Floor Joist Design

[Pappa]

Thanks for all the replys

 

2 hours ago, torre said:

Could you drop the ceiling to give more space for a deeper section joist?

Good shout, already reduce ceiling below as low as possible. I could go lower, but then I'm cutting in to the window and the beautiful ornate period window carpentry.

I'm going to have to compromise somewhere and I would prefer to put extra time and money in to installing more joists at 200 centers and do some awkward drilling.

 

 

39 minutes ago, Nickfromwales said:

Not a chance, matey. You would set all the joists out on the floor, twang a string line down each 1/3 point and then pre-drill for small bore services BEFORE installing . 3x 25mm holes down one end for electrics, 4x down the other for plumbing / other electrical / miscellaneous.

 

Oh, and yes, fit the solid timbers in at 200mm oc and move on with life. It's not going to go anywhere, and as you say, it's perfectly DIY'able.

 

This is great advice. Many thanks.
Will more holes not reduce the joist strength? Is there an upper limit to how many holes?
What if I pre-drill the minimum number of holes if I can proactively plan the plumbing/electrics?
No doubt I'll need some last minute holes... Surely I could use a multi tool drill bit or something similar to drill the holes in the tight space?

 

Thanks again

 

P

[saveasteading]

2 hours ago, Pappa said:

Will more holes not reduce the joist strength

No. Not if you keep to the rules of size, position and spacing. Later we can sort that.

[ProDave]

At 200mm spacing, you might sort cable holes etc by pre drilling, but best of luck swinging a hammer to drive the twisted nails into the joist at the joist hangers.  150mm gap is not a lot of room to swing a hammer.

[saveasteading]

29 minutes ago, ProDave said:

best of luck swinging a hammer

Use screws on whatever angle is possible or with a right angle drill attachment.

Precut service holes in the joists: mass produced outdoors before lugging them in.

[ProDave]

1 hour ago, saveasteading said:

Use screws on whatever angle is possible or with a right angle drill attachment.

Precut service holes in the joists: mass produced outdoors before lugging them in.

I keep hearing you must use those special twisted galvanised nails for joist hangers and not screws.  I seem to remember it was something BC looked at on an early visit.  And they expect every hole to have a nail in it.

[saveasteading]

22 minutes ago, ProDave said:

must use those special twisted galvanised nails for joist

So they don't loosen when a joist bounces or rotates. I expect joiners perfer the speed of nails, so must use these.

Screws don't pull out.

If the joists don't move, due to noggins (or dwangs) I can't see a problem. It might need an Engineer to say so to the bco if questioned....Best ask bco before committing. 

[dpmiller]

there are special screws for joist hangers

 

https://www.strongtie.co.uk/en-UK/product-lines/nails-and-screws/connector-screws

[Iceverge]

Ok here's a proposal.

 

Install a Filch beam of 180x30 steel and 45x170mm timbers midway on the short wall, I assume the intermediate wall is strong enough to hold it. 

 

 

 

Bolt 45mmx170mm ledgers  to the perimeter walls and use joist hangers to hold them.  Add an airtight membrane behind them and you'll have a continuous airtight barrier from the existing first floor 

 

 

 

 

Floor with 22mm Caberdeck or similar 

 

Add wall plates on top of the caberdeck and a 120 x240mm ridge beam. 

 

This will allow you to drop 220mm cut timber joists conventionally on as a roof. 

 

 

 

 

That's my detail for the eaves

 

 

 

 

 

And here's how I'd run a membrane to ensure excellent airtightness. 

 

 

This is the roof buildup i'd use. 

 

 

 

 

 

 

 

Floor thickness ~200mm. 

 

Roof U Value = compliant

 

No crane needed. 

 

Excellent air tightness, noise protection, fire performance, heat protection. rodent protection. 

 

All off the shelf materials apart from filch beam. 

 

 

[saveasteading]

40 minutes ago, Iceverge said:

I assume the intermediate wall is strong enough to hold it. 

Was discussed earlier. it is  a single brick wall, and no knowledge of foundations, but unlikely to be adequate.

Plus the  beam  would be about 350 deep which isn't wanted.

So probably no.

[Pappa]

Thanks for the replys guys.

 

3 hours ago, Iceverge said:

Install a Filch beam of 180x30 steel and 45x170mm timbers midway on the short wall, I assume the intermediate wall is strong enough to hold it.

 

Did you calculate this flitch beam or is it just a rough idea?
If we can get a beam with that depth to work, I'm in!

 

2 hours ago, saveasteading said:

Was discussed earlier. it is  a single brick wall, and no knowledge of foundations, but unlikely to be adequate.

Plus the  beam  would be about 350 deep which isn't wanted.

So probably no.

 

So Iceverge's Flitch Beam dimension prompted me to check the previous RSJ calculation performed by Deepseek/ChatGPT.
There was a flaw in their assumption which led them to oversize the RSJ to ~350mm depth. They assumed the RSJ would carry the entire floor width however in reality it probably only carries half the floor (one masonry wall will carry a quarter of the load and the other masonry wall carrying the other quarter).

Does this sound about right?

 

As far as the foundation goes, both masonry walls go right down to the foundation. the foundation is clay but compacted like iron and more than a meter deep. If we go down the RSJ route, I'll have to get structural engineer to sign off on the foundation.

 

Thanks again

Edited March 16 by Pappa

[Gus Potter]

Interesting thread this and @saveasteading makes a great point  offering up the cold formed steel joist option. They can be remarkably cost effective when you take into account the labour element.

 

On 13/03/2025 at 01:24, Pappa said:

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

Good set of diagrams, well done. Having had a very quick glance at this I can see that the primary span that needs navigation is (5.995m) say 6.1 m over the centre of the supports. I assume there will be some Velux, maybe a dormer so you have some point loads here from trimmers.

 

To go back a bit to let folk on BH know what may be going on the in the murky mind of an SE and what drives their design / risk decisions.  

 

I think.. ok it's a mid terrace as reference is made to a party wall. Do I know if there are alterations next door, can I get access? Probably not. Can I put any significant load onto the party wall. Any load I put on it is going to be off the centre of gravity of the wall (called an eccentric load) and that will cause a bending force in the party wall which could shave a lot of it's load bearing capacity and introduce a stability risk. As @saveasteading , @ETC et all have mentioned this is akin to taking down one side of a chimney breast on a party wall.. here reducing loads actually perversely often causes a problem in a brick wall.

 

What if one of the neighbours has already been messing about and they have put a beam right where we want to put one? If so then we have a fire protection problem to add to the complexity and cost risk.

 

A few folk have suggested ideas on trimming steels etc. But you need to be careful here before you implement this type of solution, you need to know where the heavy loads from main trimming beams go as you work your way down the building. Often at attic level it looks great, then you find the point loads are over an existing window lintel on the floor below! Now you end up chasing your tail and it all get out of hand / cost rises rapidly.

 

38 minutes ago, Pappa said:

As far as the foundation goes, both masonry walls go right down to the foundation. the foundation is clay but compacted like iron and more than a meter deep. If we go down the RSJ route, I'll have to get structural engineer to sign off on the foundation.

I think you need to bite the bullet here and get an SE. BC will probably ask about the founds. But to make this more manageable in proving the existing founds will work you want to spread the loads as much as possible down through the load bearing walls. It an old house so you want to spread loads as eavenly as possible.

 

I'm going to lead you down the route of considering cold formed steel joists here. Now cold formed steel joist design often entails some horrific calculations and unless you have (as an SE) a good knowledge of this and software / spreads sheets it's not your bag. For me this is in my comfort zone so it goes in my toolbox of attic / barn conversion tricks and design methods. Incedentally there are a  few companies that use "extendable " cold formed (cf) joists as a solution. Essentially you sister the existing timbers with cf joists and avoid disrupting the noggings holding the ceilings below.

 

If I was doing this I would check early that I can get a compliant stair in and headroom in the attic, see where the point loads are, say from roof trimmings. Look to see if I can increase the structural floor thickness (but then you may need that one extra step in the stair that mucks up everything!) and check the existing eaves detail to make sure I can get good support, ideally spreading the loads.. Depending on where you are in the country you have different arrangements of structural eaves detail and it is essential to get your head round this. Have a look on the internet on how a pole plate roof works and a 1/3 rafter / joist eaves works.

 

Now many posters have suggested timber type solutions but attic conversion are tricky as it's the detail that is often the issue. I can see how many posters on BH are suggesting ideas but attic conversion are a different animal.. as not least they are conversions.

 

Hope this helps a bit.

 

 

 

 

 

 

 

 

[Nickfromwales]

1 hour ago, Pappa said:

Did you calculate this flitch beam or is it just a rough idea?
If we can get a beam with that depth to work, I'm in!

If you use 2 runs of steel and 3 timbers you may be able to reduce the profile. @Gus Potter?

 

[Iceverge]

Oh by the way I'm most certainly not a Structural engineer so please only use my thoughts as suggestions!

 

If I was going to build this in post apocalyptic world where all structural engineers had been vaporised ( sorry Gus ) and was to resort to my "jump on it and guess if it's strong enough" method I think I'd be tempted to run some vertical supports all the way down to the footings to avoid too much extra weight on the existing walls. 

 

I don't know if this is an easy option with the design of the rooms downstairs?

 

 

Edited March 16 by Iceverge

[Gus Potter]

1 hour ago, Nickfromwales said:

If you use 2 runs of steel and 3 timbers you may be able to reduce the profile.

@Nickfromwales there are a few options.. but often the rub is getting an adequate bearing without a window lintel on the floor below throwing a spanner in the works. Honestly it's the bain of my life.. I come up with an attic concept design that can look great..you get a stair to fit and I feel quite clever.. I put my SE hat on and my bubble gets burst!

 

At that span to shove the loads where they can be easily dealt with.. no posi joist or timber solution is going to be economic unless you can significantly increase the floor thickness. To increase the floor thickness you invite a pile of other problems. Yes, technically you can reduce a timber joist spacing to less than 300mm but it becomes unbuildable.

 

I totally get how folk on BH are contributing their knowledge in terms of say posi or solid timber floor joist construction and then how you insulate and fire protect all of that..  but when it comes to attic conversions there are many other things like the founds, old wall stability that comes into play. You need to nut that out first before you get onto an insulation / thermal performance strategy.

 

@Nickfromwales and all.

 

Some of my bread an butter work as a designer ( SE and architectural designer) comes from builders that I work with locally. I kind of operate a "hot line" where they can say to their Clients, hey I'll give my tame SE / designer a call. To be clear folks I operate completely independantly and all the builders have to tender competetively for the job. I've spent many years proving my integrity so you have to be absolutely fair. But I want the folk I work with to do well as if they are getting on so am I. It's in my intrerest to make sure they do well.

 

Now I say to my Cleints.. here is a list of builders I know who are trustworthy.. but I always insist they get quotes under their own steam from other builders as this protects me from any accusation of a conflict of interest and lets the the builders that I work with know that they can't walk their prices up on my jobs... as that would impact on my business. For all on BH always get an independant quote as a sense check.

 

Now I can see how folk on BH are tending to go for a timber type solution.. but his may not be the best way forward. Often as a designer you do a few options.. you kind of know that some are not the best but this lets you say to a Client.. I looked at this, you are informing your Client and letting them make informed decisions.

 

And that kind of sums it up on a self build or extension. It's your money and you just need someone who will say..here are your choices.. you decide and if you do that as a Contractor nine times out of ten you will win the job and make fair profit!

 

 

 

 

 

 

 

 

 

 

 

Edited March 16 by Gus Potter

[Iceverge]

Oh, another option that dawned on me earlier would be to have deeper beam like an RSJ or Glulam as an intermediate support protruding to make a bump of the ceiling of the room below. 

 

Not ideal but it may allow for enough strength to complete a tricky job. 

[Nickfromwales]

8 minutes ago, Iceverge said:

Oh, another option that dawned on me earlier would be to have deeper beam like an RSJ or Glulam as an intermediate support protruding to make a bump of the ceiling of the room below. 

 

Not ideal but it may allow for enough strength to complete a tricky job. 

Good darts! Some may say "a feature".

 

Bloody good forum is this.

[Gus Potter]

8 minutes ago, Iceverge said:

Oh by the way I'm most certainly not a Structural engineer so please only use my thoughts as suggestions!

Hiya @Iceverge and all on BH. 

 

This is the thing I love about this site. Ok you may not be an SE/ Architect etc as the day job but you are sharing your experience and you are clearly not daft.

 

Make no mistake. My day job as an SE / designer.. I do day to day stuff, knocking holes in walls and putting in beams..  oh and raft slabs and all that puts bread on my table.

 

I also learn loads here as there are plenty folk with innovative ideas and many with decades of construction experience. For me it's a two way street.. I try and explain some SE stuff here but in return I get a lot back from all the other members efforts. I've also ended up getting to know personally some members and that in it's self makes it worth while for me. Mind you my spelling and grammer has not improved despite many giving me a helping hand.

 

BH folk are at the front end of the design curve as you are trying to make self building affordable and unique. Be proud of what you are aiming to achieve.

 

 

 

 

[Pappa]

 

Many thanks for your comprehensive thoughts.

 

3 hours ago, Gus Potter said:

I assume there will be some Velux, maybe a dormer so you have some point loads here from trimmers.

 

Yes, but none of this will be supported by the attic floor. We should be able to keep existing purlins to support the roof.

I think we can design the attic floor in a vacuum without worrying about the roof.

 

3 hours ago, Gus Potter said:

Often at attic level it looks great, then you find the point loads are over an existing window lintel on the floor below!

Good shout, never even considered this.

So the RSJ we were considering as per @Iceverge plan above... The top masonry wall there is a door in that position at ground floor (which isn't a huge problem, we can brick that door). However, on the south side of the plan... that wall doesn't actually go down to the foundation. There is an opening underneath it for a bay window! So it's likely that wall is already resting on a steel beam (did they have steel beams 120 years ago?).
We could potentially have the RSJ slightly to the left so the new spans would be 4m and 2m. Span table says we should be able to get a 75mm x 175mm joists across the 4m span. To keep the joist depth low.

 

3 hours ago, Gus Potter said:

Essentially you sister the existing timbers with cf joists and avoid disrupting the noggings holding the ceilings below.

Whoops, The ceiling/timers have already been demolished.

To be honest, that ceiling needed to be lowered either way otherwise there was no way we could do the attic conversion.

 

3 hours ago, Gus Potter said:

If I was doing this I would check early that I can get a compliant stair in and headroom in the attic. Look to see if I can increase the structural floor thickness (but then you may need that one extra step in the stair that mucks up everything!)

Yes, I can!

If I can use 170mm joists, I don't need to alter the existing ballustrades that cover the stairs from ground floor to 1st floor.

If I have to go up to 195mm deep joists, I will have to make minor alterations to the ballustrades.

Ideally, I don't want to touch the existing stairs because the Victorian carpentry is beautiful. 

 

3 hours ago, Gus Potter said:

See where the point loads are, say from roof trimmings. and check the existing eaves detail to make sure I can get good support, ideally spreading the loads.. Depending on where you are in the country you have different arrangements of structural eaves detail and it is essential to get your head round this. Have a look on the internet on how a pole plate roof works and a 1/3 rafter / joist eaves works.

Please check my understanding... If I'm keeping existing purlins, am I right in thinking I don't need to consider any of this this?

The top half of the plan there will be a dormer which will be supported by existing ridge beam (which will probably need to be upgraded to RSJ subject to SE). But the floor shouldn't need to support the dormer roof. Is that right?

 

2 hours ago, Nickfromwales said:

If you use 2 runs of steel and 3 timbers you may be able to reduce the profile. @Gus Potter?

 

 

Good shout.

Deepseek/ChatGPT believes that if I bolt together 3 Steel plates, 10mm thick, 185mm deep. It could work.
I prefer to stack modular plates sidewards as they're lightweight and can be manhandled without equipment/machinery that I'm not confident with. 

 

1 hour ago, Iceverge said:

to have deeper beam like an RSJ or Glulam as an intermediate support protruding to make a bump of the ceiling of the room below. 

Another good idea, I'm happy to consider this but I would expect 175mm joist depth in return.

We've still got the option of 195mm deep timbers at 200 centers to fall back on (unless someone can tell me why this is a bad idea).

Ultimately, any intervention involving RSJ's must be able to make reduce the depth below 195mm.

 

Thanks again for the great discussion everyone!

Edited March 17 by Pappa

[Mr Punter]

8 hours ago, Pappa said:

if I bolt together 3 Steel plates, 10mm thick, 185mm deep. It could work.

 

Bear in mind what @Gus Potter pointed out.  You won't easily get these to bear on the party walls and there is likely a lintel over a window on the outside walls which will mess you up.

[Pappa]

2 hours ago, Mr Punter said:

 

Bear in mind what @Gus Potter pointed out.  You won't easily get these to bear on the party walls and there is likely a lintel over a window on the outside walls which will mess you up.

Thanks

This new design evades windows and doors downstairs.

 

400 centers.

bottom right quadrant is made up of 72 x 170 timbers

everywhere else 44 x 170

 

I'm not sure if the top connection of the RSJ is too close to the adjacent door.

The RSJ is basically 5 x 10mm steel plates bolted together but I'm sure a SE will be able to come up with something more efficient.

 

Thoughts please?

[Iceverge]

A question I had was do you have an ensuite or anything that requires pipes in the loft conversion? 

 

Do you have a room layout for downstairs post renovation? It might be easy to add a post design dependant ?

[saveasteading]

2 hours ago, Pappa said:

RSJ is too close to the adjacent door.

Yes. The door and the corner. You will need a brick pier or a steel column.

FYI, an RSJ (rolled steel joist) is a fairly lightweight category. You will  need a UB ( universal beam), probably about 300mm deep. My gut feeling is the cost is about £450. Add column , fabrication   erecting, perhaps foundation. £2K?

 

@Gus Potter and the other SEs on here....

could we consider plating a very big timber on that cross wall and flying it across the space to pick up 4 or so joist ends.

or bolt a steel channel there?

 

Al joists then parallel to the stair opening. 

 

[Iceverge]

The more I consider this the more I wonder why we don't use bolt together steel trusses for loft conversions. 

Say something like 50x50x5mm angle iron, precut and all holes drilled. Carry them up the stairs and bolt them together in situ. Cost might be £300 per truss but say 15 of them £4500 could be absorbed in the cost of the conversion. 

 

Relatively light weight and (guessing here) should easily be able to span the 10m in OP's house. 

 

You'd need to take an airtightness layer on the outside and Insulation on the deck like a warm roof but there's no rocket science involved. 

 

Something like this. 

 

Edited March 17 by Iceverge

[ETC]

Just a thought…….why don’t you appoint a structural engineer to help you? Just saying.

[ETC]

And what nobody has mentioned is that you really shouldn’t build joists into a party wall.