Getting plastered on the white powdery stuff.
December 31, 2009 9:51 PM   Subscribe

Can someone explain the art of "back blocking" plasterboard (gypsum/gyprock/drywall/et cetera) - bonus points for a youtube vid that isn't filmed by someone not shot by someone who is actively masturbating.

I am setting the plaster in my house - yes, I should get a professional, but I can't afford one - I am aware of the above procedure, but only as terminology (my google-fu has failed me). I can imagine what it means, but this doesn't augur well for DIY activities of this nature. Thanks!

*I am about to hang the ceiling (which is where the back blocking of the butt joints is required (I think).
**I have put metal channels in to level the ceiling joists
***I am using 13mm/half inch thick plasterboard (another story)
****I have never done this before (but have necessary tools and wit)
posted by a non e mouse to Home & Garden (10 answers total) 1 user marked this as a favorite
It's essentially putting something (usually an overlapping piece of plasterboard) behind the joint where two sheets butt together. This stops the two sheets from moving relative to each other, and subsequent cracking of the paint / plaster covering the joint.

This page has a bit of info - no video, but a description & fairly clear piccy. It's not hard…
posted by Pinback at 10:26 PM on December 31, 2009

Best answer: And a nicely explanatory .pdf from CSR.
posted by Pinback at 10:49 PM on December 31, 2009

Response by poster: I saw that, and it illustrates the sheer lack of technique that I'm searching for - some of the missing info that I need:

At what point is it done?

Do I put up the second sheet and then climb into the ceiling and stick a piece of offcut plaster on it? And so on and so forth?

Or do I do it once the ceiling is complete?

What if there is no clearance between the ceiling and the roof? Do I somehow do it prior to putting up the sheet?

Should I place the back block on the sheet I've just installed, let it set and then paint cornice cement on the back block's underside and put up the next sheet?
posted by a non e mouse at 10:50 PM on December 31, 2009

Response by poster: Pinback - my response was to your previous response - this pdf is good.

Any other tips/links/vids would be appreciated.
posted by a non e mouse at 10:58 PM on December 31, 2009

Yeah, figured that ;-). Where the roofspace is inaccessible, I've seen plasterers use basically the methods described for back-blocking wall sheets as shown in the .pdf - either backstopping the back-blocking with timber/plasterboard blocks on the joists, or pulling the back-blocking down to the ceiling sheets with temporary screws.

I see you're in Aus, so I make no comment as to how acceptable that method is under AS2589.1 - except to note that the back-blocking sections of the standard (which ain't free - check your local library, or drop me an email at my profile address) is primarily concerned with finish quality, not structural. The only info which doesn't seem to be in the .pdf is that back-blocking should be minimum 400mm wide for butt-joints, 200mm wide for recessed joints.
posted by Pinback at 11:21 PM on December 31, 2009

It seldom should be necessary to use back blocking. Back blocking is only necessary when you have a floating joint that does not occur on a stud or joist. If you install your drywall so that you don't have floating end joints, you don't need to use backing. You can get drywall up to 12 feet long to span the size of most rooms. For larger rooms use two sheets of 12 foot drywall and make the joint on a stud or joist.

So the simple answer to your question is to install your drywall joints in such a way that backing is not required. That's the way the pros do it.
posted by JackFlash at 10:07 AM on January 1, 2010 [1 favorite]

Response by poster: Ah - that's the situation - no back blocking then, terrific, thanks Jackflash.

I have just noticed another problem this morning, however - the tape on one of my longer walls has bubbled all along (practically) the whole join. The plaster must have dried too much while I was struggling to bed it in (6.2 metres).

I understand that if there are few isolated bubbles, you can just cut them out with a stanley knife and fill out the gap, but do I do when it's the whole tape?
posted by a non e mouse at 1:44 PM on January 1, 2010

Response by poster: Let me try the last line again...

I understand that if there are a few isolated bubbles, you can just cut them out with a stanley knife and fill out the gap, but what do I do when it's the whole tape?
posted by a non e mouse at 1:46 PM on January 1, 2010

You may have to rip off the tape, scrape off the mud and try again. Taping is a learned skill. The pros use a device that looks like a bazooka that automatically dispenses the tape and applies the mud in one pass.

For do-it-yourselfers, an easier method is to use plastic mesh drywall tape instead of paper tape. The mesh tape has a mild adhesive so that you just stick it to the joint like duct tape. It is easy to reposition the tape until you get it just right with no wrinkles. Then you apply the mud over the top of it and the mud squeezes through the mesh. The plastic mesh tape is slightly more expensive than paper tape but worth the cost for an unskilled person.
posted by JackFlash at 2:50 PM on January 1, 2010

Response by poster: Hmm.

I've read that it's prone to cracking, but you're the umpteenth person to suggest this - as people have stared into my eyes and realised that I don't have the dark soul of a plasterer, this seems to be their consensus.

Plastering gods be damned!
posted by a non e mouse at 1:40 AM on January 2, 2010

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