Alcove shelving material thickness (and sagulator)

12 Apr 2013
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United Kingdom
Dear Forum members,

I would appreciate your advice and feedback for the following shelving project of mine.

My question is - Will 15 mm MFC or 18 mm MDF be thick enough for an alcove shelf spanning 92 cm and 31 cm deep and supported on 3 sides with battens? Full details below.

This weekend I intend to put shelving in the alcoves of the living room. The shelves will be used mostly for books.

The length of the alcove (and each shelf) is 92 cm and the depth/width is 31 cm. The mechanism of attaching to the side and back walls (all walls are brick) will be to use wooden battens - 2 battens on the sides and 1 batten at the rear.

The shelves will be white to begin with or will be painted white so the actual grain, texture and look of the sheet material matter less.

My preferred material is Contiboard/MFC chipboard and MDF in that order. I prefer MFC over MDF simply because I can get the white coloured MFC and not have to paint it. The costs for these 2 materials are similar enough for it not to be a criterion.

The most common thickness for Contiboard is 15 mm. When I put the relevant values into the sagulator it computes the sag as "EXCESSIVE" (see image below).

If I were to go with MDF 18 mm seems to be a good thickness. Higher thicknesses are either not available or not available in convenient sheet sizes. For 18 mm MDF sagulator computes the sag as "BORDERLINE" (see image below).

My question is that from what little I know of DIY it seems that these thicknesses should be adequate for non-sagging shelves. Is the sagulator showing what it is showing because it assumes that the shelves will be supported/attached only on the 2 sides but not at the back? (for e.g. if you were to use shelf brackets).

Do you think that 15 mm MFC or 18 mm MDF is okay for a shelf spanning 92 cm and 31 cm deep?

Thank you.

Sagulator - MFC 15 mm

Sagulator - MDF 18 mm

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Both will sag without support baton to front edge, even that may not be enough for mfc, depends how heavy the books.
scaffold boards old and new will be up to the job

you could introduce a center partition to support the shelves this will also help support the books to avoid leaning

you would still need a supporting front edge
or you could use 20mm timber [pine] again with a center support but no edging
foxhole, big-all - many thanks for your inputs.

I have abandoned the idea of using MFC 15 mm or MDF 18 mm because it will very likely have excessive sag without additional support battens. I am keen to avoid any additional supporting battens either at the front or at any other place.

I am now looking at MDF 25 mm whose sagging, as per the sagulator, will be within acceptable limits.

MDF 25 mm is more difficult to acquire for an amateur like me but I want to do it once and do it right. I may have found a solution at the nearby Selco branch as they also do cutting (first 4 cuts) for free so will have to get a humongous sized MDF board and cut it into smaller pieces.

Thank you.
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I always check their [Selco] damaged board area if I don't need full boards often a cheap solution.
25mm will be heavy!

Why don't you do as big-all suggested and use a center (or off-set) partition. I'd personally do away with battens as they will look ugly, as well as taking away internal height of each shelf.

This is what I did for my AV unit. All joinery was done with glued dowels:

It actually sits in the fireplace, so the top panel is screwed (countersunk) up.
I made alcove shelves for the property below in 1985, the first house purchase we made. I was more than a little surprised that the shelves are still in use, and can be seen in the property blurb slideshow #2

The shelves don't appear to be sagging yet!

I used 2440X1220X18mm dense chipboard, edges well sanded to half round profile, primed and with several undercoats, battened on all 3 sides with 18x28 planed whitewood fixed with countersunk and caulked screws and wallplugs, and screwed down through from the shelf boards at about a 30 deg angle, with "L" shaped cutouts for shelves, rounded at the corner. I cut rectangles the length of the shelf (~1000mm) plus the width of the shelf( ~240mm) by two shelf widths (~480mm), then cut out the 2 "L" shapes from that rectangle.

The "L" shape helps prevent sagging. Vertical long strips of the board, about 70mm wide were screwed into the fronts of battens and the edge of the arches to tie the whole together and hide batten ends. All the straight board cuts were made by handsaw - I could not afford many power tools at the time.

I must have had the boards delivered, I cannot imagine carrying them on the roof rack of our Nissan Cherry at the time!

The arches were marked out by drawing an ellipse in pencil on the board with string and 2 panel pins, and the elliptical cut-out was turned into a coffee table, which I still have - black and green marbled finish, lacquered with lots of yacht varnish. I must have used a jigsaw to make the cut - it was probably a jigsaw attachment on a Stanley drill in those days!

I honestly cannot remember fitting the cupboards beneath, but my wife tells me I did! The shelf supporting the TV in the corner and the corresponding shelf on the other side of the chimney breast were also made in a similar way, but with an angled cut as in the approximate cutting plan shown below. There was a front batten and a cross batten for additional rigidity.


I think that I used a 3rd board to make the cupboard fronts and doors, and possibly "L" shaped shelves within.

The only thing I would do differently today, and only because I have a better jigsaw, is to curve the internal angle of the "L" shaped shelf too.

At 1985 prices, the materials for shelving probably all cost less than £40. I think we had the flat for about 18 months, purchasing at £24K and selling for £44K.
Idoodle - introducing a center partition is a useful tactic but probably not suitable for what I have in mind. The room where these shelves will go will be a reading room / library (at least in our heads... in reality it is merely one half of our living room which originally was 2 separate smaller rooms typical in Victorian house). Because it will be our reading room we want the focus to be clean, neat shelves full of books (we have hundreds, maybe close to a thousand).

Flyboytim - That looks very nicely done! Thanks for all the detailed inputs. That is a lot of new information for me and I will digest it in the evening.

After reading the feedback on this thread I have decided to push back the project by a few days to give me time to research and determine the best materials and approach.

Thank you everybody! I will keep you posted on how the project progresses.
Cant you stick two sheets together, so doubling the thickness?

MDF is horrible to work with, in my experience, as its tough to get a smooth finish on a worked edge (such as a saw or even router cut)

What's wrong with ply?
Cant you stick two sheets together, so doubling the thickness?

I've done that with MDF to create long (~5m) floating shelves, using steel rods epoxied into the wall, and grooves in the lower MDF sheet to accommodate the rods, and a lot of countersunk screws to clamp the sheets together.

MDF is horrible to work with, in my experience, as its tough to get a smooth finish on a worked edge (such as a saw or even router cut)

It planes up ok though, and takes nice chamfered edges, and paints over with oil alkyd paints if well primed and undercoated, and sanded down with a fine grit.

Nothing wrong with ply, especially bamboo ply, but it's expensive.
What's wrong with ply?

When I did my AV unit I flipped between Ply (which was my first choice) and MDF.

The Ply at the sheds was horrible - peeling top layer, broken cores etc. etc. And was £12 per sheet more.

I don't have any timer yards near me who have prices on their website. And I'd feel a bit of a prat driving to one to find their Ply is overly expensive!

MDF needs more 'work', but using the right primer is key.
Jumping on the end of this thread.
I spent last weekend building an alcove unit in 15mm mdf. I realise the shelves will sag too much and I'm going to replace them this weekend with much thicker stuff but my question is... Is 15mm mdf thick enough for the sides of the unit, which will have about 4 shelves each a metre wide and with plenty of books on top?

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