OSB sheathing, and the need for double stud at wall head.

[saveasteading]

1. Recommendations for OSB thicknesses please.

I have designed a section of rebuild in timber, using the Scottish document for it.

 

For the sheathing of the walls and roof they require minimum 9mm OSB class3.

That doesn't seem a lot for what is holding up the building.

 

But the cost saving compared to say 18mm is significant.

(if I can save 25% on every element of construction, then it becomes 25% of a lot.)

 

What do the experienced roofers and Engineers (and clients) among you think? 

 

Easy to say 18mm when specifying and someone else is paying.

 

Walls are 140 x 50 stud, timber clad.

Roof raised tie trusses 220 x 50 rafters, and a metal clad surface.  (The design guide assumes concrete tiles)

 

2. Between you , in other posts, you have recommended using double stud at foot and head of the new timber walls. The design guide doesn't mention that as a necessity. In most sketches it shows single  and in one sketch it shows double. If the roof trusses align with the wall struts, I cant see why double helps. It does of course stiffen the whole building and provide a substantial longitudinal tie, but that should not be necessary.

And there will be treble C24 timbers at lintels, which will pick up intermediate trusses.

 

 

Comments on the practicalities as well as design theory will be welcomed.

 

 

 

 

[markc]

I would never use 9mm OSB on walls or roof, it’s just too flimsy and drum like, no need for 18 as that’s overkilll and too much hard work.

you don’t need to double headers and footers unless specified but it does make the install a lot easier especially when working alone … put a single footer and header in first, make the wall to suit and then rock it up into place and align, nail through! Much easier than singles.

Edited October 28, 2021 by markc

[ProDave]

I generally see 13mm OSB used as sheathing.

 

Use double headers and footers.  the way it normally works is you fix a single footer the the wall plate or whatever is sits one.  You then build the stud walls in sections with single header and footer to each wall section.  Then you fix a second header across the top with the joints staggered to tie it all together.

[saveasteading]

12 minutes ago, ProDave said:

and footer to each wall section. 

 

So this is for ease of construction. Let me paraphrase to check my understanding.

 

Fix bottom plate to foundation masonry. Make a section of wall on the ground, including top and bottom timbers  (as if in a factory though this is to be stick-built) and lift panel into place. Nail down and prop in place. When enough panels in place  fix continuous header over the top of them all.

 

Hence 13mm osb reduces the weight  for the heave into place. Strength of wall increased  by extra timber top and bottom.

The timber price this way has gone up by £150, but the OSB down by £300 (cf 18mm) and the joiner will surely be very happy, so reduce his quote (or not increase it.)

 

and do you think 13mm on the roof too?

 

I can't see osb3 at 13mm. 11 or 12 depending on source.

 

What size of panel is normal, assuming 2 workers, or if 3?

 

 

[ProDave]

You can probably see the Norboard factory or pass it every time you go to Inverness, you would think it would be cheaper here.

 

I wouldn't want to do more than 2.4M wide sections or it will be hard to lift.

 

Beware, OSB is available in metric or imperial sheet sizes.  You really want metric or your stud spacings at 2ft won't match the plasterboard.  God knows why they still make it imperial sizes.

[Gordo]

9mm osb is plenty adequate for structural stud walling. It is only to provide racking resistance ie diaphragm/bracing. Many people recommend ply which offered a little more moisture resistance. TBH I wouldn’t bother

[saveasteading]

33 minutes ago, ProDave said:

God knows why they still make it imperial sizes.

For export to US? Because some joiners still work that way, then have to find imperial pbd too?

But I agree that it should only be sold in multiples of 600 except by request.  Not much fun cutting 20mm off every board.

 

Yes the Sterlingboard factory is visible from a long way away. There is a constant plume of.....is it smoke and steam mixed? As it dissipates fairly quickly I like to think it is largely steam. Presumably they use their own timber waste for heating.

They say that pilots like it being so close to the airport as they can see what the  wind is doing.  Have had an aborted landing there recently.

 

It is said that a lot of the timber is coming in from Norway, and most of the product is going to US and China. The new owner of Norbord is a Canadian company.

It would be nice to drive in there with a truck and get a few pallets at factory prices.

 

World market cooling, but I wonder if they will reduce production and maintain demand.

 

The main reason I asked about thickness was that I see several old threads where people swear by 18mm. Perhaps when it was much cheaper it was a consideration, and allows a nice nail bite for battens.

[Gordo]

9mm osb is plenty adequate for structural stud walling. It is only to provide racking resistance ie diaphragm/bracing. Many people recommend ply which offered a little more moisture resistance. TBH I wouldn’t bother

[ProDave]

7 minutes ago, saveasteading said:

Yes the Sterlingboard factory is visible from a long way away. There is a constant plume of.....is it smoke and steam mixed? As it dissipates fairly quickly I like to think it is largely steam. Presumably they use their own timber waste for heating.

They say that pilots like it being so close to the airport as they can see what the  wind is doing.  Have had an aborted landing there recently.

It's also a useful giant wind indicator for sailors in the Moray Firth.

[saveasteading]

52 minutes ago, ProDave said:

wouldn't want to do more than 2.4M wide sections or it will be hard to lift.

Not much chance of using the wrong centres with this method. I think I would set up a jig on the floor to avoid constant measuring.

 

I've watched the factory process and it isn't sophisticated, other than  computer marking the timbers for cutting and positions. Then it is men with nail guns, and man-handling.

Factory panels have a stud at both ends. Presumably site build omits the leading edge stud to save wood and maintain the centres.

[saveasteading]

1 minute ago, ProDave said:

sailors in the Moray Firth.

Proper sailing. I don't fancy it much.

[Gordo]

8 minutes ago, saveasteading said:

 The main reason I asked about thickness was that I see several old threads where people swear by 18mm. Perhaps when it was much cheaper it was a consideration, and allows a nice nail bite for battens.

Good point. If using 9mm osb best use a large head nail at 150mm max crs. If using a nail gun I’d suggest must closer crs best.

[saveasteading]

It is written. My circumstances require 100cc.

 

3.1 mm x 50 mm wire nails at 100 or 150 mm centres as calculated

 

That makes 5000 nails in the external walls alone, so a nail gun is rather important.

Edited October 28, 2021 by saveasteading

[saveasteading]

Sterlingboard class 3 is available in 9, 11, 15 and 18.

 

For now I am going to specify 9mm as the guidance, but allow 'greater thicknesses as desired and available'.

 

 

[DragsterDriver]

I’m currently buying 12mm ply cheaper than 11mm osb. Strange times!

 

I use 9 or 11mm osb usually- if stick building on site I tend to build the wall-brace it- then board it once upright. 

[SteamyTea]

3 hours ago, saveasteading said:

As it dissipates fairly quickly I like to think it is largely steam

And CO₂.

It is almost certainly the deaerator for the boiler feed water.

 

Got a coupe of threads going on about mould on plywood.

Edited October 28, 2021 by SteamyTea

[markocosic]

Use 18 mm for making structural box beams?

 

9/10/11/12 mm is PLENTY for sheathing to give shear resistance. It's also walkable on 600 centres for the purposes of adding insulation / putting on a roof etch as long as you're not a particularly fat bastard or jumping up and down etc. 

 

For headers...I would do this next time:

https://bygghouse.com/wp-content/uploads/2017/02/Summary-of-Swedish-Wall-Element-Construction.pdf

http://blog.lamidesign.com/p/swedish-platform-framing-info.html

 

 

 

(single header, bring the wall studs all the way up to the level with the floor, then put your "second header" vertically against the inside of the wall to carry the floor load - after you've lapped your airtightness membrane over the top of the header ready to sandwich it under the floorboard.

 

Tape the bottom piece of airtightness membrane down onto the floorboards before you build the next level of wall on top. It'll then be squashed down by the sol plate of the floor above.

 

Roof trusses can be treated the same way. Those little diamond tiedowns pick up the roof truss at the top, then the header on the two side ears, and (if aligned) the wall stud on the bottom ear...but from the outside not from the inside. so that there's nothing to interrupt your airtightness membrane...that you remembered to lap over the top of the header, taped to the existing wall membrane, before you sat the trusses on top of it. Use some little tiling battens to squash the taped joint flat for extra security if you like.

 

 

[markocosic]

Would also build frame without OSB. Just a diagonal on the opposite side. Add OSB afterwards. Minimises weight to lift and you can square up the the house this way then nail it all before it moves. You can also let it get wet (the solid timber) if you must before having a couple of dry days going crazy with the OSB and getting it covered up before it turns to weetabix?

 

Walls were built in max 6 x 3 metre sections here. 10 studs. Header and sole plate. 2x8s so approx 200 kg a section. 2 men to lift; 1 man to prop and stabilise with rope until secured.

Edited October 29, 2021 by markocosic

[SimonD]

20 hours ago, saveasteading said:

in other posts, you have recommended using double stud at foot and head of the new timber walls.

 

The only time you need to double up sole and top plates is when studs and rafters don't line up.

 

2 hours ago, markocosic said:

Would also build frame without OSB. Just a diagonal on the opposite side.

 

14 hours ago, DragsterDriver said:

if stick building on site I tend to build the wall-brace it- then board it once upright

+ 1 to that. Don't add any osb until you've got at least some cover from the roof.

 

I wouldn't brace until it's upright as it can then be trued up in all directions.

 

As for 9 or 11mm it would typiclly depend on what the SE specifies. Same for the the roof.

 

20 hours ago, saveasteading said:

Walls are 140 x 50 stud, timber clad.

 

And to risk being picky, you'll either be building using 38 x 140 cls (50 x 150 unfinished) or 47 x 150 (45 x 145 nominal finished) carcassing, these differences will make a difference to your frame and subsequent dimensions. The cls can be ideal because you can get it at 2.7m lengths too.

 

2 hours ago, markocosic said:

For headers...I would do this next time:

https://bygghouse.com/wp-content/uploads/2017/02/Summary-of-Swedish-Wall-Element-Construction.pdf

http://blog.lamidesign.com/p/swedish-platform-framing-info.html

.

That's very neat. Still lots of stick framing going on in Sweden mind you, it's seen more as the diy route nowadays but it's understandable given the climate why they like to go indoors to make their houses. I'm still bemused why there isn't more in the UK. Instead there seems around us to be a massive growth in the use of tin hat scaffolds at major cost to the customer.

 

I have a slight hybrid of the framing method where I have a glulam ring beam and my 1st floor joists hang off that rather than penetrating througth the tf.

Edited October 29, 2021 by SimonD

[saveasteading]

So great to hear from people who have done it.

 

The design is clear, that 9mm osb is enough, and it doesn't need double footer or header.

 

The reality on site can be different so I  didn't, for example, want to show 9mm then find that "everybody uses 18mm" at contractor's insistence and all the dims be out immediately  by 9mm (they all add up).

More importantly the double header and footer needs to be drawn , to avoid any errors in height. (This will join into existing and so has constraints).

I would like the drawing to building control to be close to site practice.

 

I have done this before with 3 storeys and factory panels, but it was a package and these details were not of concern.. It was interesting how different the 2 suppliers were in details at foundation level, (one required a pre-fixed timber and utterly level footer and the other wanted a concrete kerb, also dead level) but there was no use of double headers.

 

I am thinking that the extra footer can be laid dead level to take out inevitable wobbles in the masonry base, and the extra header is a tidy way to build it if making the panels on the ground.

But that may not be necessary if all made in place.    

 

Have not chosen a joiner yet, so no doubt they will all have different ideas of 'the only way'.

 

I will ponder and report back.

 

Don't stop the suggestions and advice coming though.

[SimonD]

4 minutes ago, saveasteading said:

I am thinking that the extra footer can be laid dead level to take out inevitable wobbles in the masonry base

 

I wouldn't worry, get the wall up and just shim and use non-shrink grout where necessary, like you would finish under the base of steel columns.

[saveasteading]

5 minutes ago, SimonD said:

use non-shrink grout

Just in passing. In many hundreds of columns I never specified non-shrink grout.  I think the very term allows people to be casual and the bases are often only partly filled.

I preferred to use a dryish mortar, forced in to refusal. Just me perhaps.

 

But your point is a good one, thanks.

[markc]

9 minutes ago, saveasteading said:

Just in passing. In many hundreds of columns I never specified non-shrink grout.  I think the very term allows people to be casual and the bases are often only partly filled.

I preferred to use a dryish mortar, forced in to refusal. Just me perhaps.

 

But your point is a good one, thanks.

Semi dry mortar forced in will not shrink, wet grout poured in under a column does shrink  away and allow rocking if it’s not a non shrink.

for domestic and average commercial buildings the base plate sizes do not require poring grout. We used to have some columns with 2m diameter bases and portals with 1200x800 bases and often ended up with a grout thickness of 15-30mm so liquid was needed even when vibrating the plate