Does this MDF carcass top need support? If so, how?

leemkule · Monday at 10:11

I'm making a 3 carcass built in shaker wardrobe for my house which has quite a pitched roof so the ceiling is sloped.

Each carcass is about 1125mm in width and will be made in 19mm oak veneered MDF (including the backs). I've linked some screenshots to the design and the space that it will go into.

My main concern is that at that width, the top of the carcasses may sag under its own weight. So I was wondering if anyone had any recommendations on how I could go about preventing that?

Any advice from those with more experience would be greatly appreciated, thanks!

ChaiLatte · Monday at 10:30

How much space is there between the sagger and the ceiling?

You could install a stiffener there - wood on edge, steel on edge (in a routed groove secured using a gripfill-like adhesive), maybe a piece of 20x 20 or 25x 25 steel angle screwed on, maybe a piece of unistrut (41 x 21).

Locate the rafters and drive a 6" nail in from the underside. Only go in about half way and then bend it over flat. Call it a feature.

leemkule · Monday at 10:52

Thanks for your reply. I have about 100mm above the carcass to the wall.

I like the idea of the steel edge secured in a groove.

For the sake of clarity, do you mean something like this? Or do you mean the other way round where the steel would be laid horizontally in a shallower groove?

ChaiLatte · Monday at 11:30

If you have 100mm, I would not bother with the groove. That was a fallback measure in case headroom was really tight. With 100mm, wood will be fine - you can look up 'torsion box' to see the structural principle.

But yes, if used, the steel would be on edge as it is stiffer that way. Groove absolute maximum of 5mm deep in an 18mm panel, just to stop the steel flapping about. Think about trying to bend a plastic ruler; one way is easy, the other way is hard.

An advanced and probably unnecessary step with a stiffener is to pre-camber it upwards 2-3mm so on the day of install the roof of the cupboard is bowed upwards. As time progresses, it is never going to rise further, but may droop a little.

leemkule · Monday at 11:43

That's great thanks. I have seen people use the torsion box method but I had perhaps mistakenly thought that that was for use under the bottom of something, say a table, rather than on top. I had perhaps wrongly assumed that the benefit of adding it on top might be reduced because you're adding more weight? What thickness of wood would you use on my 100mm of top?

I am leaning towards the steel idea as routing a 5mm groove should be fairly easy and I'd like to keep as much space up top for ventilation as possible.

Thanks for all your help.