Marrying joist levels

[chris_d]

Dear Members,

I've been building an extension to my house over the past few months and I'm almost at joist level (intermediate floor). Unfortunately, the joists on the existing building are out of level by 38mm over approx 7.5m. Furthermore, building regulations now dictate that new joists must bear on 'complete masonry units' so blockwork cannot be notched to allow for discrepancies.

I am trying to decide how to lay the next few courses of blocks/coursing bricks to bring the new walls up to joist level. The old and new joists run parallel to each other at 400mm centres and the transition point between old and new will form a single (larger) room. Due to the difference in level of the old joists, I believe that I have four options:

1). Sit new joists in the same plane as the old hence no shimming required to regularise floor planes.
2). Sit new joists at or above the highest point of the old joists and shim above old joists to regularise floor planes;
3). Sit new joists at or below the lowest point of the old joists and shim above new joists to regularise floor planes;
4). Equalise the error, sitting the new joists half-way between the highest and lowest point of the old joists and shim new flooring as appropriate;

Option 1) makes me recoil in disgust as this would severely skew my coursing levels which I would then need to make up for the wall/roof plate in just three courses (215mm / course) and the 38mm discrepancy seems just too big to live with given I usually work to 3.25mils in the day job, hence dismissed! Option 2) has been dismissed to save the hassle of lifting and shimming the existing floor boards. Option 3) is reasonable except that taking the lowest point means that I must cut-down 65mm thermalite coursing bricks to 40mm which is PITA given the 21m course run. Hence I favour option 4) as this can be achieved without the cutting of blocks/coursing bricks but this results in a discrepancy of between 0mm and 19mm over approx 3.75m. Remember that I would have to progressively resolve that discrepancy across joist centres (400mm).

Could anyone with more experience or wisdom please validate or contradict my conclusion?

Now, I freely admit that I have spent way too much time over analysing this and most builders would just pick the nearest convenient coursing height and defer the problem to the floor fitter! However, since I'll be the floor fitter, installing engineered oak, then I'd like to make my life easier with a bit of forethought now!

Thanks in advance,
C

 

[hanser]

Chris

Discuss the options with the Building Control bloke. They are generally both helpful and practical.

 

[RogerS]

Hi Chris

I second what Hanser says.

You are very fortunate to live in Herefordshire! Apart from the glorious countryside, you'll find that the guys at Building Control are some of the most pragmatic and helpful that you will find. The Herefordshire website lists the name of the BCO responsible for your area..but you may already be on the case there given that you're up to joist level!

Good luck

Roger

 

[chris_d]

Dear Roger & hanser,

Thank you for responding. My BCO has been very helpful and in this case I actually agree with his view that notching the relatively weak thermalite block is unacceptable due to the potential for internal voids/faults within the block being accentuated by the notch and point load resulting in a later failure. He went on to say that since the pull-out force of thermalite is very low then lag bolting a ledger joist congruent to the existing floor plane is not acceptable as the bolts can easily work loose.

The BCO has suggested either laying the new joists 'on the wonk' to match the old or shimming the new flooring to meet the old.

I was hoping that an old hand might at least steer me down the least painful route with the benefit of hindsight.

Cheers,
C

 

[Jacob]

Can't you put some sort of wall plate in? Or notch some heavier non-thermalite blocks under each joist?
38mm in 7.5 metres is not a lot - maybe just ignore it, nobody will notice (unless it's the billiards room :shock: ).

 

[RogerS]

.....He went on to say that since the pull-out force of thermalite is very low then lag bolting a ledger joist congruent to the existing floor plane is not acceptable as the bolts can easily work loose.

.....

Would that be bolts that are screwed in as opposed to bolts and a nut? If so then couldn't you bolt your ledger joist using bolts and nuts ...even maybe put a steel plate running along the outside of the thermalite blocks and then cover it with a 'feature' to hide it?

 

[Carlow52]

Not sure I am following the different levels here but are steel joist hangers an option.

The other suggestion is to cast an in-situ concrete ring beam with some rebar, that would negate the need to notch anything

 

[chipchaser]

As I understand your post:
The junction between old and new floor will be within the new larger room, possibly down the middle.
The two floors as finished must be level with each other, at least where they meet.
You do not want to disturb the old floor.
It is not clear whether you intend overlaying the old floor with matching engineered floor.
I don’t understand how your new masonry coursing will relate to the levels required to support the joists but this is probably not necessary as you will resolve that.

Things I would consider if it were my house:
You don’t say whether the existing out of level floor is a problem to you, Maybe it was never an issue until this problem arose?
38mm in 7500 is very little, less than 1 degree.
Do you want to progressively remove the out of level in the new floor as you move away from the junction with the old?

I would avoid cutting bricks/blocks on site based on my experience of bricklayers site work, e.g. hack it off and make up the gap with mortar. However, I think you are not likely to find significant voids in Thermalite and you can saw them easily.

There are many ways of skinning the cat and circumstances alter cases.

If you don’t intend to overlay the existing then adopt either solution 1 and forget the day job or solution 3 and fix firrings on top of the new joists say 6mm tapering to 44mm. You could also do option 4 and cut two firrings in one go

If you are overlaying the existing floor then either option 2 or 3. I suggest option 2 but you don’t need to lift the existing floor as long as it is securely fixed. Simply cut firrings and lay those on the top of the existing floor over the existing joist lines. Because they may taper from 38 mm to zero you might want to glue the thin ends to the existing boards.

Obviously based on how I understand your post and really your choice depending on how comfortable you feel with the various solutions.

Regards
Graham

 

[Digit]

Dunno. But if the Squirrels are now getting tooled up I'm gonna have to get some serious weaponry!

Roy.

 

[chris_d]

Dunno. But if the Squirrels are now getting tooled up I'm gonna have to get some serious weaponry!

Roy.

Surely you have an old copy of the Daily Mail to roll into a cosh?

 

[chris_d]

Can't you put some sort of wall plate in? Or notch some heavier non-thermalite blocks under each joist?
38mm in 7.5 metres is not a lot - maybe just ignore it, nobody will notice (unless it's the billiards room :shock: ).

Thank you Jacob. A wall plate (assume timber ?) is not possible as this is a solid 9" load bearing wall. Sadly, anything but a 'complete masonry unit' is contrary to my BCO's interpretation of current BRs. The degree to which the old floor is out of level across its 7.5m span doesn't actually bother me. It is the potential 38mm in 400mm discrepancy that could occur between the old and new floor (at one end only) if for example I sit one end of the new joists at the high/low point of the old and the new joists truly level themselves.

Would that be bolts that are screwed in as opposed to bolts and a nut? If so then couldn't you bolt your ledger joist using bolts and nuts ...even maybe put a steel plate running along the outside of the thermalite blocks and then cover it with a 'feature' to hide it?

Thank you Roger. Yes, a lag bolt is a glorified screw and your proposition is technically viable. However, drilling multiple holes in 4N thermalite blocks at height doesn't appeal neither does the need to hide the plate/nut projection since I'm intending to use external insulation with EML and polymeric render over.

Not sure I am following the different levels here but are steel joist hangers an option. The other suggestion is to cast an in-situ concrete ring beam with some rebar, that would negate the need to notch anything

Thanks Carlow. I haven't decided whether I'm building the joists in or using hangers since that is dependent on the relative ease/difficulty in obtaining the actual coursing level required for either method. Furthermore, hangers must also bear on 'complete masonry units' as per BRs. I've tried to source adjustable masonry hangers but to no avail as they would solve my problem since I could choose a level for coursing convenience and adjust for the exact joist level/plane on the hanger.

The ring beam would work but is more hassle/costly than shimming flooring.

As I understand your post:
The junction between old and new floor will be within the new larger room, possibly down the middle.
The two floors as finished must be level with each other, at least where they meet.
You do not want to disturb the old floor.
It is not clear whether you intend overlaying the old floor with matching engineered floor.
I don’t understand how your new masonry coursing will relate to the levels required to support the joists but this is probably not necessary as you will resolve that.

Things I would consider if it were my house:
You don’t say whether the existing out of level floor is a problem to you, Maybe it was never an issue until this problem arose?
38mm in 7500 is very little, less than 1 degree.
Do you want to progressively remove the out of level in the new floor as you move away from the junction with the old?

...I think you are not likely to find significant voids in Thermalite and you can saw them easily.

Regards,
Graham

Thank you Graham - you have a thorough understanding of the problem space! I do intend to overlay the old floor and also fit weatherproof boarding to the new joists before installing the engineered oak flooring on top.

Sadly, I've been using a thin bed mortar system (a pre-mixed fine sand/cement mortar that gives a 3mm bed) which means that I haven't got much scope for adjusting coursing levels. I can do the next few courses in normal mortar but I left it a bit late before discovering the old joist levels and when coupled with some interesting coursing issues around steel Catnic lintels of various depths my main variable to make up levels is cutting blocks. The coursing levels/combinations dictate that options 1-3 require the cutting of blocks/bricks, however, option 4 by complete fluke doesn't! Your suggestion to glue the thin firrings has pretty much solved the shimming pain with option 4.

You are correct, I didn't previously identify / don't have a problem with the existing floor and I have no real desire 'to progressively remove the out of level in the new floor as you move away from the junction with the old'. I do however wish for there to be a seamless join between the old and new flooring.

I have indeed found many voids in the 4N thermalite blocks and they are an absolute PITA to saw - I mistakenly chose 4N blocks which are significantly more dense than the standard 2.9N type that you are probably thinking of. I was swayed by the sound deadening properties of Hanson's Partywall 4N blocks; the 7N high-strength type that I used below DPC to bear the suspended concrete floor were near impossible to saw. Before someone suggests to simply switch blocks to a less dense type - that is frowned upon due to associated differential expansion problems except where separated by a natural slip plane eg DPC / expansion joint.

This has to be the longest post I've ever written - thank you for getting this far through my pedantry!

Cheers,
C

 

[RogerS]

Chris

Just went back to the OP. Am I missing something? 38mm in 7.5m. OK - you're using a thin bed mortar mix. Just make it a bit thicker over the last few courses and build up/lose that 38mm. Over that length, to alter the top of the course by 38mm is no-brainer, surely. Job done.

 

[chris_d]

Chris

Just went back to the OP. Am I missing something? 38mm in 7.5m. OK - you're using a thin bed mortar mix. Just make it a bit thicker over the last few courses and build up/lose that 38mm. Over that length, to alter the top of the course by 38mm is no-brainer, surely. Job done.

Sadly not that simple - wish it was!

I think that I have explained the situation poorly. The old joists run in two separate co-planar spans, where each span bears on an external wall and shares a bearing on an internal partition wall. The 38mm difference in level of the old joists is between the two opposing external walls which are 7.5m apart. Now the external walls of the extension run parallel to the old. The new joists also run in two spans, bearing on the new external walls and a shared bearing on a uinversal beam between them.

Therefore, the 38mm difference is wall to wall not along a single linear course that you are describing. Making up a difference isn't a problem with thin-bed mortar as you have explained but losing it is. Since I have just two and a bit courses left to joist level and if I wanted to marry up exactly with the wonkly plane of the old joists then in my case I would need to loose some difference on one side. Since new joists must bear on complete masonry units then my only option would be to cut blocks.

Hopefully that has made it clearer. The situtation is such that I can fit the new joists truly level with three courses of 215+3+65+3+215 without the need to cut blocks/bricks but resulting in a difference in level between the old and new joists of between -19 and +19mm. That would only be across the 400mm separation. Hence at the transition point between old/new joists I'll have to do some shimming to give a smooth continution of flooring.

Kind regards,
C