Leveling upstairs floor, found a hard base

15 Feb 2015
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United Kingdom
Hi, I am looking to level the floor in an upstairs room. I lifted the floorboard expecting to see joists so that I could sister them. The floor is a little out especially in the corner of the room.

On lifting the boards up to asses I found that the floorboards are sitting on 3x2 beams and beneath them looks to be a hard set base.
This all looks untouched from when the house was built in the 50s.

Downstairs has had a recent garage to room conversion. The downstairs room was extended out 30 years or so ago with steel beams. You can see where the beam is spliced and boxed in.

1. What material is the base below the 2x3 beams? Is there anything below this hard base ? Something must be holding this hard base and beams up?

2. How would I level the floor if I can't sister. I have heard using hardwood is a good idea to shim the top of the 2x3s?

Look forward to hearing



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I’d be going 6mm plywood. A decent grade. We use Sp101
then use fibre based smoothing compounds like Mapei renovation
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Looks like pugging. In some buildings (e.g. officers, schools, multiple occupancy buildings, large houses, etc) it was used as a form of insulation (sound, heat). Basically something like 2 x 1in laths were nailed to the sides of each joist, short lengths of planking were dropped onto these (to form a continuous "bridge") and a mixture of rubble, ash and sometimes lime mortar were poured in on top to make a thick layer. Perfectly safe to leave in situ. If you take it out, replace with 4in of mineral wool batting cut dead to size (may require replacement of the bridging pieces with 6mm plywood)
Hi there, thanks so much for your reply. The room below was a previous garage so it makes sense the pugging was there as insulation.

Our builder has had a look at the floor and says the tongue and groove is tight and untouched. They plan to use some leveling compound straight on the floorboards without ply. Normally the floor has underlay and carpet on.

Do I need the plywood on top of the floorboards before the SLC?



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Do I need the plywood on top of the floorboards before the SLC?
I've always laid plywood before the floor layers apply the SLC in the past - at least 5.5mm ply nailed at 100mm centres or screwed at 150mm centres (100mm centres on the rows around the edges). On occasion I've lifted old flooring where the SLC was laid directly onto the planks - it has always been cracked along the joints between the boards, so I'd be concerned about this potentially telegraphing through the flooring if it is somethingh like vinyl
Thanks for the great feedback. What screws would you advise? Floorboards are 20mm and ply 5.5mm, would 20mm screws/ nails be ok so to avoid any potential services?

I mostl nail these days (pneumatic coil nailer) although I do sometimes hand nail if I don't have my kit to hand and the area isn't too big (25mm ring shank flooring nails), For screwdriving ideally use 25mm flooring screws, if you can get them, but make sure you sink the heads flush (which is a lot easier with nailing TBH). As @dazlight says use a levelling compound afterwards

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