Worth digging more than 200mm under floor joists?

[House man]

Hi guys,

 

Building regs want minimum 150mm with air bricks for underfloor ventilation

 

Is it worth digging to 800mm or so below with a hatch for future inspection or is there really no need?

 

This is my first property with an extension in working on so please forgive the "naiveness"

 

 

[TonyT]

Is this a new build or renovation?

can you get rid of the extra soil?

 

Do you plan on running services under the floor?

[House man]

Hi thanks for taking the time to respond

It's an extension doubling the house size, and renovation too as I will add new joists and roof to old part of house

 

Soil is removable, and my new foundations have already been put in at 900mm

[Marvin]

Digging down to 800 inside could cause heave if you know what that is...

 

My understanding of heave is like you pushing your foot down in a muddy field: the ground squeezes out under the load letting your foot down and the ground up around the sides, only on house foundations this happens slowly.

 

Also you have lateral pressures from the ground around....

Not sure about frost protection or water creating an under floor pond, or finally, what use the gap would be? 

It's usually used forairflowto keep the area dry.. .

 

Good luck

 

Marvin

Edited December 25, 2021 by Marvin

[tonyshouse]

Simple answer is ‘no it is not’ it is worth insulating the floor to better than current regs and that will likely drive a slightly lower subfloor. 

[PeterW]

Why are you using joists and not backfilling with hardcore, insulation and screed ..?

Do you need a ventilated underfloor due to ground conditions ..?

 

Could you use an insulated beam and block rather than timber ..?

 

@House man no point in excavating now any deeper to give access as the founds won’t have been designed to resist the lateral loading but it would be good to get the questions above answered. 

[ProDave]

There is no reason why a suspended timber floor cannot be insulated properly, ours is.  A solid slab is not the only way to get a well insulated floor.

 

The original question seemed to be about access and crawl space to access services.  Our previous house had nearly 2ft crawl space with access gaps through the sleeper walls so you could crawl around under the entire house, and we had 2 trap doors to get down under the floor.  That house has some pipework under the floor and some cables, think building a house the "old way"

 

Our new house has a similar void under the floor but no trap doors to get down there because we wanted a better level of air tightness.  But this time there are no services down there.  Obviously mains water electricity etc come up and into the building, but straight up in ducting.  There is just one short run of drain pipe not under but within the depth of the floor beams to serve the utility room.  That should not need access ever.  There are no water pipes or cables under the floor, those are all in the service void around the walls or in the inter floor space between ground and first floor.

[joe90]

Yes if you have no services that need access then why? A lot of digging fir nothing. Like said above why not a solid floor with loads of insulation.

[Onoff]

 I've the weirdest DIY floor build up (more like cock up) in my lounge; concrete slab of unknown depth, a 4x2 sole plate, 4x2 floor joists on that then 5/8" chipboard flooring. Vented via external air bricks it's super cold. Picture of a repair in progress when I went through the floor one time. 

 

 

This is adjacent to the dining room that has a traditional suspended timber floor. That will need building up as there's such a void underneath. The lounge slab will be dug up 

 

It will all go and be replaced by a solid, insulated floor. 

 

Best bit is they knocked through after they'd done the lounge slab and floor so there's a height difference of about 100mm and a ramp between rooms!

 

[ProDave]

3 hours ago, Onoff said:

 

It will all go and be replaced by a shiny new build house

Corrected that for you.

[Onoff]

4 minutes ago, ProDave said:

Corrected that for you.

 

That's my ever waking dream. It'll never happen as it takes two to want to do it.

[House man]

Hi guys, not gonna lie i am grateful and very moved by the amount of comments and time to try help me out.

 

I was under the impression that having there a space would be ideal incase of any leaks or if I want to make some small basement area in the future or even just a small underground storage. After reading these comments and speaking to some builder friends I've been advised that the odds of solid tested copper work leaking water is next to zero and although space would be ideal if it was there, there's no use taking out tonnes of soil at extra cost, and the house is not very wide so any digging under could cause a foundation structural issue.

 

And to answer peterW, im using joists and floorboards to match existing house and the inspector said we ideally want an airbrick every 2m

 

 

[joe90]

34 minutes ago, House man said:

im using joists and floorboards to match existing house

That’s not a requirement, all my floors are concrete/insulation, it’s what I would do for an extension even if the rest of the house was timber etc.

[PeterW]

9 hours ago, House man said:

odds of solid tested copper work leaking water

Right - first off use Hep2O and there are no joints under the floor. 
 

10 hours ago, House man said:

using joists and floorboards to match existing house

No point in doing that - just creates a problem later down the line. You may need to look at how you maintain ventilation under the existing floor if you’re blocking any air bricks.