MortarThePoint

Ply sheathing on flat roof (35mm trusses)

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I'm installing 25mm SE plywood sheets on a small area of flat roof approximately 3.5m x 1.75m in size. I'm using 47 x 50-0 firring strips under the ply to create a gentle fall. The trusses are only 35mm timber. I've laid the sheet perpendicular to the trusses. I'll be screwing the plywood down with 5x100mm screws (coungtersunk) so that the screws always pass through the firring into the truss. The end of the sheet doesn't fall on a truss so I believe I have two options:

  1. Cut it back to the centre of the truss and have the join between sheets there. I'd be happy with that if the truss timber was wider, but having a join 35mm timber feels a bit bad
  2. Add some noggins to support where the boards join. I'd make this out of 4x2 so am happy there would be room for the join.

I'm inclined to go with option 2. Which of A or B would you go for? The yellow trusses are 600mm c/c.

image.png.07656e5983359d287b4303dc8e0a36dc.png

Also, should I be adding noggins between the trusses along the long edges of the plywood at the perimeter of the flat roof?

 

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I think the boards, whereever their edges are need to be supported all along.

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8 hours ago, Marvin said:

I think the boards, whereever their edges are need to be supported all along.

 

It's going to be difficult as there are gang nails on the trusses at this point and the truss timber is only 72x35 on both the top chord and rafter.

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Posted (edited)

if you think truss is too narrow to have sheet joins on them 

--then add some width too those trusses by nailing a strip the side of  them?

 

Edited by scottishjohn

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I am not a fan of always joining boards on rafters/joists,(even plasterboard) I would instal noggins at 400mm centres and lap some off cuts of ply between the noggins across the ply joins underneath, glue it as well if you want.

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Would this design help to reduce the work? However all edges will need supporting. 

 

If I really couldn't make it work I would put 2 layers on and bond them together, but this may cause other problems.... 

20210501_081833.thumb.jpg.6c17fbd29cbb0e68a0bf7c2e2b9ac6f9.jpg

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It does not matter where the joins are as long as they are supported, trying to cut sheets accurately to fit joists is very time consuming, bung in noggins and off cuts and it will be fine, you can overthink it!!!

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If your joists are on 600 or 400 cut the sheets down to 2400 or get the merchant to before deliver. All short joints to be supported on joists. Long joins ideally t and g. Think you're ok without joins or noggins if 18mm on 400s or 22mm on 600s. I nogged my garage roof for the long joins only needed a few rows, set the joists on 16" imperial as was easier. 

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your strips you intend to fit to get a run on the roof should be placed UNDER the edge of the roof beams on one side  like a wall head plate  -that way it will be a simple slope from one side to other .

 

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I've circled here where it's going to be a pain to install noggins due to the gang nails. On the other end of the top chord there is some space. 6 noggins to install along that edge. I think the two timber sections shown at either end of the top chord in the drawing are diagonal bracing that obviously can't (and doesn't need to) be there when the sheathing is on.

 

image.png.f720da27d0e40822035471139dcca9a2.png

image.png.7a286dd21d38f0a33820072af900d6d8.png

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2 hours ago, scottishjohn said:

your strips you intend to fit to get a run on the roof should be placed UNDER the edge of the roof beams on one side  like a wall head plate  -that way it will be a simple slope from one side to other .

 

 

I've fitted firring strips like this (exaggerated obviously :-):

image.png.6f2d0309ea63959f7015479f34ddb08b.png

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Posted (edited)
2 hours ago, joe90 said:

I am not a fan of always joining boards on rafters/joists,(even plasterboard) I would instal noggins at 400mm centres and lap some off cuts of ply between the noggins across the ply joins underneath, glue it as well if you want.

 

I like this approach, but wish there was more meat to the top chord as putting 4no. 5mm screws through it doesn't feel kind. (4no. at the ends of the top chord, 2no. in other areas). It's only 35x72 timber.

 

I'm sure I'm probably over thinking it though. The red lines below show where noggins need to be added.

 

image.png.8e6364f74c0cd31d41fdb8dd5fcd09ce.png

Edited by MortarThePoint

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1 hour ago, MortarThePoint said:

It's only 35x72 timber.

Yes, just make sure your screws are in the middle (pencil line to follow👍). That’s why I don’t like joining sheets on a joist that narrow. Those noggins will be fine, 

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Posted (edited)
8 minutes ago, joe90 said:

Yes, just make sure your screws are in the middle (pencil line to follow👍). That’s why I don’t like joining sheets on a joist that narrow. Those noggins will be fine, 

 

Still have to work out how to cope with the gang nails. I guess I'll jus put the noggins as close to the edge as possible but not on the gang nails.

Edited by MortarThePoint
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7 minutes ago, joe90 said:

Yes, just make sure your screws are in the middle (pencil line to follow👍). That’s why I don’t like joining sheets on a joist that narrow. Those noggins will be fine, 

Yes designed so thin that you can't propperly fix to them without risking splitting and or undermining the structural integrity is a non holistic approach.

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Posted (edited)
3 minutes ago, Marvin said:

Yes designed so thin that you can't propperly fix to them without risking splitting

I tend to use longer but thinner screws to reduce the splitting risk. Not ideal but if you have to join sheets on a joist this thin they must be put in at an angle.

Edited by joe90

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1 minute ago, Marvin said:

Yes designed so thin that you can't propperly fix to them without risking splitting and or undermining the structural integrity is a non holistic approach.

 

Yes that's my beef really. 25mm plywood will be plenty strong enough without edge noggins, but the wisdom is to have the edge noggins so...

 

The sheet join noggins is a different matter.

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1 minute ago, MortarThePoint said:

25mm plywood will be plenty strong enough without edge noggins,

Then why not screw strengtheners to the edge of the ply at the edge (noggin slightly short of joists/gangnails) this will stop any flexing, 👍

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1 minute ago, joe90 said:

Then why not screw strengtheners to the edge of the ply at the edge (noggin slightly short of joists/gangnails) this will stop any flexing, 👍

 

I like that idea. It's logical, but would it satisfy BCO etc?

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1 minute ago, MortarThePoint said:

but would it satisfy BCO etc?

Can you honestly tell me a BCO is going to count the nails you used ??? I have a reputation for over engineering stuff and I would be very satisfied with that fix, in fact I have done it often, does not need a structural engineer to work it out. Make the “noggin a tight fit between gang nails and fit after the roofing sheet is fitted, 4 off screws along its length down through the ply, job done.

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Well it's looking good and sturdy, but only 2/3 done as the latest batch of plywood is 2mm thicker than batch I got last year. Crazy!

 

I started off predrilling and countersinking, but gave up on that whilst doing the first sheet as it worked well without. The second sheet though needed the countersinking as it created pushed up splinters without doing so. I wasn't worried about lack of predrilling not pulling the sheets down as I was using a screw with partial thread. 4.0 x 70mm Screw-Tite, an excellent screw IMHO. Where the firring is thicker, I'll be using a 5.0 x 100mm screw. No glue, though I would for a floor.

 

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The scaffold is about 400mm above the flat roof and at 11:30 last night I was putting in the last couple of screws when one jammed breaking by screwdriver bit. Couldn't get the screw out so had to grind it off, which I did this morning.

 

Felt pretty cramped up there in the dark with only the light of my drill. Got the job 99% done though 🤣

 

image.png.f6aef715dacacb366a7bec0620ed9f23.png

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