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When using furring strips on top of the roof joists to provide a slight slope for a flat roof, my current assumption is that you need 2 sets of noggins, one for the roof OSB and another for the ceiling plasterboard.

This to make sure that the OSB and plasterboard edges have support. Is my assumption correct?

Here is a rough sketch of what I mean

illustration of noggins between rafters

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  • How far apart are the joists/rafters with the tapered piece on top? If they are close enough, you will not need any extra blocking. – Jack Jun 19 at 5:20
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@muttonup, to specifically answer your question: Yes; where both edges need support, it is best practice to provide blocking in plane with both upper and lower surfaces.

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The rest is to expand a little bit:

Yes, that works to provide backing for the two planes. Although, doing the same thing in a different way is better for preventing differential movement between planes as well as being typical in the trades; you're going to get more stiffening out of an oversized piece of lumber beveled at the correct angle on the top edge so you have full height blocking for both planes.

Keep in mind that blocking, in the context of load bearing, is typically not necessary when the span between joists does not exceed that required for meeting the needed load rating for the sheet good.

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Can I ask what drove you to decide to add blocking? Are you working off plans? Generally for new work rather than renovations, blocking for stiffening would be an unnecessary expense unless you're over-spanning your materials, and one would hope not to mess around with that unless you're working off engineered plans...?

Some notes to chew on:

  • You wouldn't use H-Clips for a flat or low slope roof that has engineered plans calling for solid blocking. H-clips will transfer some of the deflection from one sheet to another, but it will still allow for cavities to form with a great enough load difference between sheets.
  • If you would still need to block a tongue and groove sheet to meet the right load rating, you should just by conventional plywood and save the money. If T&G does meet the load requirement without blocking, you should save yourself time and skip the blocking, going with the T&G subfloor (after all a low slope roof is a deck, just one for the birds) to save the money on the blocking and skip a step. There is a learning curve to installing T&G.
  • whatever route you take, make sure your fasteners are long enough to penetrate through the sheet good, through the sleepers, and bite the required depth into the joist itself.
  • In the event that you are working off plans, a common issue for laypeople is to find that you are misinterpreting those plans. Where it calls for blocking, or noggins if that's what you are hard on calling them, the blocking may just be for stiffing the joists, and not extend up to the plane of the top of the sleepers. Take a look at the picture under the header "Step 3" in this article.

Good luck!

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  • Thanks. In answer to your question, I am using spanning tables with 400mm gaps, with 18mm OSB, so as you say I probably don't need to use Blocking at the top, just at the bottom for the Plasterboard. Just wanted to sure. – muttonUp Jun 21 at 12:38
  • Sounds like you know what you're doing! Happy framing! – The Ghost of Jon Jun 21 at 21:42
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Noggins are not meant to be used as edge backing. Its intended use is for bracing and stiffening joists and studs. If anything you will lose strength when improperly placed.

I normally use H-clips and/or backer clips for OSB or drywall as needed.

H-clip a 1" square flat piece of metal with tabs bent into an H shape designed to hold the edges of plywood and drywall sheets together

H-clip installed between two sheets of OSB

Backer clip

T-shaped clip used for adding nailing support for drywall

installation example of backer clip

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    How is adding lumber (however misguided) going to reduce strength? – isherwood Jun 21 at 12:55
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T&G OSB does not require edge blocking. Drywall always needs edge blocking everywhere.

9.23.15.3. Edge Support

  1. Where the edges of panel-type subflooring are required to be supported (see Sentence 9.30.2.1.(2)), such support shall consist of tongue-and-groove panel edges [or!] not less than 38 mm by 38 mm [1-1/2" by 1-1/2"] blocking securely nailed between framing members.

bcpublications.ca

I thought you were asking if it needs cross-bracing, which I've no idea: how tall can a furring strip be before it itself needs cross-bracing? The sheathing holds the top, so that's why they're more concerned with strapping or cross-bracing along the bottom. But if it's going to end up with an entire 2x12 sticking up at one end, it should probably be blocked cross-braced and tie-plated.

Given the small size of the cross-bracing they're talking about, you're probably supposed to treat the whole thing as its own member and fill the entire space with actual crosses; any blocking made from available lumber would be too short; I'm not sure if it's acceptable to stack blocks.


What I'd really be looking at is if the roofing manufacturer expects it to be blocked regardless of the type of sheathing. But not really, because I'd be using Class 3, 3/4" T&G OSB, 16" OC.

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