Installing cabinet on thick sound/thermoproofing layer and concrete

Question

I need to mount IKEA Metod wall cabinets on a sound/thermoproofing layer that is itself glued on reinforced concrete walls. These cabinets are installed on a rail

The layer consists of 1cm of plaster and 6cm of high density fiber glass wool, the specs are here in French (should be the 10+60 variant). Unfortunately it does not seem to indicate the plaster wall maximum load as far as I can see (probably because you are not supposed to mount stuff on it!) Behind it are 20cm of reinforced concrete.

I did not expect that that layer would be that thick and am having now doubts on how to best proceed. The person from the construction company recommended to use cavity dowels, but I am doubtful.

The specs of the cavity dowels I have indicate 20kg of load bearing capacity. A 60cm wide wall mounted cabinet weights about 20kg, and maybe 50kg fully loaded. It seems that I would need to put in quite a few screws to make this work (especially assuming some safety margin), and I suspect this will compromise the structure of the plaster wall.

The other option is to drill into the concrete, but given the lever forces in play here the required screw size would be quite big.

What do you recommend?

Answer 1

Whether you like it or not, you're going to need to mount those rails into the concrete if you want the cabinet to stay on the wall. There's not much structural integrity or load bearing capacity in 1cm of plaster.

There are a variety of ways of attaching things to a concrete wall, from screws (like Tapcon™ that are specifically designed to hold in concrete), to expanding anchors.

You indicate you'd had "cavity dowels" recommended to you. I'm not familiar with that term, but if they're something designed for the purpose, they should do just fine. If you expect your load to be 20Kg cabinet + 50Kg of "stuff", then you're only looking at 4 of these "cavity dowels" which doesn't seem terribly excessive to me.

You'll need to go to your local hardware store of choice, take a look at the options there and pick anchors that meet your expected load rating. It's possible that you'll find "cavity dowels" that are rated higher than 20Kg per fastener, so you may not need to use as many. You'll also need to select a masonry drill bit of the appropriate size for the anchors you'll be installing. Once you have those, drill the holes and insert the anchors according to the manufacturer's instructions.

Odds are good that you'll really only need 2 anchors (of whatever type) to hold your 50-70Kg load. Anchors are designed to hold a lot of weight. It's quite possible that you'll find anchors that are rated at 50Kg each which means that 2 of them (to hold the rail level) will be more than enough to support your expected weight. Again, though, you'll need to look at the packaging of the anchors you're choosing to determine which ones are appropriate.

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There's another possibility (but it's probably not something you're after or you wouldn't have chosen a wall-mount cabinet). Come up with some sort of leg affair to support the majority of the cabinet's weight from below, then your anchors into the wall are simply there to keep it from tipping over.

Answer 2

You'll need to cut out a strip of insulation and fill it with a piece(s) of timber/lumber and maybe a layer of plywood to achieve proper flush mounting of the rail(s). This buildup will be the only thing secured into the concrete and will be plenty solid.

Once achieved, then attach your rail(s) to the buildup for your previously expected installation to proceed without a hitch. Future cabinet replacements will now also be completed very simply.

Answer 3

In the end, we went with the cavity dowels. The most solid ones we found are called TOX Acrobat and we placed them in a distance of 20 or 30cm, depending on the expected load.

For what it is worth, we asked a kitchen installation professional working in an adjacent flat and this is what he did as well; moreover, the dowels he used were less solid (he was using those spiral shaped ones that are not anchored behind the plasterboard).

I will update this answer should the cabinets come down :-)