DPC level and floor joists

[nebjamin]

Hi folks,

Sure this has been answered before but after a bit of googling cannot find any info.

I have a 1930s solid wall house which is rendered all over and also has a rendered plinth along the lower section of the walls which finishes above the air bricks. The top of the plinth is aligned with all external doors.

Due to the plinth I cannot see the location of the DPC, but the house definitely has one (sleeper walls supporting the floors have a visible bitumen DPC running along the bottom of the timber).

Anyway - quick question for my interest (as potentially having some paving work done).

I have heard that the DPC in many cases runs along the same height as finished floor level.

Can someone that knows explain how the floor joists are then protected from sitting in damp walls? I.e. if the FFL is above the floor joists and at the same level as the DPC then if joists are set into external solid walls they'll be exposed to damp (since they must be below FFL).

I don't know for sure if my ground floor joists are set into the walls - haven't had any boards up against the walls yet!

Thanks!
Ben

 

[dann09]

If you would care to research on here, there is a mass of info, pics and diagrams ref your Q's.

Briefly:
1. your plinth should stop short of the ground level.
2. the DPC should sit 150mm above the ground level
3. and the joists should sit on the DPC

often, the above detail is not the case.

Do not increase ground level unless you are certain that the finished surface will end up below the DPC, and not be in contact with the plinth/render.

Locate your external DPC.

 

[^woody^]

With a timber floor, the floorboards are generally higher than the DPC in the external walls.

Also the floor joists are commonly built off small "sleeper" walls and not built into the external walls. There will be another DPC between the sleeper wall and wallplate which the joists sit on. In this way the floor is isolated from the external walls and dampness

If the joists are built into the walls, they tended not the be wrapped in a protective damp barrier, and relied on ventilation keeping them rot free.