Floor Joist Design

[Pappa]

Hi guys

 

I'm attaching a sketch I've made of the structure of my first floor.

I want to build an attic floor above this.

Please can you advise which orientation of the floor joists is the most structurally sound?

 

All of the brick walls in red are structural walls that go down to the foundation.

 

I will refine and share my joist calculations once I commit to a design. Provisionally... I think 8 x 2 might work.

 

Thanks again

 

P

[nod]

Chuck it out to one of the joist companies They will quote and design off cad for free Building Control will require this also 

[kandgmitchell]

It's normal practice to span the shortest distance and space at 400mm centres so that plasterboard for the ceiling and boarding for the floor works better. Spacing at 600mm centres means you need thicker plasterboard and floor boarding to avoid deflection. Spacing at 300mm centres just uses more material.

 

The earlier versions of the Building Regulation Approved Document for Part A - Structure used to have very useful load/span tables for timber joists, rafters and purlins which could be used quickly to determine a joist size. You probably can find the archive versions on line. They stopped providing them as it favoured one material over others which became much wider available.

[Nickfromwales]

9 hours ago, Pappa said:

Provisionally... I think 8 x 2 might work.

Are you just installing solid timber or have you looked into posi joists (open metal web) too? There not huge money, and would attract a free design in most cases. Much better for 1st fix electrical and plumbing, so consider the downstream savings to be had there vs drilling and notching into solid timber for pipes and cables. 

[Pappa]

Thanks for the comments guys.

 

5 hours ago, nod said:

Chuck it out to one of the joist companies They will quote and design off cad for free Building Control will require this also 

Any recommendations please?

 

1 hour ago, kandgmitchell said:

It's normal practice to span the shortest distance and space at 400mm centres so that plasterboard for the ceiling and boarding for the floor works better. Spacing at 600mm centres means you need thicker plasterboard and floor boarding to avoid deflection. Spacing at 300mm centres just uses more material.

 

The earlier versions of the Building Regulation Approved Document for Part A - Structure used to have very useful load/span tables for timber joists, rafters and purlins which could be used quickly to determine a joist size. You probably can find the archive versions on line. They stopped providing them as it favoured one material over others which became much wider available.

 

Great advice, I will change to 400 centres on the top half.
Bottom half won't be possible for 400 centres, I'll have to stick with 300 centers as the span is too long.
I've had a look at span tables and they don't give any advice when the span is this long. I'm having to manually calculate.

 

 

1 hour ago, Nickfromwales said:

Are you just installing solid timber or have you looked into posi joists (open metal web) too? There not huge money, and would attract a free design in most cases. Much better for 1st fix electrical and plumbing, so consider the downstream savings to be had there vs drilling and notching into solid timber for pipes and cables. 

 

I didn't even know they existed !

Can you send me some recommendations please?

 

Thanks again

 

P

 

[Nickfromwales]

We used these for a 3 storey shop conversion. Happy with price and service.

 

https://www.mitek.co.uk/products/posi-joists/

 

No affiliation, so get a few quotes obvs. 
 

+1 to changing the joists to run the shortest span. With all posi joist designs I do I let them calculate deflection with joists at 600mm centres, then drop that down to 400mm but with the same spec joist. By design there’s a bit of deflection that’s too close to the wind for me aka ‘bounce’ and using the beefier joists at 400mm spacing seems to manage that well.
 

It’s only a few extra joists for a much better job, but also always use 22mm P5 flooring and not 18mm. 

[kandgmitchell]

Must admit, the open web joists do make life easier. Used them for the first floor in a large two two storey extension at the last house. Apart from being lighter to work with, it's so much easier to run pipes and wires through them. The suppliers provide the supporting calcs so you don't have to bother, for, as you mention your span is off the end of the AD Part A tables.

[saveasteading]

Try James Jones. Timber flanges and osb webs. Their website has an interactive span calculator 

[Nickfromwales]

2 minutes ago, saveasteading said:

Try James Jones. Timber flanges and osb webs. Their website has an interactive span calculator 

Is that for I-beam vs posi? Much of a saving to be had? 
 

A bit more of a PITA if fitting MVHR or soil pipes within the voids though. 

[Blooda]

We used posi-attic joists for our loft. [Harlows Timber as they are fairly local]  Giving us the extend in the future upwards.  

We added loft legs and boarded the centre giving us a 600mm gap to fill with 500mm insulation and 100mm gap.

So lots of accessible storage. 

 

 

 

 

 

 

 

[Iceverge]

Is this an attic or a first floor? 

 

As you'll have intermediate bracing and triangulation with an attic the span tables will be different. 

 

Also is the space above the floor going to be inside the conditioned envelope? 

 

If Yes than a cut roof or an airtightness layer above the roof trusses is much easier to detail. 

[Pappa]

20 minutes ago, Iceverge said:

Is this an attic or a first floor? 

 

As you'll have intermediate bracing and triangulation with an attic the span tables will be different. 

 

Also is the space above the floor going to be inside the conditioned envelope? 

 

If Yes than a cut roof or an airtightness layer above the roof trusses is much easier to detail. 

 

Thanks for the reply.

Yes, this is for an attic.

The space above the floor will be habitable space and therefore within the insulated and airtight envelope. I will be installing the appropriate vapour barriers and PIR thickness against the roof rafters.

 

[ProDave]

23 minutes ago, Pappa said:

Yes, this is for an attic.

The space above the floor will be habitable space and therefore within the insulated and airtight envelope. I will be installing the appropriate vapour barriers and PIR thickness against the roof rafters.

Pause right there.

 

For room in roof, I 100% recommend a warm roof construction (insulation above rafters) and air tightness layer following the roof line of the whole house, so even the eaves space that may not be part of the rooms upstairs is air tight and warm.

 

This is SO much easier to detail and get right and so much better without lots of cold parts of loft to be separated from the warm rooms.

 

This also then makes it easy and desirable to separate the function of floor joists and roof structure.  We have posi joists for the floor upstairs and then a cut roof hung from a big ridge beam.  So not only is all the roof space insulated and air tight, there are no joist members intruding, so you have a completely open loft area to do as you like in terms of what is accommodation and what is storage and how they work together.

[Nickfromwales]

Room in roof would have to be done that way? Wouldn’t pass regs otherwise. 

 

Would still need acoustic insulation in the bottom of the trusses, underfoot. 

[Pappa]

1 minute ago, ProDave said:

Pause right there.

 

For room in roof, I 100% recommend a warm roof construction (insulation above rafters) and air tightness later following the roof line of the whole house, so even the eaves space that may not be part of the rooms upstairs is air tight and warm.

 

This is SO much easier to detail and get right and so much better without lots of cold parts of loft to be separated from the warm rooms.

 

This also then makes it easy and desirable to separate the function of floor joists and roof structure.  We have posi joists for the floor upstairs and then a cut roof hung from a big ridge beam.  So not only is all the roof space insulated and air tight, there are no joist members intruding, so you have a completely open loft area to do as you like in terms of what is accommodation and what is storage and how they work together.

 

Thanks for this.

So I'm going to have a dormer at the rear of the house which is going to be warm roof construction. However, on the front side, its a pitched roof which is going to give me limited head height. I'm going to unfortunately have to stay with cold roof insulation on this side.

[ProDave]

Just now, Pappa said:

 

Thanks for this.

So I'm going to have a dormer at the rear of the house which is going to be warm roof construction. However, on the front side, its a pitched roof which is going to give me limited head height. I'm going to unfortunately have to stay with cold roof insulation on this side.

Draw it on paper.  It makes no difference.

 

You either set the rafters high and insulate below them, or set them lower and insulate above them.  The thickness from plasterboard inside to surface of tiles on the outside is the same.

 

If you want minimum thickness, which it sounds like you do, then a hybrid roof, some insulation above the rafters and full fill insulation between the rafters.  That is actually what we have. It just needs a little more design.

 

[Iceverge]

30 minutes ago, Pappa said:

and PIR thickness against the roof rafters.

 

Sucks through teeth.........

 

Shakes head...........

 

There are better ways to skin this particular cat. PIR between timber is only a good idea if you're selling the stuff. 

 

 

 

 

[Iceverge]

Looking at the top sketch you made I'm assuming that the house is mid terrace with the ridge running centrally along the short axis? 

 

Any sectional views or sketches would be appreciated. 

 

I'm also guessing that a crane isnt on site or easily available so perhaps trusses would be harder to get into position ( unless a delivery mounted truck crane is available? )

 

 

[Pappa]

4 minutes ago, Iceverge said:

Looking at the top sketch you made I'm assuming that the house is mid terrace with the ridge running centrally along the short axis? 

 

Any sectional views or sketches would be appreciated. 

 

I'm also guessing that a crane isnt on site or easily available so perhaps trusses would be harder to get into position ( unless a delivery mounted truck crane is available? )

 

 

 

Mid Terrace

 

No other sketches I'm afraid.

 

No cranes, everything will have to be placed by hand I'm afraid

 

[Nickfromwales]

1 minute ago, Pappa said:

 

Mid Terrace

 

No other sketches I'm afraid.

 

No cranes, everything will have to be placed by hand I'm afraid

 

You will need the mother of all scaffold to be able to man-handle those up there (without damaging the neighbours properties etc). 

[ProDave]

1 minute ago, Pappa said:

 

Mid Terrace

 

No other sketches I'm afraid.

 

No cranes, everything will have to be placed by hand I'm afraid

 

Strong argument for cut roof and ridge beam separate to floor joists.  Everything can be lifted one at a time by hand at those sizes.

[Nickfromwales]

Scaff mounted hoist may help out considerably, but you’ll need to go another lift higher or make a scaffold jib for the hoist and do it like the Egyptians did 👌

[Nickfromwales]

1 minute ago, ProDave said:

Strong argument for cut roof and ridge beam separate to floor joists.  Everything can be lifted one at a time by hand at those sizes.

Yup. Defo simplify the bejesus out of this job, and prob drop the cost a lot to go cut roof. 

[Pappa]

2 minutes ago, ProDave said:

Strong argument for cut roof and ridge beam separate to floor joists.  Everything can be lifted one at a time by hand at those sizes.

Sorry, could you clarify what this means.
Do you mean putting a steel beam in to split the span? That way the joists will be half the size?

[saveasteading]

2 hours ago, Nickfromwales said:

that for I-beam vs posi?

Yes. A straight competition. I've used John James  once and it will have been on price. But I remember  the service was good too.

I think they were 7m or so spans and I compared them also with rolled steel (Z)  joists.

The video of manufacturing is impressive ...stunningly efficient.

2 hours ago, Nickfromwales said:

if fitting MVHR or soil pipes within the voids

Yes. You have to cut holes.

 

Much lighter than a solid timber but still very heavy. The span tables give lots of options on price v depth v weight.