Help building patio up to DPC level

[TA14]

Equus said:

No it's not: it's permanent bridging of the DPC by the ground itself that's the potential problem. Rain gets driven onto the outside face of a cavity wall all the time (and pisses straight through surprisingly quickly, according to the tests the BRE have done). A bit extra splashing back up from the paving flags will have no effect whatsoever.

The water running down the inner face of the outer leaf is why I like to see a concrete cavity fill, angled to the outside, to just below DPC, with weep holes, to shed this water away from the building. With this in mind I'd be wary of the exact height of the membrane in this sketch:

Too Late said:

I used the plan on paving expert

[48k]

Too Late said:

i finished last year exactly what you want to achieve






We finished with this:

Really like this. Looking to do something very similar this year. May I ask from where you sourced the slabs, and also how is the patio constructed - is it a single skin of brick infilled with MoT?

[SAB888]

Equus said:

037 said:

Equus said:

037 said:

A reason that building regs dictate the 150mm below dpc rule is to reduce any rain splashing above the dpc level where there is most risk of damp bridging to inside.

No it's not: it's permanent bridging of the DPC by the ground itself that's the potential problem. Rain gets driven onto the outside face of a cavity wall all the time (and pisses straight through surprisingly quickly, according to the tests the BRE have done). A bit extra splashing back up from the paving flags will have no effect whatsoever.

The 10mm. gap solution suggested by SAB888 is just fine, so long as you have no existing problems.

A reason not the reason.

THE reason in terms of the 150mm above ground level requirement for DPC.

To suggest that it's 'splashing' of water bouncing back up from ground/hard surfaces is absolute nonsense. If it's raining hard enough for any splashing to be occurring (which is fairly rare), then the entire face of the brickwork above DPC level on any windward elevation will be absolutely saturated.

Go take a reality check, and come back when you're feeling better.

I've always been baffled by the theory that rain splashing up from the ground surface is a reason that the dpc is normally 150mm minimum above ground level. A heavy rainfall will soak the exposed wall and make any splashing from the ground completely insignificant. If the detailing is done properly, there should be no problems.

[037]

SAB888 said:

Equus said:

037 said:

Equus said:

037 said:

A reason that building regs dictate the 150mm below dpc rule is to reduce any rain splashing above the dpc level where there is most risk of damp bridging to inside.

No it's not: it's permanent bridging of the DPC by the ground itself that's the potential problem. Rain gets driven onto the outside face of a cavity wall all the time (and pisses straight through surprisingly quickly, according to the tests the BRE have done). A bit extra splashing back up from the paving flags will have no effect whatsoever.

The 10mm. gap solution suggested by SAB888 is just fine, so long as you have no existing problems.

A reason not the reason.

Assuming you have a cavity wall and also assuming that the cavity isn't full at the lower level

THE reason in terms of the 150mm above ground level requirement for DPC.

To suggest that it's 'splashing' of water bouncing back up from ground/hard surfaces is absolute nonsense. If it's raining hard enough for any splashing to be occurring (which is fairly rare), then the entire face of the brickwork above DPC level on any windward elevation will be absolutely saturated.

Go take a reality check, and come back when you're feeling better.

I've always been baffled by the theory that rain splashing up from the ground surface is a reason that the dpc is normally 150mm minimum above ground level. A heavy rainfall will soak the exposed wall and make any splashing from the ground completely insignificant. If the detailing is done properly, there should be no problems.

[Too Late]

48k said:

Really like this. Looking to do something very similar this year. May I ask from where you sourced the slabs, and also how is the patio constructed - is it a single skin of brick infilled with MoT?

Hi

NuStone is where we got the slate slabs from
The wall in a double skin, with concrete blocks at the back, infilled with MoT type1. the outer wall (built along the house wall is a single skin concrete block wall. I laid armoured cables for single up lights sunken into the corners. Brick lights were added to the front highlighting the garden

Build thread here:
http://www.pistonheads.com/gassing/topic.asp?t=159...

[Equus]

037 said:

Assuming you have a cavity wall and also assuming that the cavity isn't full at the lower level

Both irrelevant.

Even with a solid wall, the same principles and reasoning apply: the DPC level is set to avoid permanent bridging, and 'splashback' would have negligible influence (if you get water penetration as a result of 'splashback', you'll sure as hell be getting it as a result of driven rain penetration). Rising damp occurs as a result of failed or faulty DPC or external bridging of the DPC. I've never known a case of the latter where clearing away whatever detritus was causing the bridging - even if to only a few mm below the DPC level - didn't solve it. The 150mm. is B.Regs erring on the side of caution. The truth is that anything that prevents standing water or ground moisture bridging the DPC is sufficient in reality.

If your cavity is full at lower level, you've got a problem anyway. As I've mentioned before, the BRE did tests where they filmed a (well pointed) cavity wall as water was sprayed at the outer leaf. Within minutes the inside face of the outer leaf was literally streaming with water. If your cavity is blocked, a bit of splashback near the DPC is the least of your worries.

[Sheepshanks]

One of my daughters has a house built into a hill with decking down one side which ends up several feet above the dpc as the house and ground drops away. They're having problems with damp in a corner of a room where the corner is under the decking.

They've been told the cavity is probably filling with water but that seems unlikely to me - sure there may be a bit of bouncing rain, but water doesn't drain towards the wall, and half the height of the wall is sheltered anyway even from driving rain.

Oddly the surveyor didn't mention it all, and, to be fair, the rest of the survey was pretty detailed and, unusually, very certain about what was good and bad.

[Equus]

Sheepshanks said:

... a house built into a hill...

We'd need a lot more information to attempt to diagnose the problem, but these six words are enough to start alarm bells ringing all on their own.

[Sheepshanks]

Equus said:

We'd need a lot more information to attempt to diagnose the problem, but these six words are enough to start alarm bells ringing all on their own.

I don't think it would be possible to diagnose remotely, it's just the notion that the cavity is filling up seems unlikely to me.

I think it's coming up through the floor in the affected corner as it's a bit wet but on the wall it looks like condensation damp.

[georgefreeman918]

Too Late said:

Sorry i missed this question

Membrane was from a local builders merchant and it was held there as i poured concrete in to make the gully. Once set, i poured stones in and cut it back so it wasnt visible...

Sorry another question regarding the membrane, (hoping to start this weekend!).

So it is not glued or attached to the wall. If you fill the channel with gravel, does the membrane not stick out of the top and permanently on show?

[48k]

Holy thread resurrection!

OP, did you have significant difference in levels between the interior floor and the patio that you had to compensate for? Just started planning mine and the kitchen floor is quite a bit higher than the DPC and that's before we've put UFH in the kitchen and tiled it. Not sure how to design the patio to be same level as kitchen floor whilst not bridging the DPC.

[TA14]

48k said:

Not sure how to design the patio to be same level as kitchen floor whilst not bridging the DPC.

Have a read of this thread

[48k]

TA14 said:

48k said:

Not sure how to design the patio to be same level as kitchen floor whilst not bridging the DPC.

Have a read of this thread

I have, I'm just not 100% certain what to do. The OP said:

georgefreeman918 said:

If a patio was to be built up to the internal floor level it would be a few mm under the dpc, not adhering to regs.

My situation is that if the patio were to be built to the internal floor level it would be about 4cm over the DPC level.

Can I simply do a "deeper" version of what he's done, attaching membrane up the exterior brickwork (over and above the house DPC) to meet the internal floor height and have the patio at that height with a trench filled with stone between the patio and the house wall?

[TA14]

48k said:

My situation is that if the patio were to be built to the internal floor level it would be about 4cm over the DPC level.
Can I simply do a "deeper" version of what he's done, attaching membrane up the exterior brickwork (over and above the house DPC) to meet the internal floor height and have the patio at that height with a trench filled with stone between the patio and the house wall?

See my reply above of 4th of April. Most dpc are below floor level so this is always an issue. Use single size stone for the drain - pebbles are best rather than the crushed stone shown above.

[mamba101]

I recently had a porcelain slab patio laid by a paving contractor.

Porcelain patio https://imgur.com/gallery/Rx7bVn5

Where I am, in Ireland, building regulations are that the outside ground level should be 150mm below the dpc (damp proof course). Mine is now 20-30mm below, it starts at the wall render).

The contractor installed a dpm (damp proof membrane) between the patio and the house (along the outward facing edges only) and the patio falls away from the house.
Are these measures sufficient to prevent damp issues?
If I sell the house, will a surveyor take issue with this?


If it's bad news, what are my options? I've been reading a lot about installing things like a aco or a French drain between the patio and the house.
My 2 questions around these options are
1) How difficult would it be to cut porcelain slabs in place? (On a full mortor bed) to install a drain.
2) Will this satisfy a surveyor?

Thank you!

Edited by mamba101 on Monday 8th June 13:41

[TA14]

Hi mamba,
1) quite difficult
2) probably not

Even if you cut the slabs OK it will be difficult to install the drain. I think that you would have to lift the row of slabs and relay them (expect a high percentage of broken slabs)

Depending on the prevailing wind and rain direction, how bad is it in foul weather? Or have they just been installed? Given that they fall away from the house it's a pity the they weren't put in four inches lower at the house end

[mamba101]

TA14 said:

Hi mamba,
1) quite difficult
2) probably not

Even if you cut the slabs OK it will be difficult to install the drain. I think that you would have to lift the row of slabs and relay them (expect a high percentage of broken slabs)

Depending on the prevailing wind and rain direction, how bad is it in foul weather? Or have they just been installed? Given that they fall away from the house it's a pity the they weren't put in four inches lower at the house end

Thanks for replying. Prevailing weather is at the front of the house.

I think my image link didn't work before but here it is: https://imgur.com/gallery/Rx7bVn5

The contractor will be out tomorrow to look at 2 slabs I found to be flat (towards the grass end) that is resulting in some standing water, but I'm trying to decide if I want to push for bigger work. Really, should I be telling them the patio needs to be relaid?

While the patio falls away from the house the fall is very shallow imo, it's approx 1:200 (I believe the recommended is like 1:80??) - He has been adamant that the fall is fine but I'm not convinced.

[TA14]

Hi Mamba, this is one of those 'I wish we didn't start from here' situations. Realistically, I think that all you can do is take up a row of flags near to the house and install a french drain soak away, say 200 mm wide, narrowing to 100 or 125 at the surface.

The fall should be 1 in 40 to 80 so that's about 50 mm fall from the outhouse whereas you have about 15 mm fall. Water will stand at 1 in 200. At about 1 in 140, if the ground settles slightly or if a few flags are placed slightly out of level then water will pond hence the 1 in 80 minimum. The contractor isn't going to relay the whole job though.

Good luck with your meeting.

[mamba101]

TA14 said:

Hi Mamba, this is one of those 'I wish we didn't start from here' situations. Realistically, I think that all you can do is take up a row of flags near to the house and install a french drain soak away, say 200 mm wide, narrowing to 100 or 125 at the surface.

The fall should be 1 in 40 to 80 so that's about 50 mm fall from the outhouse whereas you have about 15 mm fall. Water will stand at 1 in 200. At about 1 in 140, if the ground settles slightly or if a few flags are placed slightly out of level then water will pond hence the 1 in 80 minimum. The contractor isn't going to relay the whole job though.

Good luck with your meeting.

"this is one of those 'I wish we didn't start from here' situations." Story of my life right now!

The contractor came out and said everything is fine in his opinion, that even though the fall is shallow it still falls and therefore is OK. And that some standing water is expected.

The other part was about the patio being close to the DPC, relaying is as you expect not really an option, but he said he would put in a aco drain/french drain (he told me I need to decide exactly what I want) and will only charge for the labour.

He said he could cut the slabs in place, rather than taking them up as that would break more.

Given the picture above, do you think a drain around the house is the right thing to do?
My hopes for that would be 1) A safeguard against any wind driven water/flooding in extreme or prolonged rain and 2) Allow the house be OK'd for regulations as the 'ground' would be 150mm below.

[mamba101]

I've created a new thread to discuss my patio https://www.pistonheads.com/gassing/topic.asp?h=0&...