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Measuring for composite front door and threshold design

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I'm measuring for 2 replacement windows and a composite front door in my 1960s bungalow. I've been putting it off as I can't find anyone to install only (I supply) but I really need to get on with it so I can get the failed CWI sucked out and replaced ... looks like I'll have to learn how to DIY.  I think I'm going to order from Russell Timber. They're good value/spec and the windows look pretty similar to my other (expensive) windows. Door says it's 58mm Insulated Timber Veneered Composite Door hung in a 80*57mm Hardwood Laminated Frame.
I'm stuck on how to work out the threshold details so I can measure the height of the door - and the width.
I think I have to measure from the top of the existing door frame where it meets the horizontal wood bit (not sure what you'd call this? Soffit maybe). I've measured to the bottom of the sill outside at the threshold which is almost at the level of the finished floor inside (I'm not planning on a new floor, it's solid bamboo laid on top of old parquet with something like Timberlay gold underlay). I need to work out where the bottom of the door will end if that makes sense. The new door just has a thin aluminium threshold bar (see drawing) - Russell say they have different sill options.
The existing door seems to be plonked on top of messy bricks and cement/mortar (photos).  I’ve done a sketch but I don’t know what’s inside the threshold! The composite door will need to sit on something? I'd like something with better thermal properties than concrete. Do I need to remove some/all of the bricks under the existing door? I've been looking at insulated thresholds and came across Partel and Triotherm insulated door thresholds (there's also Compacfoam). Anyone used any of these products? see https://www.prodomo.ie/thermal-brackets/triotherm-as-thermal-base-for-doors/
So where do I want the door to end in terms of measuring for the height of the unit?  It's not going to be a level threshold because there's a step down outside. What would you do?
the wall is 280mm deep on the left (from outside) so will I need to place the door further back so it spans more of the cavity?
I've measured the door width outside - basically just brick to brick.  The smallest measurement is 928mm. However the internal width is 898mm and if you look at photos there's not much room to play with internally. If I ordered 928mm less 20mm tolerance = 908mm wide this is still 10mm wider than the internal measurements. Does this matter? I don't know how thick the plaster is. The plaster is wet plastered  straight on to block walls.
Any suggestions/comments would be appreciated. This is all new to me. Or suggestions for people who could install (J9 of M25). Thank you

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It's a bit difficult to draw as I don't know what's inside. It would be good to know what to expect because I don't want to be without a front door for too long when I come to replace it. Please keep me posted if you gather any knowledge on how to fit a front door. Just looking for somewhere to buy one of these thermal supports. You can get them in Ireland but they don't seem to ship to UK. https://www.prodomo.ie/online-shop/#!/Thermal-Support-70-80-100-120-140mm-x-85mm-x-1-175mm/p/163970650/category=45495629

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Sometimes you have to leave it to the experts. If you get the wrong door it is an expensive mistake.

You're trying to save money,  fair enough. But expertise has a value.

What would you think if the advice on here turned out not to work for you because of some misunderstanding?

Maybe a fitter will let you pay direct for the door, but do pay for the advice.

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Thanks @saveasteading.  I totally agree! Expertise does have a value and I'd much rather leave it to an expert. However I can't find anyone round here to fit a timber frame door that I supply - I really have tried very hard. And I'm too far away for the suppliers to install. The builder who quoted £1750 was no expert when it comes to fitting windows - he fitted some of my other windows and they are leaky (I have a Flir camera) so they're on my to do list. He'd heard of Compriband but not airtightness tape. However he wasn't keen to rake out the cavity - that would be extra. All the FENSA people didn't want to know which I understand. I think part of the problem is that currently there is a real lack of experts for retrofitting anything.  So I've concluded that I need to get on with it and give it a go but you're right - it could be an expensive mistake. If I get it wrong and have to buy a second door I could perhaps console myself that it would still work out cheaper than supply and fit from the swanky companies AND I will have learnt a lot and gained confidence.

Apart from that I am currently suffering from a job done incorrectly by experts - I paid over £4k just for the install of an MVHR system (rigid metal). I queried the condensate drain going into the gutter and pointed out that building regs and manufacturer said pipes in a loft must be lagged but he dismissed this. He did eventually lag some of the drain pipe after water was leaking out of the bottom of the machine. Then it all froze solid in the extreme cold weather and water was leaking out of the bottom of the machine. He said he'd never known something to freeze in a loft!!  This is an expert.  Anyway, the machine is damaged - heat recovery is very poor and it's not boosting - so I'm in a courteous battle with him. Also my new ASHP is not working properly so having similar hassle with that.  Do I want to go through this again with a window/door fitter? Not really.
But genuine thanks for making me reconsider and look back through all the old quotes. I've found that helpful.  And I can see I need to ask my questions in a different, more specific way as people don't want to give me advice that leads me to expensive mistakes.
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You really want the company supplying the door to do the measurement. It can't be that hard, fairly standard in the industry. Doesn't matter who then fits it, just make sure the supplier takes the liability of measuring.

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Thanks @Conor for your reply. I'm now calling this an experiment so I can allow myself to fail and risk wasting money (strange but works in my head somehow!).

I don't want to have any more people to blame/chase for bad jobs. I just want it done and done well with no shortcuts, so I'm ok if I get it wrong. The potential gain is a warmer front door, being able to progress my retrofit and boosting my confidence and ability in general building.  I'll start with the two windows down a side passage so by the time I come to the door I'll have had some practice.  I've attempted lots of DIY jobs in the past and generally manage to get there even if it takes me a while.
I'm sorted on the width and I'm getting my head round the threshold design, allowing for the doormat etc. And a timber frame is a bit more forgiving/adaptable if it all goes wrong!
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On 24/02/2024 at 21:01, Conor said:

You really want the company supplying the door to do the measurement. It can't be that hard, fairly standard in the industry. Doesn't matter who then fits it, just make sure the supplier takes the liability of measuring.


Which is great in principle. 


But they simply wont come here. So self measurement is necessary, or the windows/doors wont get done.


Yes, im limited who i can use because i need something that looks traditional, nt your run of the mill UPVC. But that just where we are.


@novice becky please take pics of the threshold when you do it.

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@Roger440 Yes, I'll take pics but you may beat me to it. I'm very slow. In which case you'll have to take pics!

I'm also wondering whether it might be a more attractive job for someone if I've measured and ordered so I can't blame them for getting it wrong (but now I'm having visions of a brand new badly fitted leaky front door).

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