Levelling joists with plastic spacers?

7 May 2012
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United Kingdom
I am converting part of a garage to utility area and want to raise the floor to house floor level. (The area is to be sealed from the garage with fire break.) My idea is to make footings from two 45x95 timbers resting on mortar beds onto the existing concrete floor then lay joists over these (more 45x95 being sufficient for the span). So a 10mm mortar bed would give a total of 150mm before boards and covering. First question - is a footing like this acceptable?

The problem is that the existing concrete is not level, with a corner/corner height difference of 21mm. Ending up with over 30mm of mortar in places seems a bit thick and tamping things down to achieve a level is always difficult. So I would lay the two timbers down in the mortar, accept any irregularity then individually set the height of each joist using spacers. The plastic spacers used around window frames seem like ideal candidates. Any reason why they are not? If not, any suggestion?
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If I am doing anything like that I use strips of lead because you can easily nail or screw through it.
30mm is a lot to use spacers on, so what you need to do, is pack the joist to the required level with whatever spacers you have, and then pack a dryish concrete mix underneath it where you need to support it. It'll shrink slightly as it dries, and then you can pack it back up if necessary. You'll need to fix the joist to the outer wall so it doesn't move, and then put noggins in the floor joists to keep them stable. The plastic spacers are fine, or you can use slate, or lead if any to hand as Catlad suggested.
Thanks. Do you really mean concrete, not mortar? The 30mm is the combined mortar and spacer lift so the aim is start with 10mm of mortar with no spacer at one corner and thicken the mortar elsewhere, reaching perhaps 15 + 15 at the worst point.

Perhaps the thing that concerns me is not the simple weight load but how a wood/mortar connection copes with movement from a washing machine and whether it is likely to abrade down over time.
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We usually have a plethora of packing material to hand ranging from concrete slabs and roof tiles to fibre cement slates, cement board, lead etc.

I prefer dry packing to wet packing on the joist seat.

We had to make sure that the gap above the joists was soundly packed on our latest job as some of the joists will be needed later on as propping needles.
A good thing for DIyers and people who tot all their lead;)
Having read the instructions, they advise one every 400mm "depending on the loading". If I've worked out you're method correctly, you're laying the footings on their side, and then the joists vertically across the span of the garage, and if that's the case, then you couldn't get one every 400mm. Correct me if I've misunderstood you're intentions.
Those screwy thing made me think, especially at £1.6 each! I have used this system for double height oven units with only 2" spare headroom - no room for standard plinth fittings. So it goes like this:- Drill a hole 1" deep in the bottom of the joists, get some stainless steel coach bolts (round headed). Put a nut on the coach bolt run it up under the head, put a washer on the nut. Put fixings into the hole in the bottom of the joist. Invert the lot. Unscrew the nut, holding the coach screw by its square. The nut will now carry the weight of the joist. Twiddle the nut for best effect. It would be nice to use a 2" square washer under the joist and under the coach bolt head, but I could not find any quickly.

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