How do you layout/position floor joists?

[Gizzydash]

Hi Everyone,

I'm sure this is relatively straightforward for someone out there. Planning on replacing existing suspended timber floor with deeper joists and insulating. I've looked at joist spans/spacing etc and understand all this but I can't find anywhere about how you practically layout and position the joists - I've got a 3mx3m room, at 400 centres I'm at 7 joists with ~100mm left at either side of the room. Is it ok to leave this 100mm unsupported finish floor at either side, or do you put another joist at the ends effectively with like a 100mm centre? Also, what is the best approach for laying out, is it start with a joist in the centre and work out, or basically work from the room width, find how many joists at 400 centres and see what's left when you get to the wall?

Thanks in advance!

 

[Swwils]

From NHBC on joist spacing;

https://nhbc-standards.co.uk/6-supe...-floors/6-4-10-construction-of-timber-floors/

1. be in accordance with the design and not increased
2. account for the decking material to be used
3. be a maximum of 600mm
4. have a clearance of 25-75mm between the first joist and the wall face to aid the installation of services and the fixing of floor decking.

Premier noggins may be required to support decking.

 

[cdbe]

Check the span tables (but if you're deepening the joists anyway it shouldn't be an issue) and make the spacing 420mm. If you're using chipboard flooring sheets it helps to try and space the joists so the ends land on joists.

 

[Gizzydash]

Thanks - so is it common practice to space the joists to suit the situation and not stick rigidly to 400, 500 or 600 exactly? In my 3m room, going by the guidelines of leaving 25-75mm from the wall, none of those centre distances would work unless I'm totally confusing myself at this stage which is entirely possible!

 

[cdbe]

They are maximums, you can go less, see how 450 works for you on the tables with your chosen joists.

If you go to the wider center of 600, you need to check the suitability of your floor boards.

 

[Gizzydash]

ok, thanks. Therefore, leaving 25mm each side to wall on a 3m room, I can either have 8 joists at ~370mm centres or 7 joists at ~420mm - as long as the spans are ok per the tables? Also, in terms of treated v.s. untreated joists, local builders merchants seem to carry a much larger range of untreated. Assume I can use untreated but ideally get treated?

 

[^woody^]

Set the end joists in from the finished plaster face, not the wall.

Normally you would set the joists at equal spacing (400/450/600) and adjust the last one - this allows for plasterboard to fit properly without cutting every board to match the joist spacing. This would be good practice even on ground floors.

100mm at the wall is not an issue

Untreated timber timber should be OK with a normal insulated cavity wall if bearing on the wall or off hangers for upper floors, but ground floors should use treated timber. Treat the ends with preservative regardless.

 

[Gizzydash]

Thanks for all the info. Couple of final questions - what fixings are suitable for the joist sitting on the timber wall plate (nailed/screwed down)? Is a mutli-purpose mortar mix that comes bags ok to use to set the new timber wall plate & DPC underneath?

 

[^woody^]

How can floor joists be on a wall plate?

 

[Gizzydash]

Apologies if I'm using the incorrect terminology - setup is a suspended timber floor, with joists resting on dwarf walls on inner leaf (with honeycomb walls in-between). The dwarf walls on inner leaf have a bit of timber on top which the joist ends rest on.

 

[^woody^]

If the joists are not resting on the structure, but on dwarf walls, then you would just spike nail the joist to the plate, and fit noggins at the bearing, and at mid span

 

[noseall]

How can floor joists be on a wall plate?

You're getting as lazy as me for not reading posts properly.

 

[^woody^]

 

[Gizzydash]

Promise final question on joists - does the clear span in the tables and notching guidance only apply to the full length of the joist regardless if there is a sleeper/multi sleeper walls in-between? for example one end of the room to the other is 3m, but when you put a sleeper wall in the middle you've got a span now ~1.5m. For the tables/notching are you still looking at 3m span or 1.5m? Not entirely sure if sleeper walls in middle count towards reduction in span or if that's dictated by end to end joist span

 

[cdbe]

It halves it.