Subfloor fixing to stringers

[TandC]

Morning all, 

 

What's best practice for fitting subfloor to the steel stringers (the horizontal steel angle floor bearers creating the bilge)?   My old boat had the plywood subfloor screwed directly to the bearers. Good for maximising the headroom but not great for making the boards easy to raise when required as invariably the screws would shear off in the steel etc.  That also meant that the subfloor wasn't level due to old broken scews not cut out etc.   

 

In the new boat there is decent headroom so I could fit timber to the tops of the stringers, and then the plywood subfloor can be screwed to those.  I was thinking 18mm or 12mm ply strips drilled and screwed, perhaps even some grab adhesive too for solid adhesion along the entire length.   This would give a level to lay onto and solid enough base to easily screw the floor into.  All painted to seal from under-floor condensation, as will be the underside of the ply subfloor. 

 

suggestions appreciated. 

[BEngo]

Treated (tanalised) 3x2 ish timber battens on top of the stiffeners.  Screw them down.   Either drill and tap about M6 or use no8  self tappers if they will drive through your stiffeners.  I used self tappers, but put one through all the holes before fitting the batten.

 

I would not bother with adhesive.

N

 

[Thames Bhaji]

Morning. When I did ours I hired a nail gun for the day which saves a lot of drilling steel. The type of gun with explosive caps on the nails… it was quite fun! 

[blackrose]

Personally I wouldn't screw the battens down as again you might find that the screws shear due to differential thermal expansion/contraction of the wood & steel. Then you've defeated the whole object of putting battens down in the first place. As long as the steel underneath is painted I can't see any reason you'd ever want to take the battens off so I'd just stick them down with a flexible structural PU adhesive/sealant. Stixall, Sticks like Sh*t, Marine flex, Sikaflex, etc. They'll never come off.

[blackrose]

17 minutes ago, Thames Bhaji said:

Morning. When I did ours I hired a nail gun for the day which saves a lot of drilling steel. The type of gun with explosive caps on the nails… it was quite fun! 

 

I just wouldn't trust any nails to last. When I was fitting out my sailaway I used to hear loud bangs which was the screws that the builder used shearing on hot days. Less likely down at floor/bilge level where everything is cooler but still a possibility.

 

In my opinion any joint or fixing between different materials on a boat should be flexible and screws/nails aren't.

Edited January 12, 2023 by blackrose

[Thames Bhaji]

5 minutes ago, blackrose said:

I just wouldn't trust any nails to last. When I was fitting out my sailaway I used to hear loud bangs which was the screws that the builder used shearing on hot days. Less likely down at floor/bilge level where everything is cooler but still a possibility.

Yes, I’ve realised in the 17 years since that some frown on the nail gun method.  The saving grace might be that since it was quick I put in twice as many nails as I would screws, and if a couple go then floor still isn’t going anywhere. We’ll see, no problems so far. 

[jonathanA]

Self drilling self tappers used by roofers. Screwfix do them. They are typically stainless or galvanised and designed to last.

 

I don't see why expansion would be a problem the wooden bearers would have a little bit of give if needed. 

 

Personally I think the use of no nails or similar makes no sense in this situation.

 

[BEngo]

1 hour ago, jonathanA said:

Self drilling self tappers used by roofers. Screwfix do them. They are typically stainless or galvanised and designed to last.

 

 

These are good, if you do not need countersunk heads.  I suppose you could always counter bore the battens.   Being hard,  Tek screws  can be brittle and they dont like very thick material.  Get good quality  heavy duty ones, not far east cr@p off the web.

 

  I put a load into  5 mm thick steel last year and discovered their peculiarities the hard way. Fortunately the ones that sheared did so flush with the steel.   A driver with a variable torque setting is essential.  Start low and work up till you find a setting where they just drive through. 

 

N

[nb Innisfree]

I fixed 3x2 timber bearers to steel angle bearers by trapping the.top horizontal steel bearer part between the 3x2 timber bearers and pieces of 18mm marine ply, pulled together with ss screws, same for hull sides, that way steel is unbreached.

Floor was 18MM ply screwed to timber bearers, again with ss screws, in fact entire boat had them. 

Edited January 12, 2023 by nb Innisfree

[David Mack]

And do you run your timber floor bearers along the top of the steel stringers across the boat, or lengthways along the boat spanning between them?

The former will divide the bilge area into a series of fairly unconnected spaces, meaning little underfloor ventilation and any water down there will take a long time to evaporate.

Run the floor bearers lengthways and there is much more scope for underfloor ventilation.

[Thames Bhaji]

4 hours ago, nb Innisfree said:

I fixed 3x2 timber bearers to steel angle bearers by trapping the.top horizontal steel bearer part between the 3x2 timber bearers and pieces of 18mm marine ply, pulled together with ss screws, same for hull sides, that way steel is unbreached.

Floor was 18MM ply screwed to timber bearers, again with ss screws, in fact entire boat had them. 

 I like that. Wish I’d thought of it 17 years ago. 

[nb Innisfree]

22 hours ago, David Mack said:

And do you run your timber floor bearers along the top of the steel stringers across the boat, or lengthways along the boat spanning between them?

The former will divide the bilge area into a series of fairly unconnected spaces, meaning little underfloor ventilation and any water down there will take a long time to evaporate.

Run the floor bearers lengthways and there is much more scope for underfloor ventilation.

I an them along steel bearers to allow easier access to ballast, no sign of damp after a couple of years.