Support under joists

slate

Use a cut down concrete paving slab? They come in various thicknesses.

Install a secondary wooden frame pillar to pillar to provide more than a point load support.

I would have though folding wedges would do the trick. That is folding wedges as in this definition, rather than some sort of sleep aid, which Google seems to throw up for the term:

you mean cement & slate or just dry slate ?

You mean put in a paving slab with mortar top & bottom ? ....... just pitting in dry would likely be not 100% supporting

To difficult to add any framing, no room to work

dry late...hammer it in tight

makes a good DPC as well

In case description not clear this sketch may help (quick rough sketch

- no CAD masterpiece)

There are some slabs (intended for council use) that are

Or use a car jack to lift the joist slightly to avoid hammering it in

A strongish fine concrete. If you want do it all in one fill, then don't make the mix too wet. That should reduce the shrinkage.

The traditional way would be to fill close to depth, and then pack the last bit with slate (it slides in nicely, and is very strong in compression).

If you want a "dry" solution, the pairs of large folding wedges:

Jack up the decking *very slightly* at each pillar with a car jack and appropriate length of 2x4; slide into the gap bits of slab and/or slate until snug (they don't have to be neatly trimmed - any old bits will do); remove jack; repeat on next pillar. The weight of the hot tub will ensure the packing doesn't go anywhere.

Thnx

Fill close to joist, and push slate is after its set or while 'wet' (agree it would be a dry mix)

No travis perkins? They sell grano as well as grano&dust

Don't have any slate, years ago it was being ripped of roofs everywhere ..... will ask around

After it is set, that way you can get a good interference fit - plus you can "tune" the amount of packing to suit the gap and or the sag of the joist.

Yes because the joists will flex, and warp, anymix will sooner or later crack away. Then you get the bouncing back and a clunk when it hits the pillars. Brian

You can get adjustable supports to help level decking. Wickes sell these but there are plenty of others available to suit all kinds of gaps

Richard