d.i.y french drain help needed

byrneos

hello.
am getting external insulation fitted soon and want to take it down below the floor slab.i want to put in a french drain at the same time.so any advice? whats the best piping to use?what grade of gravel? degrees for slope that type of thing...thanks in advance.

glenntec

byrneos wrote: »

hello.
am getting external insulation fitted soon and want to take it down below the floor slab.i want to put in a french drain at the same time.so any advice? whats the best piping to use?what grade of gravel? degrees for slope that type of thing...thanks in advance.

why do you want to go below floor level?? are you trying to stop cold bridging?are you doing this around all sides of your house? there would be an issue with your damp proof being covered..... a little more info or a link to the product you are using would be helpful

byrneos

glenntec wrote: »

why do you want to go below floor level?? are you trying to stop cold bridging?are you doing this around all sides of your house? there would be an issue with your damp proof being covered..... a little more info or a link to the product you are using would be helpful

yes its to advide cold bridging and around all sides
there is no damp proof course in the house 1930's mass concrete

don't have link handy will try find it ,its only the drain im doing diy the rest is contracted

Graaaaa

I've been looking into options for the same thing myself - 1930's mass concrete corporation built house. My concern with external insulation was what to do with at the edges with the neighbours (mid-terrace), at the eaves (cast eaves, with ends of timber trusses cast-in) and whether or not the walls need to breathe as ventilation/condensation is an issue.

For these reasons, and the relative cost, I've opted to go with dry-lining. Hack off the plaster, fix 25mm timber battens with a vapour barrier and insulated boards fixed onto (not between battens). Add to this 25mm holes drilled at 600mm centres just above each floor level to permit trapped moisture to drain and hopefully the effervescence and mould will not come back.

Back to your questions: I agree to insulate to foundation to prevent cold-bridging. However many of these houses had floor boards laid onto bitumen/asphalt as a DPM - this may not be functioning now and I would suggest if you can that you take up the floors to have a look. In my case, linoleum was glued to boards which were nailed through the ashphalt with the damp rising through the nails, rusting them to bits and rotting the timber around. The linoleum had the effect of trapping this moisture so it couldn't evaporate, exacerbating the whole mess.

French drain - unless the perimeter of your house is huge, a 400-450mm wide trench lined with a geomembrane (to keep the stone and natural soil separate) should be sifficient, with a 100mm perforated uPVC pipe (normal land-drain pipe). You need to know where your drain is going to connect to and the level of the connecting drain in order to know how deep you can go with the french-drain pipe; the deeper the better but no less than 150mm bedding under the pipe, in order to prevent as much standing water as possible. Falls in the pipe of 1 in 150 should be fine. Stone should be clean 14mm stone (i.e. no sand or silts) also known as Clause 505 filter material.

Cover the drain with about 150mm topsoil or whatever surfacing you are intending, with slopes away from your walls of at least 1 in 80 for the first half metre. If you can, I would recommend putting inspection chambers at the ends of straight runs and junctions on the pipes so that you can check for silting up if problems arise later in, and also so that it can easily be flushed/jetted clean if you do find siltation.

Check with the insulation supplier if the boards are suitable for backfilling against with stone, or if you need to protect them with filler boards too.
Hope this helps, it's not as complicated as it sounds, I post/link to a sketch if I can find one.

byrneos

Graaaaa wrote: »

I


Back to your questions: I agree to insulate to foundation to prevent cold-bridging. However many of these houses had floor boards laid onto bitumen/asphalt as a DPM - this may not be functioning now and I would suggest if you can that you take up the floors to have a look. In my case, linoleum was glued to boards which were nailed through the ashphalt with the damp rising through the nails, rusting them to bits and rotting the timber around. The linoleum had the effect of trapping this moisture so it couldn't evaporate, exacerbating the whole mess.

thanks for advice on the drain.....

just wondering how you remedied the above.
as we have the same problem only it was the new fake wood laminate with foam underlay instead of linoleum that trapped the moisture.

thanks .

Graaaaa

To be honest, I didn't really remedy it fully...
I cleaned what I could, spent a day on my hand and knees with a wirebrush stripping back all rust and rotten boards, then lashed on a full tin of preservative. After it soaked in (I worked it in well at hollows and the nails) I probably did the unforgivable, which was to lay underlay and kitchen laminate (i.e. green from preservative treatment underneath).
As I'm renting the house for the moment and hope to sell in more sane times, I'm reasoning that the damage will be minimal until such time as somebody not strapped for cash takes it off my hands and renovates properly.
If I was to do it again, the only thing I would do differently is to apply a rust treatment to what remains of the nail heads.
Ultimately the boards need replacing, and probably if I was to go that route I would look at laying insulation and renewing the damp proofing too - not for the faint hearted!

byrneos

ok thanks for reply...yeah i think ill have to do some major work to sort mine.
but thats after the job in hand.want to see how the french drain effects it aswell.

Leopold of Ballina

I am installing French drains around a very old house. As an alternative to a soak away, would it be possible to run the drains into one of the unused wells? Can a well take a high volume of water from on top in a short space of time without overflowing? Thank you.

BryanF

Leopold of Ballina wrote: »

I am installing French drains around a very old house. As an alternative to a soak away, would it be possible to run the drains into one of the unused wells? Can a well take a high volume of water from on top in a short space of time without overflowing? Thank you.

What's the water table and soil like? as in when its really soaking, in the worst of the season, will the water have somewhere to go?

This is a sitre specific question and you need professional advice on site, I would suggest you get an opinion from whoever is looking at your sewage treatment system