How to stop MDF sag?

paperclap

Hi all,

I’m starting to build some wardrobes in our bedroom. 

It’ll comprise of three carcasses made up of 18mm MR MDF. Bottom, left, right and top. There won’t be a backer board. The panels will be scribed to the wall. Each carcass will be 440mm deep by 1060mm wide.

The whole thing will sit on a plinth scribed to the floor. The outer sides will be offset from the wall by ~60mm to allow for door swing (filler strips scribed to the wall). To aesthetically balance the entire thing, the top of the carcasses will be offset from the ceiling by ~60mm (with another filler strip, scribed to the ceiling).

But I digress. My concern is that, even while no weight will be on it, or be pulling on it, the top panel of the carcass (440mm by 1060mm) will sag.

I’ve had a few thoughts (some will work, some won’t).

Thought of some sort of metal banding over the outside edge of the top panel of the MDF. But, the doors will be overlaid… so won’t work. Don’t want to stare at metal banding!

Thought about a real wood (or maybe even MDF) across the front of the top panel (within the carcass). Could potentially be a way to conceal some nice lighting, if I take it that far. But unless I add another batten of sorts along the back, the back may continue to sag.

Thought about screwing the top panel to the ceiling (either with plasterboard plugs, or, adding noggins between the ceiling joists and screwing into that (we’re in a bungalow, so accessible)).

Any thoughts?

chris_n

There is nothing to stop you fitting as many MDF braces as you want across the top, if your scribed front is 60mm then one in the middle and one at the back can be 55mm. You could brace the middle one with steel angle or flat bar if you really wanted. 

paperclap

Thanks, but (visible) braces are pretty ugly! So I’m trying to find the most attractive / most concealed way of doing it

ThisIsWeird

The top panel will be ~60mm from the ceiling? So fit 60mm spacers between the ceiling and the top of the panel, in the centre of its run, and screw it up. An 18mm thick panel around 1m long won't sag if secured to the ceiling in its middle.
You'll need one at the back, too, unless the top panel will be bonded to the back wall - you could run a neat quadrant bead along the back on the wall, which will support the panel there and prevent sagging.
Or, this scribed filler panel along the top - why not use that? Once it's all cut and scribed, mark on the ceiling where it runs, and drill a series of 3mm holes (4 or 5?) up through the ceiling in the centre of where the 18mm thickness will run (clearly checking there's no wires and stuff up there). From above, it should be enough to PVA a timber or MDF batten to the top of the p'board ceiling, nicely snug between joists. Then screws down from above, through the battens, and into the filler strip. The filler will need pilot holes if it isn't to split. Before doing up, add PVA to the filler's top edge to glue it to the ceiling. 
The 60mm thickness, coupled with the PVA and the screws, will mean that filler ain't going to flex a mm. So neither will the top panel that'll then be screwed to it.

So, that's all doable. Are you going to fit an MDF shelf? Not that's the saggy problem

chris_n

paperclap said:

Thanks, but (visible) braces are pretty ugly! So I’m trying to find the most attractive / most concealed way of doing it

I was talking about havthe braces above the top board same as your scribed infill panel so not visible 

travis-powers

I’ve just put this through the https://woodbin.com/calcs/sagulator/ and it’s coming up with acceptable.
 Also after you have fitted the infill this will also add strength.

ThisIsWeird

Akshully, since you are fitting a scribed top filler panel in any case, the easiest way to use it as additional strengthening for the top panel would likely be:

Mark where the filler panel will run along the ceiling. Mark inwards the filler strip thickness - 18mm? Glue and screw a timber batten to the ceiling, catching all the joists - 1"x1" will do.

The filler strip gets screwed and glued to this batten - or just glued (panel pins optional) if you don't want screw heads to fill, and also glued (all PVA) to the actual ceiling - prevent movement and cracks. Then glue and screw up through the top panel, 9mm in from the front lip, into the filler strip.

Jobbie almost certainly jobbed.

Mutton_Geoff

Since you have a 60mm gap between the top of the wardrobe and the ceiling (I assume that's what you mean by "offset"), the I would run two spans of something stable across the top left to right set back 100mm from the front and another 100mm from the back. Two pieces of a pair of 60mm by 1060mm of your 18mm mdf glued together as battens for the span. Glue and screw the top panel to these battens. Maybe scribe them to the ceiling but don't fix them to the ceiling, any movement there may twist the wardrobe and muck up the doors closing etc.

The new upstand set back on the top of the wardrobe will hardly be visible and even if you stood on the bed to look, would be presentable rather than a void going all the way back to the wall (impossible to clean).

Make up the top sections with the twin mdf cross braces glued and screwed, prime and paint it all before it goes up. Bear in mind screwing into the edge of mdf isn't a good enough joint on it's own, but the (thin screws) will hold it all together while the glue dries.

paperclap

Hi all,

I’m building some built-in units in our bedroom. 3 sets of 1060mm wide carcasses, each with double doors.

Inside, I’d planned to fit some shelves. The shelves will be ~1020mm wide by ~400mm deep.

I know an 18mm MDF shelf over this span will sag.

Would lipping the front and back of each shelf prevent sagging? If so, is there a go-to type of wood for this sort of thing?

Or, is 18mm MDF out of the question altogether? Perhaps 18mm plywood? Ply is damn expensive though!

Any thoughts?

Thanks!

ThisIsWeird

The back of the shelf is easily supportable against the back of the unit or wall - just fix a decorative quadrant bead along there.
Can't think of any way to support the front other that adding strong enough support section running either in front of the panel lip a 'lip', or under the front edge, set back ~50mm to make it less obvious. I guess that would need to be box-section steel or similar. Or to add a central partition to the upper storage section to anchor the shelf to the cupboard roof, and hence the ceiling.
I can't see a timber 'lip' doing much, unless it's hard, inflexible, timber, and reasonably sizeable.

Oldernowiser

Twin Slot Shelving system on back wall? very flexible design, can be taken down and re-used for ever (very MSE !).
I have reused many sections from our prev. home - these are over 20 years old now, and still as new. 
In our walk-in wardrobes i have fixed the verticals so spaced to accommodate clear/translucent storage boxes between the spurs, and shelf height as appropriate. 
ThisIsWeird's suggestions will i think look a lot neater - box-section particularly so,  i have built box-section storage in our cellar, takes a lot of weight!