DPM / walls in integral garage

[raine]

Hi,

I'm going to build an extension to include an integral garage, and I'm confused about the DPM detail.

I understand that the garage FFL has to be at least 100mm below the FFL of the rest of the house. This means that the garage FFL will also be 100mm below the DPC in the walls.

Am I right in thinking that the garage floor needs a DPM? And that this needs to be lapped up to the DPC in the walls? If so, what's the standard practice for hiding the DPM where it's lapped up the walls in a garage?

Unless the wall between the main house and the garage has to be a cavity wall, insulated as an external wall? Maybe that's it? Then the skin that makes up the internal walls of the garage could have a lower DPC to match the garage floor?

Have I just answered my own question?

Is the outer wall of the garage then single-skin? Or uninsulated cavity maybe?

Thanks in advance. Hope this makes sense!

 

[DevilDamo]

Just out of interest, where in Surrey are you? I'm on the Surrey/Hampshire border in Farnborough.

I understand that the garage FFL has to be at least 100mm below the FFL of the rest of the house.

As per AD B of the Building Regulations, if a door is provided between the house and garage, then a step of min. 100mm is required, or you can provide a level floor at the location of the door but making sure the garage floor falls away from the door to the main garage door (i.e. to prevent fuel entering the house).

Unless the wall between the main house and the garage has to be a cavity wall, insulated as an external wall?

What will the external wall to the garage be... single skin/cavity? You have to provide a cavity wall (or achieve required u-value's) between the house and an unheated space (e.g. garage). So you would either make the wall between the house/garage thermally efficient or the external wall to the garage.

 

[noseall]

Am I right in thinking that the garage floor needs a DPM? And that this needs to be lapped up to the DPC in the walls? If so, what's the standard practice for hiding the DPM where it's lapped up the walls in a garage?

Yes, the garage floor requires a dpm, but we tend to not bother lapping into the dpc as it is showing and subsequently gets damaged.

We just trim it off at floor level once the concrete is set.

 

[raine]

Brilliant, thanks for the replies.

So is the standard procedure to have the garage as uninsulated space, with the walls between it and the house insulated instead? I guess so, as garage doors are not famous for their insulating properties!

In that case, the insulated walls between the house & garage wouldn't necessarily need to be cavity, as there's no weather to be kept out? We could instead maybe fix insulation-backed plasterboard onto these walls, or something? What's the usual construction of walls between house & garage in a modern regs-compliant house?

 

[noseall]

So is the standard procedure to have the garage as uninsulated space, with the walls between it and the house insulated instead? I guess so, as garage doors are not famous for their insulating properties!

In that case, the insulated walls between the house & garage wouldn't necessarily need to be cavity, as there's no weather to be kept out? We could instead maybe fix insulation-backed plasterboard onto these walls, or something? What's the usual construction of walls between house & garage in a modern regs-compliant house?

The walls AND floor of the house (integral garage) that join with the garage need to be fully insulated as the garage space is regarded as the same as external.

Any other walls in the garage, i.e. those that are forming the external walls to the garge, need not be insulated, though it may be practical to do so should you ever wish to convert in the future.

Even if all the walls in the garage were up to spec, the garage door would still render the space 'cold'.