Tolerances to expect

[MortarThePoint]

I've worked in the world of precision engineering for many years and as a self builder know that I need to adjust my expectations from dealing in fractions of a mm when it comes to tolerances. I'd be interested in people's views as to tolerances on the following (my current naive expectation in brackets):

• Foundation lateral placement (ignoring due to local side collapse) (+/-50mm over 5m)
• Foundation level (+/-5mm over 10m)
• Brick or block lateral placement (+/-10mm over 5m)
• Brick or block level (+/-5mm over 10m)
• Brick or block plumbness (+/-5mm over a floor)
• Lateral pipe of socket placement (+/-25mm)
• Window of door opening dimensions (+/-5mm for normal sizes)

Feel free to add to the list as there are doubtless many areas I haven't thought about. My expectations have come from what I feel is practical, but I can't defend them much beyond that.

[Tennentslager]

https://www.building.govt.nz/assets/Uploads/projects-and-consents/guide-to-tolerances/guide-to-tolerances.pdf
 

Here is a guide from NZ

They have earthquakes so ...

[Jeremy Harris]

I had much the same problem with tolerances, and when I set a 2D positional tolerance on the position of our soil pipe, where it comes up through the slab, of +/- 30mm the ground works chap thought I was joking.  I should have made the tolerance even tighter as it turned out, as it ended up being too close to the inside face of the wall and caused me a fair bit of work putting in a small offset to allow the internal soil stack to fit OK.

 

Our window openings were specced to be 5mm bigger all around than the frames, something that the window supplier came around to check.

[MortarThePoint]

8 minutes ago, Tennentslager said:

https://www.building.govt.nz/assets/Uploads/projects-and-consents/guide-to-tolerances/guide-to-tolerances.pdf
 

Here is a guide from NZ

They have earthquakes so ...

That's some serious bedtime reading!

[MortarThePoint]

5 minutes ago, Jeremy Harris said:

I had much the same problem with tolerances, and when I set a 2D positional tolerance on the position of our soil pipe, where it comes up through the slab, of +/- 30mm the ground works chap thought I was joking.  I should have made the tolerance even tighter as it turned out, as it ended up being too close to the inside face of the wall and caused me a fair bit of work putting in a small offset to allow the internal soil stack to fit OK.

 

Our window openings were specced to be 5mm bigger all around than the frames, something that the window supplier cam around to check.

 

I want to be sure I have realistic expectations as it saves grief and means that I can tweak to accommodate the appropriate tolerance in advance.

[nod]

You can do a lot of the setting out yourself 

Ive set out with lasers EDM etc for years 

It’s quite surprising how many trades still use chalk lines and string lines 

Just because they have always done it tgat way 

[JFDIY]

If you've worked in precision engineering (as I do), then unless you get a very good builder then book a long holiday and come back once it's done, you will be mostly disappointed otherwise. 

 

I've ended up re-visiting so much of our place, getting there now though, and I've learnt not to expect (total) perfection. 

 

 

 

 

[MortarThePoint]

I've found some NHBC guidance for some of these:

    http://www.nhbc.co.uk/NHBCpublications/LiteratureLibrary/Technical/filedownload,15912,en.pdf

but it's to do with finishes really.

• Brick or block level: +/-8mm over 5m, +/-12mm over further
• Brick or block plumbness: 8mm over 5m, 12m for higher but 8mm per storey

 

[MortarThePoint]

4 minutes ago, nod said:

You can do a lot of the setting out yourself 

Ive set out with lasers EDM etc for years 

It’s quite surprising how many trades still use chalk lines and string lines 

Just because they have always done it tgat way 

 

The groundworks crew used something like that to spray out the foundations. I measured it with a tape measure over the following weekend and was really impressed. It was about +/- 30mm typically and I was using a tape measure over rough ground so some of that will be on my measurement. The sprayed line has a width anyway. I did notice that they had misread a dimension though and lined up two walls that weren't supposed to be.

[MortarThePoint]

6 minutes ago, JFDIY said:

If you've worked in precision engineering (as I do), then unless you get a very good builder then book a long holiday and come back once it's done, you will be mostly disappointed otherwise.

 

I'd prefer to readjust. It's always been a pet peeve of mine when people have not thought about tolerances and default set very tight. I saved a previous company I worked for >£30k on a 10,000 fps high speed video camera that was proposed to confirm that an event happened within less than 100ms. I was able to show it using my phone at 120fps and we went with a rig using a 1200 fps point and shoot camera with an LED attached to a calibrated signal generator setting a reference timing. The event was always faster than 50ms.  Would have been cool to play with the 10,000 fps camera though.

[Vijay]

22 minutes ago, JFDIY said:

If you've worked in precision engineering (as I do), then unless you get a very good builder then book a long holiday and come back once it's done, you will be mostly disappointed otherwise. 

 

I've ended up re-visiting so much of our place, getting there now though, and I've learnt not to expect (total) perfection. 

 

 

 

 

 

Couldn't agree more, maybe an idea to meet some builders you're thinking of using and ask what tolerances they work too. Ones I've known have thrown their levels around like it wasn't an important tool. I've seen work by "professional" brickie gangs that the walls looked straight enough until you put a level next to it - shocking!

[Russell griffiths]

1. Reasonable 

2. No chance. 25-50 if they can be bothered. It will be better if the lads laying the concrete are also laying the blocks. 

3. Should be spot on if you use a surveyor, bricky and a tape, could be anywhere I have seen a wall built completely on the wrong side of a line, so 100 mm out of place,  check it before proceeding. 

4. Are you talking below dpc or face work. 

5. As above

6. I’ve actually seen pipes the wrong side of a wall so toilet pipe in the room next door. 

 

My answers are based on 30 years of working in the real world 

not downloading the latest government guidelines. 

I would do two things in your case

1. forget your an engineer, it’s a house not a spaceship 

2. Get all critical points marked on a set of plans including pipe locations, then get a surveyor to mark these out as the job proceeds, choose the most local company to your site so a visit to site can be done in under an hour, I had 4 site visits for setting out and the bill was about £370. 

 

You are looking at some points with to much detail but have not listed things that in my opinion are far more important. 

 

Footing level is not not important at all, a bricky can cope with 50mm out of level and in 3-4 courses of blocks underground he will correct it no problem. 

 

However you have no mention of floor level, this is a far more critical point, there are many on here that have been dissatisfied with the finishing on a floor with many bags of levelling compound needed. 

Also door threshold levels, these are crucial to getting a seamless finish. 

 

I would relax a bit on tolerance until you get up to dpc

 

get it set out correctly and triple check it. 

 

 

A simple thing but something people may not be aware of, your architectural drawings come in layers that are drawn on at stages as the architect designs the house. 

For a couple of extra quid you can get the layers removed to give you simplified drawings 

So you can get all layers removed to just give you a foundation layout

then one that just shows corners for blockwork, which can incorporate pipe locations

[MortarThePoint]

4 minutes ago, Russell griffiths said:

1. Reasonable 

2. No chance. 25-50 if they can be bothered. It will be better if the lads laying the concrete are also laying the blocks. 

3. Should be spot on if you use a surveyor, bricky and a tape, could be anywhere I have seen a wall built completely on the wrong side of a line, so 100 mm out of place,  check it before proceeding. 

4. Are you talking below dpc or face work. 

5. As above

6. I’ve actually seen pipes the wrong side of a wall so toilet pipe in the room next door. 

 

My answers are based on 30 years of working in the real world 

not downloading the latest government guidelines. 

I would do two things in your case

1. forget your an engineer, it’s a house not a spaceship 

2. Get all critical points marked on a set of plans including pipe locations, then get a surveyor to mark these out as the job proceeds, choose the most local company to your site so a visit to site can be done in under an hour, I had 4 site visits for setting out and the bill was about £370. 

 

You are looking at some points with to much detail but have not listed things that in my opinion are far more important. 

 

Footing level is not not important at all, a bricky can cope with 50mm out of level and in 3-4 courses of blocks underground he will correct it no problem. 

 

However you have no mention of floor level, this is a far more critical point, there are many on here that have been dissatisfied with the finishing on a floor with many bags of levelling compound needed. 

Also door threshold levels, these are crucial to getting a seamless finish. 

 

I would relax a bit on tolerance until you get up to dpc

 

get it set out correctly and triple check it. 

 

 

A simple thing but something people may not be aware of, your architectural drawings come in layers that are drawn on at stages as the architect designs the house. 

For a couple of extra quid you can get the layers removed to give you simplified drawings 

So you can get all layers removed to just give you a foundation layout

then one that just shows corners for blockwork, which can incorporate pipe locations

 

I've seen there is quite a big gap between theory and practice so it is very useful to have real world numbers. My "brick or block level" was thinking above DPC, but I would hope the plumbness applies below DPC too.

 

I don't find it stressful dealing with large tolerances as long as I have a feel for what they should be. Measuring something say 30mm out and having no idea what is reasonable is where the stress comes in. I'm cool if a room ends up 30mm smaller, but not if a doorway does. Some things are largely aesthetic (brick level and perp alignment every other course), some things functional (having a crapper in the corner of the living room rather as intended next door in the toilet) and some structural (plumbness).

[Jeremy Harris]

My view was that there were some tolerances that were super-critical and some that really didn't matter too much.  A factory manufactured panel timber frame build needs more thought than block and brick. I think, as there's not much scope for being able to change things once the frame is built.  This is what caused my soil pipe problem, as this poked up through the floor slab it needed to come up into a clear space close to a wall, but it ended up about 20mm too close to the wall, just because the tolerance wasn't tight enough on its position.

 

My experience with window openings was that the window company didn't start making them until the house was up and they could come around to double check the sizes of all the openings.  My guess is that they do this because they have learned over the years that the as-built dimensions may not tally with the drawing dimensions.

[ProDave]

I dug my own foundations. I set it out by marking first with pegs and string and then with ground marker paint, the centreline of each trench.  The builders doing the pour and blockwork said it would never be accurate enough and predicted they would have to do some rework.

 

As it happened when they got the surveying equipment out there was just one trench a little off line, and that was because I could not get the digger in quite the right place.  5 minutes with a spade had the side shaved off that slightly errant trench and all was well.

 

Slightly digressing I watched an episode of Grand Designs Australia yesterday.  There the building inspector found the formwork for the pool was "50mm outside the building line" so he made them dismantle it all and re do it.  I would be mighty pi$$ed off if my building inspector was such a jobsworth.  It would have made no practical difference if the pool had ended up 50mm too big.

[Jeremy Harris]

We had a complaint about the height of our ridge once the house was up.  Luckily I still had access to a Total Station and the OS steel pin from when they surveyed the lane was still visible by the drive entrance.  The planning chap came out, we set up the Total Station over the pin and showed that the ridge height was slightly lower than as shown on the approved plans.  During the course of the conversation with the planning chap, I asked him what sort of leeway they allowed and he reckoned that anything up to about 100mm out would generally be absolutely fine.  No idea if that's a widespread rule, or just his personal judgement, but it seems reasonable to me.

[MortarThePoint]

16 minutes ago, ProDave said:

I dug my own foundations. I set it out by marking first with pegs and string and then with ground marker paint, the centreline of each trench.  The builders doing the pour and blockwork said it would never be accurate enough and predicted they would have to do some rework.

 

As it happened when they got the surveying equipment out there was just one trench a little off line, and that was because I could not get the digger in quite the right place.  5 minutes with a spade had the side shaved off that slightly errant trench and all was well.

 

Is it normal to survey between dig and pour of the foundations to check that they are correctly dug?

 

16 minutes ago, ProDave said:

Slightly digressing I watched an episode of Grand Designs Australia yesterday.  There the building inspector found the formwork for the pool was "50mm outside the building line" so he made them dismantle it all and re do it.  I would be mighty pi$$ed off if my building inspector was such a jobsworth.  It would have made no practical difference if the pool had ended up 50mm too big.

 

That's nuts!

 

 

[ProDave]

2 minutes ago, MortarThePoint said:

 

Is it normal to survey between dig and pour of the foundations to check that they are correctly dug?

 

That's nuts!

 

Probably not normal, but it was an unusual situation that I did the digging and the builder did the pour, foundation build up and timber frame.

 

They wanted to be sure the trenches were in the right place, othwerise they would not be able to build it correctly.

[Miek]

4 minutes ago, MortarThePoint said:

 

Is it normal to survey between dig and pour of the foundations to check that they are correctly dug?

 

 

That's nuts!

 

 

Yes, the BCO will want to check the subsoil before pour, but they won't care about dimensions, that's your job

[MortarThePoint]

Just to be clear the "That's nuts!" comment was about the swimming pool ?

 

My groundworks guys dug to sprayed lines and the only measuring going on was for level/depth. BCO didn't go down and measure any stiffness, but gave things a good eyeball.

Edited May 9, 2020 by MortarThePoint

[Pete]

I had the timber frame company do the Passive slab so everything fitted but that did not stop me from checking everything. When I told them that one of the drains was outside the wall ( the lower roof has two vertical down pipes hidden inside two columns so they cannot be seen from the outside so had to be bang on) the guy was convinced and checked it again and said it was fine. Numerous discussions later I told him to move it and I would stand the cost of altering it if I was wrong, which I wasn't. I think a lot of self builders know their house back to front and inside out which can be a bit problematic with some builders but we are a different  breed I think. Be vigilant is allied can say!

[dnb]

1 hour ago, Pete said:

Be vigilant is allied can say!

 

That is good advice. Anyone can make a silly mistake at any stage. It's your job to spot the ones that will make life miserable for you. My build has had its fair share of these. One sticks in my mind where the architect added up the blocks etc in the foundations and wrote the finished floor level on the plans as 500mm from the ringbeam top, but had 600mm of material in the buildup! We spotted it early enough, fixed it and had a good laugh about it and how building control hadn't seen it.

 

I am trying to get all my blocks to support the SIPS panel sole plate level to within 3mm and the lateral placement to within 3mm for lengths and 5mm for diagonals. This is very slightly tighter than the SIPS team have asked for but it gives me scope for measurement error. The ring beam was spot on on average, but undulated +/-8mm in height (worse than the 5mm I desired, but by no means a problem) and 15mm laterally (I budgeted 25mm, but it really wasn't critical since the beams were 600mm wide for a 400mm wall) We got the first row of blocks down to +/-3mm in height and +/-5mm laterally then found the floor beams were really inaccurate and errors accumulated really quickly.

[Pete]

27 minutes ago, dnb said:

 

That is good advice. Anyone can make a silly mistake at any stage. It's your job to spot the ones that will make life miserable for you. My build has had its fair share of these. One sticks in my mind where the architect added up the blocks etc in the foundations and wrote the finished floor level on the plans as 500mm from the ringbeam top, but had 600mm of material in the buildup! We spotted it early enough, fixed it and had a good laugh about it and how building control hadn't seen it.

 

I am trying to get all my blocks to support the SIPS panel sole plate level to within 3mm and the lateral placement to within 3mm for lengths and 5mm for diagonals. This is very slightly tighter than the SIPS team have asked for but it gives me scope for measurement error. The ring beam was spot on on average, but undulated +/-8mm in height (worse than the 5mm I desired, but by no means a problem) and 15mm laterally (I budgeted 25mm, but it really wasn't critical since the beams were 600mm wide for a 400mm wall) We got the first row of blocks down to +/-3mm in height and +/-5mm laterally then found the floor beams were really inaccurate and errors accumulated really quickly.

You have just reminded me of our architects **** up. We had the exterior drawings for quite some time and the internal measurements and I noticed there was no shadow on the cantilevered vertical section which equated to 300mm of floor space which the architect had missed completely. It was sorted before the timber frame was made thank God!

[MortarThePoint]

I measured how they had sprayed the foundations out, but can only imagine what they would have made of it if I had started measuring the trenches as dug before the pour. Not particularly practical with a tape measure as well due to tench side collapse potential. Well since it has been poured and gone off I have been out with a tape measure ? I think I'd better start a new thread for thoughts on that.

[Brickie]

On 09/05/2020 at 09:45, Russell griffiths said:

Footing level is not not important at all, a bricky can cope with 50mm out of level and in 3-4 courses of blocks underground he will correct it no problem. 

I’d really rather not,though. 
Seriously,armed with extremely basic kit (a dumpy & two pieces of 4x2srewedto make an inverse ‘T’) is there any reason footings shouldn’t come in within 20mm for level?