued down its side, and this should provide support for weight racks and machine legs that go near the wall. 22mm chipboard would be preferable to 18 if any of your equipment is particularly heavy and point loads it's legs. You may want to use load spreaders under point loaded legs too. Ventilate the place well when in use, and natural draught through the door will be sufficient when empty. The nice thing about decking flooring as opposed to chipboard is that it absorbs sweat without becoming slippery (this is typically why squash courts etc are not sealed), unlike moisture resistant chipboard, so if you sweat a lot while working out, consider decking style boards instead

Start from zero if you're trying to get the flooring underneath the level of the side dooor. This may mean you need to buy some narrow batten for the first couple of rows of supports, and have the Batten fully supported by the concrete floor

If you're considering turning the garage habitable in future, fees a lot more upgrade required than the quick job you're doing now, and I ink there is nearly no point in laying prep work now for that event then. instead, take your floor up at that time, and re do it habitable style

 

[BucksDIY]

Thanks for your replies tomfe and cjard. If I understand you correctly, you basically mean a bit of a hybrid between a suspended and a floating floor, with left-to-right joists supported by legs? So there'll be a shrinking gap underneath them (which is a lot less than 150mm but the ventilation in an unconverted garage will be perfectly fine)?

I agree that 22mm chipboard is a good idea - I had already thought of that as an improvement. When you say "decking style boards" can you point me at a link - I'm not 100% sure what you mean. Do you mean regular timber floorboards or something else?

Thanks in advance

Trev

 

[Nige F]

I've heard of duckboards but sweatboards is a new one

 

[cjard]

Decking boards, like what you lay outside. They're about 5x1 and ribbed differently each side, for your laying pleasure