Construction of Dorma

[paulandfrodo]

Under the new (well 2008) PD regs you can build a dorma upto 40m3 onto a mid-terrace.

I've always constructed Dormer's using the following :-

OUTSIDE

Slate
Horz. Batten (2" x 1")
Breathable Membrane
Vertical Batten(2" x 1")
18mm WBP Ply
2"x4" Stud wall (With Celotex between and over studs)
Vapour Barrier
Plasterboard
Plaster

INSIDE

I've seen a few builders remove the vertical batten (not too concerned about that) BUT, instead of using WBP Ply they are using OSB3.

Surely this is not fit for purpose as it's not a "structural" sheet material.

I know the last Dormer I did BC were insistent on using WBP Ply.

Would appreciate your comments on the substation of WBP Ply for OSB3.

Paul.

 

[DevilDamo]

I usually specify the construction as you've mentioned.

However, remember the thermal reg's changed on 1st October, so there will be a slight increase to insulation thicknesses.

 

[freddiemercurystwin]

You can do it with OSB to save a few quid but its shortsighted as it just won't last as long as ply.

 

[Deluks]

OSB3 does the same job as ply in the majority of applications.
What could possibly go wrong with it when used in a dormer?

 

[freddiemercurystwin]

It will delaminate far quicker than WBP ply if it ever gets wet.

 

[Deluks]

Not really, and how will it get wet under tiles and breather membrane? osb3 swells slightly if it gets a one off good soaking but I've seen it used as hoarding for months on end without falling apart.
Only advantage of using ply is if you build a structure that's gonna leak
ie: you do a crap job.

(Not suggesting that of you though FMT )

 

[paulandfrodo]

My concern was prob. more aimed at it's structural capabilities in handling lateral load strains. I would guess lot's of small bits of wood (OSB3) would not be as strong as large continuous sheets of ply (running in different directions)... However, with all the glue that goes into OSB3 to make it moisture resistant (not proof)...

Paul.

 

[Deluks]

Plenty strong, it's used in SIPS as well as i-joists, both structural applications.

 

[paulandfrodo]

Fair point. Quite agree.

Do SIPS panels just use normal nails (from air gun/nail gun) or do they use skewed nails (as you would on a joist hanger) ? Sorry, just thinking out loud !! Not at all relevant...

Paul.

 

[Deluks]

No idea, but I'd have thought galvanised rink shanked fixed in with a nailgun, air or paslode.

 

[chappers]

You are incorrect in thinking OSB is inferior, in fact it is graded to certain standards, whereas wbp ply usually isn't.
OSB 3 is graded for structural use and is particularly suitable for humid areas and must pass certain specifications to achieve that grade

 

[freddiemercurystwin]

You are incorrect in thinking OSB is inferior, in fact it is graded to certain standards, whereas wbp ply usually isn't.
OSB 3 is graded for structural use and is particularly suitable for humid areas and must pass certain specifications to achieve that grade

Just like you can get moisture resistant MDF! BTW 12mm WBP is normally more than adequate.

 

[geraint]

You are incorrect in thinking OSB is inferior, in fact it is graded to certain standards, whereas wbp ply usually isn't.
OSB 3 is graded for structural use and is particularly suitable for humid areas and must pass certain specifications to achieve that grade

Just like you can get moisture resistant MDF! BTW 12mm WBP is normally more than adequate.

and that will delaminate in the same circumstance....

 

[freddiemercurystwin]

You are incorrect in thinking OSB is inferior, in fact it is graded to certain standards, whereas wbp ply usually isn't.
OSB 3 is graded for structural use and is particularly suitable for humid areas and must pass certain specifications to achieve that grade

Just like you can get moisture resistant MDF! BTW 12mm WBP is normally more than adequate.

and that will delaminate in the same circumstance....

Yes but I could say grass is green and you'd disagree .........

 

[thomasw]

the main reasion osb boards are used if fire regs for lofts ply will burn much quicker were osb board takes longer every one knows when it is wet its just like weaterbix if for the fire regs