18mm OSB3 T&G for roof deck on dormer

[olley]

What's the general opinion on gluing the osb down to the joists?

I intend to glue them together but getting different views on whether to glue as well as screwing them to the joists, using a foaming glue to take up any slight gaps in the firings.

[Nickfromwales]

Belt & braces wins every damn time afaic....  You'll only get one chance to do this properly, and I cannot possibly see why adding glue to the mix would be of any detriment?

 

When installing Egger (P5) floorboards I always use D4 PU glue and 5 screws per joist run, with screws that don't have thread all the way to the head of the screw so there's no risk of 'jacking'.

 

Oh, and don't forget to NOT let that D4 glue get all over your hands, it's a PITA to get off anything, including tools / tape measure etc.

[olley]

Thanks Nick, I know what you mean about poly glue, it's a pita if you get it on you as I always seem to do.

The plan is ordinary D4 on the T&G and the poly on the joists.

[olley]

Decks on and covered with plastic, now to start on the floor, just had a price of £9.5+vat, for 18mm P5 chipboard, about another £1 for 22mm

Area is about 50sqm.

[Nickfromwales]

7 hours ago, olley said:

Decks on and covered with plastic, now to start on the floor, just had a price of £9.5+vat, for 18mm P5 chipboard, about another £1 for 22mm

Area is about 50sqm.

22mm!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

 

[Gus Potter]

On 02/03/2025 at 12:09, olley said:

What's the general opinion on gluing the osb down to the joists?

I intend to glue them together but getting different views on whether to glue as well as screwing them to the joists, using a foaming glue to take up any slight gaps in the firings.

An important thing to remember is that firrings get thin at one end. One golden rule is to make sure your fixing is going at least 2/3 of the thickness of the thing your are fixing (OSB in this case) into the strucural timber (the roof joists) until the firring gets to a sufficient thickness that it can be counted on to act as a proper substrate. For this to happen the thicker end of the firring needs to be properly fixed to the top of the joist. If I was to glue anything it would be the firring pieces to the tops of the supporting joists.

 

This matters as often we use the roof as a stiff element that stops the roof and say dormer from twisting sideways.

[olley]

I glued and screwed the firings to the joists. The deck is 18mm T&G OSB3 glued together and screwed down to the firings, four 5x50 screws in every joist. 

On top of this goes 200mm pir insulation followed by more OSB or marine ply, that's so I can fit solar at a later date.