DPC ?

[robgil]

Ive just received a quote for some work, kind of testing the water to see if its worth getting a firm in.
One quote was from a specialist firm who do restoration work. They seem pretty good and ive seen some of their work which was fine, and their prices are very realistic.
However, a brick plinth rebuild and sole plate with scarfed in studs, easy enough job. They specified reclaimed brick , good , so soft reds I would presume, lime mortar with nhl 3.5 , again good as this would be a direct replacement of the original with good water vapour control as well as making the mortar the weak point and not the brick. Green construction oak, ok , I would prefer seasoned as it will be under render, then they specified a DPC in the brick and a slate DPC under the sole plate. :shock:

Here is a scenario , the green oak will shrink , the render will crack , moisture will get in and pool on the slate , not good for the sole plate.
Moisture will rise through the brick plinth then get halted by the DPC thus saturating the brick and mortar below the DPC.

What do you guys think?

 

[Penners]

A DPC in the plinth and another under the sole plate seems excessive to me. Why two?

I'm a believer in having a simple plastic membrane DPC under the sole plate, to stop the oak getting damp. Or rather, to stop it staying damp for ever and a day. I don't see that this interferes with any breathability functions, and I believe it's good for the structure. You can of course have slate if you prefer, but I reckon the considerable extra cost won't pay you back. Better to spend that money on more important things.

Yes, green oak will shrink and move a bit, but so long as you deal with any render cracks promptly, this won't really be a problem.

 

[middi]

I agree with Penners.

I wouldn't worry about the oak plate shrinking, the downforce of the weight above will keep it in permanent contact with the masonary and dpc.

Middi

 

[robgil]

Thats my point though guys , i dont want to have to repair cracked render, it looks pants under limewash , especially when it rains.
Dpc's arnt entirely necessary in this instance surely? The brick plinth will remain un-rendered to aid moisture evaporation.

 

[Penners]

But the brick plinth is still likely to be damp, thus it will keep the sole plate damp.

But your house - your choice.

 

[robgil]

Yes but surely if this is the case then engineering bricks and cement would be better suited to deal with any moisture?

 

[Penners]

I'm not sure whether engineering bricks/cement mortar will form an effective damp barrier. I suspect not nearly as effective as a completely impermeable plastic membrane.

Middi - if you're tuned it, what's your view?

 

[robgil]

My point is if a DPC goes into soft brick it might stop the natural drying process which might damage the brick in the long term. Engineering bricks are not nearly as absorbent.

 

[middi]

I think I explained on another one of your threads how and when to use DPC in base walls of timber buildings and how it works Rob.

Scenario 1

If a soft brick dwarf wall is situated under an oak sole plate and this oak plate is exposed to moisture and/or rain running down the exterior walls of the building, it would be useless to insert any kind of DPC as this would be deterimental to the wellfare of the timber. This DPC ( of any form) will act as a water trap running down the walls, delaying the natural drying process at the point of contact between dpc and oak and the capillary action will only serve to divert the moisture into the building. In this scenario and all things being equal, ie:- breatheable fabric above and below the wall plate, will NOT require DPC.

Scenario 2

If you take control of any water that runs down the building walls onto the oak plate to protect it , ie:- protected walls and studs ( timber cladding as example) and with water dispersal at a levels below the oak plate ( lead flushing ??) , then the addition of DPC below and in contact with the oak plate would be of great benefit. A totally dry and protected area, no moisture will reach this plate from above or below.

I hope this clarifies the situation.

Middi

 

[middi]

......scenario 3

If an engineering brick base wall is constructed, this, used without a dpc, will only serve to trap moisture below the plate.

An engineering bricks approach would be usefull ( abeit not aesthetically pleasing) only where a dpm and dpc are lapped and total control of moisture is prevented from penetrating into the building. I personally do not see the need for the use of engineering bricks in a timber frame building. The same end result can be obtained with more pleasing bricks, ie:- hand made or reclaimed soft reds or whatever period bricks are endemic to the region.

Middi

 

[robgil]

Totally agree Middi, I guess I am just trying to justify adding things to the original construction that were never there in the first place. You know I am a bit of a purist. Its just that alarm bells rang when I read the spec for two DPC'S so i thought I would compromise and put none in at all.
Surely the addition of a DPC in this instance would be a better safe than sorry approach?
Withought them, and with the wall working as it should and allowing any , if any , moisture to evaporate then the timbers should be ok? The wall isnt in a place that will be under excessive damp conditions.
I guess I have to figure out why the plate and plinth rotted in the first place and if i cannot get to the source of this process I will have to try and prevent it from reoccurring by minimising the damage.

 

[middi]

Your sole plate rotted, ROB, because it was encased in cement render and not much ventilation was getting to it.

A a matter of fact, even when these plates were ( eventually) encased in lime plaster their decay will be speeded up , albeit at a slower rate than if it was encased in cement.

These sole plates were originally left open to elements where plenty of ventilation could get to them.

If you intend to stick to the original construction design of the building, you must also be prepared to live with unacceptable living conditions of that period . Unfortunately that is the only way to guarantee that copious ventilation is achieved throughout the building therefore slowing down the decay process.

Another way of course , take control of the situation by judicious use of modern materials to complement and preserve whatever is left and live in modern conditions

Middi

 

[Penners]

take control of the situation by judicious use of modern materials to complement and preserve whatever is left and live in modern conditions

Beautifully expressed, Middi. A succinct summary of good period property conservation today.

 

[robgil]

Right then ,DPC under the plate it is (even though I came here kicking and screaming ). Now , if I was to use slate would it need to be a long continues run of slate? I know this would be ideal but ive got some slate in my garden that i can trim and use hopefully, if two pieces are to be used I would need to over lap them in the middle?

 

[Penners]

You really are a kicker and screamer, aren't you Rob? :wink: Why would you go to all that trouble and difficulty instead of just buying yourself a roll of good quality DPC membrane?

It's because plastic is modern, isn't it? And you just can't stand the idea of anything modern in your house.

 

[robgil]

No , its because the slate is free Penners.
Ive got a dish washer, thats fairly modern.

 

[middi]

Rob, be a purist if you like and use slate dpc. By the way you will need a double row and staggered in order for these to work properly. ( actually this is not purism, I would categorise it "old fashioned"

Take Penners advise, buy youself a roll of 220mm wide plastic dpc for 6 or 7 quid and save yourself a lot of time. It will also do the job much better. Also the inevitable movement of the dwarf wall and oak plate will not induce any cracking and splitting of slates which will fail to do their job.

Middi

 

[robgil]

Ooooh Ok then. Plastic it is.
Now , you know when you rebuild the plinth up to the oak plate? The dpc will go directly under the plate ,what do I build up the gap with under the dpc? Anything I suppose so long as its bonded with mortar?

 

[middi]

The easiest way to fit flexible dpc under the plate is to staple it ( sparingly !!!) under the plate. build your dwarf wall with bricks and mortar or whatever you wish to use but leave a gap of at least 12/15 mm between the last brick and the dpc.
By leaving this gap it will be much easier to pack the gap with mortar with a small trowel and a 10mm thick "ramming" wood stick. Leave it to dry out 24 hours( or longer) and you can remove any supports that hold the sole plate.

ps: strictly speaking the dpc should not be perforated and lapped where necessary along it's length. In the case of few odd staple holes I usually make the exeption to facilitate the job

Middi

 

[FamilyWiggs]

Ooooh Ok then. Plastic it is.
Now , you know when you rebuild the plinth up to the oak plate? The dpc will go directly under the plate ,what do I build up the gap with under the dpc? Anything I suppose so long as its bonded with mortar?

Well since you've given in to the plastic, why not just fill the gap with expanding foam? :shock: