thinking of a basement

cj maxx

hi yall.
i am (hopefully) weeks away from gettin planning for a timber frame bungalow but was thinking of putting a basement in it and wondered if anyone had thoughts/experience of same. basically i was thinking of shuttering a tank underneath the house. walls 300mm with steel @200mm with a spine wall to carry concrete slabs (hse floor). im on a hill so think i can stone around the tank and pipe water away from it (plus internal drainage). if i can configure a staircase down to it into the foor plan it could double the hse size. i am worried about damp though so any thoughts would be appriciated. its not in the planning application , would i have to reapply???

rayjdav

Yes,
Planning WILL BE required prior to you building it. Advisable to do this 1st then, when in the bag, turn your sod but not until then.
Also, an Engineer would be needed to specify the structure of the basement as it also will naturally have knock on effects to your foundations etc.

DOCARCH

As rayjdav suggests, if considering a basement you should engage an engineer experienced in the design of basements.

Normally it is advisable to dig a trial holes (to at least the depth of the proposed basement) and a number of bore holes, to assess where the water table may be, and have soil (character) tests carried out. Depending on the outcome of these tests, then a suitable besement can be designed/specified.

Also as rayjdav says, any basement really should be part of your planning application.

unclebill98

cjmc wrote: »

hi yall.
i am (hopefully) weeks away from gettin planning for a timber frame bungalow but was thinking of putting a basement in it and wondered if anyone had thoughts/experience of same. basically i was thinking of shuttering a tank underneath the house. walls 300mm with steel @200mm with a spine wall to carry concrete slabs (hse floor). im on a hill so think i can stone around the tank and pipe water away from it (plus internal drainage). if i can configure a staircase down to it into the foor plan it could double the hse size. i am worried about damp though so any thoughts would be appriciated. its not in the planning application , would i have to reapply???

As others have suggested i'd agree. I did a 2k s/f basement.

My 2 cents is really make sure your engineer/builder has plenty of experience with basements. From my experience the following is paramount. They have to know there stuff, from concrete spec to steel layout/order forms etc. Are you going to have all the walls shuttered or doing interior walls with blocks layed flat. A good engineer will get it right.

At every stage get the engineer to inspect. This will cost more but save you thousand if errors are found. Shutters can move leaving some interior walls slanted, expensive to correct.

Tanking, the single most important thing to get right. No room for error with this. Use the highest spec tanking you can afford. Get it done by a pro, do not do it yourself or get someone to do it for cash etc. If this fails your f'd for putting it right cheaply. Make sure your engineer inspects it before back filling it. I used insulation boards and plywood to cover mine during back fill and spent the 5-6 hours on site when the digger was filling it it. If done wrong or with the wrong grade fill you can have sinking paths etc. Its possible to puncture the tanking but if noticed it can be patched up. Make sure its over lapped correctly and that the top of the basement is correctly set up to take the house above in regards to membranes.

It can take up to 2 years to fully dry out. I used 2 de-humidifiers for 2 months. Stopped and just let nature to its thing. All fine now....!

If your tiling floors above the basement then basic sound proofing of the basement ceiling will help take the noise out.

Basically, fantastic when done right and a complete waste of money if done wrong......

cj maxx

thanks guys. expected planning requirement so no surpise there !
getting an engineer experienced in basements could be tricky as i dont know anyone who has put a basement in ,so if you know an suitable engineer in north monaghan could you pass details on ( not sure of forum rules etc re this )
as for the shuttering , would plan doing the work myself ,( some experience), but specs i would hope to get from the engineer. (keys, waterbar, steel, concrete grade, channel, etc .
the 4 exterior walls and spine wall would be concrete and any additional walls block ,
i would probably run 2 lines of girders length wise (either side of the spine) packed to support the slabs and above the slabs insulation + underfloor heating.
insulation around and below the tank also
an idea(though getting expensive) and any/all help accepted !

galwaytt

I have an r/c basement, under my timber home, with a suspended deck (no internal walls). R/c deck is 200mm iirc.

First things first: it's expensive.
Second, Under no circumstance should you take any shortcuts on the tanking/bituthene/etc.
Thirdly, if at all possible, I'd go with a 300Kpa XPS insuation outside the tanking, and backfill against it. All that concrete is cold, even in the hottest of hottest weather.

cj maxx

thanks for the advice. insulation and drainage are biggest worries, plus radon barrier. think i need to see one and go through problems/pluses

Euripides_

galwaytt wrote: »

I have an r/c basement, under my timber home, with a suspended deck (no internal walls). R/c deck is 200mm iirc.

First things first: it's expensive.
Second, Under no circumstance should you take any shortcuts on the tanking/bituthene/etc.
Thirdly, if at all possible, I'd go with a 300Kpa XPS insuation outside the tanking, and backfill against it. All that concrete is cold, even in the hottest of hottest weather.

Apologies for resurrection!

Thinking of going DOWN for a basement on a house build. Rear of house will be on a decling hill, so extra(basement) floor when viewed from rear.

What is the expensive part of a basement? Is it just the extra inspection/ planning/ water table inspection, or something else?

Also when u guys say say tanking, presume u mean septic tank drainage/ perculation etc?

Sorry for q's, not a builder!

Euripides_

cjmc wrote: »

thanks for the advice. insulation and drainage are biggest worries, plus radon barrier. think i need to see one and go through problems/pluses

Did you ever see an engineer??

Johnnyhpipe

jay_k wrote: »

Apologies for resurrection!

What is the expensive part of a basement? Is it just the extra inspection/ planning/ water table inspection, or something else?
!

Cost of inspections and planning are almost negligible in comparison to actual basement construction costs. Consider the sheer volume of earth to be excavated and carted off site, the temporary works required to stabilise the ground during excavation, the massive extra over if you encounter bad ground, bedrock or a high water table, the sheer volume of steel reinforcement and concrete, the underslab drainage and services, maintaining local groundwater pressures so as not to affect ground conditions beneath nearby structures etc etc etc etc etc.

Design/planning/inspection costs are a drop in the ocean compared to the above mentioned construction costs.

As for tanking...its basically a waterproof membrane around the entire surface area of the basement. Nothing to do with a septic tank. Your structural engineer will spec this at design stage.

Euripides_

Johnnyhpipe wrote: »

Cost of inspections and planning are almost negligible in comparison to actual basement construction costs. Consider the sheer volume of earth to be excavated and carted off site, the temporary works required to stabilise the ground during excavation, the massive extra over if you encounter bad ground, bedrock or a high water table, the sheer volume of steel reinforcement and concrete, the underslab drainage and services, maintaining local groundwater pressures so as not to affect ground conditions beneath nearby structures etc etc etc etc etc.

Design/planning/inspection costs are a drop in the ocean compared to the above mentioned construction costs.

As for tanking...its basically a waterproof membrane around the entire surface area of the basement. Nothing to do with a septic tank. Your structural engineer will spec this at design stage.

Cheers for the very helpful reply my friend. Yeah thats understandable. I dont mind costs I can plan and account for.

Water table and unforeseen bedrock issues are the ones that would make me apprehensive. Obviously still in the early stages. But we are fairly positive in the design of the house we want

Johnnyhpipe

No probs, once you appoint an engineer the first thing he will recommend will be a site investigation. This will determine soil types/water table level/bedrock depth (if relevant) etc. This will allow you to proceed to tender with greater cost certainty.

galwaytt

I'm not on a hill, and went down for mine. But hit rock almost straight away and needed a rock breaker for a week. More €€€€.

Still worth it imho - will do it on the next one as well.

Euripides_

galwaytt wrote: »

I'm not on a hill, and went down for mine. But hit rock almost straight away and needed a rock breaker for a week. More €€€€.

Still worth it imho - will do it on the next one as well.

Absolutely. Makes perfect sense in Ireland with land so scarce/expensive.

hexosan

My basement is fully enclosed on all sides. We dug down into peat and grey marl clay for 3m before reaching a sand gravel base. We laid compacted hardcore, binding and a layer of sand.
Then shutters for the raft base were erected and then the tanking contractors arrived supervised by a sika employee and Sika Proof A-08 (approx €20/m2) was laid on top of the base.
Steel to the engineers spec along with starter bars were installed & the necessary drainage pipe work.
Instead of the hydrophilic strip or PVC water bar I used Sika's new metal strip waterbar (it installs in a fraction of the time and the contractors swore by it claiming they'll never use either of the other two types again). Then concrete was poured and floated.
Shutters were erected for the walls and I used oktagon plug (German product rated to 10bar pressure, PM for details) to seal the shutter bolt holes (much much easier than the rubber grommets). I also had a water resistant admixture included in the concrete mix.

Sika do their own certified water tight concrete mix but this is approx €130 per cube nearly double the cost of standard concrete.

After the walls cured the tanking installers returned (again supervised) they rolled bitumen on the walls followed by Sarnivap 500 (approx €20/m2). Only thing they didn't make clear was this had to be back filled within a week which put me under severe pressure.
I used 6mm cori board placed against the walls and taped to protect the membrane during backfilling.
I installed a French drain all around the perimeter of the raft foundation fed back to a 6500L tank buried below the basement. This pipe was wrapped in a geo textile membrane which runs all the way up the wall and I back filled approx 400mm out from the wall the full height with washed round drainage gravel.
Internally in the basement I have a 200mm cavity which will be fully filled with bead insulation and then a 100mm block wall.
I placed an 80mm drainage pipe again this wall which in buried in 200mm eps floor insulation and it feeds back to a sump pit in the plant room.
The cavity has holes drilled in a few location to drain into this drainage pipe below the floor screed level should there ever be a leak.

So hopefully with this belts and braces approach I shouldn't have any issues with water ingress.
As others have said there is nothing and I mean nothing cheap about building a basement. You really need to make sure everything is spot on as it'll cost a lot more to fix than it ever cost to install correctly.
Whoever pours the raft have them do the walls as well, make sure you use a registered company to install the tanking and get receipts and save all emails.

I have contact details by PM for the tanking installers who work nationally & the sika rep who was a pleasure to deal with (he even fitted the water bar on site with my shuttering contractors)

Hope all this helps as I found it near impossible to find info when I started out.
Mods I've named the products as Sika is a large multinational company and doesn't need my free advertising hope this is ok.

Hex.