Stupid garage DPM/DPC question

VDubDan · 11 Aug 2026

My house currently has a very grim, 1950's precast garage. My intention is to knock this down and build a lean-to garage.

For future proofing, and because I intend to store a classic car, I'm thinking to do a traditional extension build - i.e., rendered block cavity wall (rendered to match house), trench fill foundation, then concrete slab floor.

The bit I can't get my head round is the DPM detail for the floor. The DPM would normally come up to lap into the DPC, with the FFL matching the DPC.

Obviously in my garage that isn't the plan, I guess I'll do the DPC 150mm above ground, but I need the slab at the ground level. What do I do with the DPM? I don't want it lapped up, that'll be silly

noseall · 11 Aug 2026

VDubDan said:
My house currently has a very grim, 1950's precast garage. My intention is to knock this down and build a lean-to garage.

For future proofing, and because I intend to store a classic car, I'm thinking to do a traditional extension build - i.e., rendered block cavity wall (rendered to match house), trench fill foundation, then concrete slab floor.

The bit I can't get my head round is the DPM detail for the floor. The DPM would normally come up to lap into the DPC, with the FFL matching the DPC.

Obviously in my garage that isn't the plan, I guess I'll do the DPC 150mm above ground, but I need the slab at the ground level. What do I do with the DPM? I don't want it lapped up, that'll be silly

Lower the internal DPC by a course of brick (or whatever) and maybe add a cav’ tray at external DPC level.

valuablealter · 20 Aug 2026

VDubDan said:
My house currently has a very grim, 1950's precast garage. My intention is to knock this down and build a lean-to garage.

For future proofing, and because I intend to store a classic car, I'm thinking to do a traditional extension build - i.e., rendered block cavity wall (rendered to match house), trench fill foundation, then concrete slab floor.

The bit I can't get my head round is the DPM detail for the floor. The DPM would normally basketball stars come up to lap into the DPC, with the FFL matching the DPC.

Obviously in my garage that isn't the plan, I guess I'll do the DPC 150mm above ground, but I need the slab at the ground level. What do I do with the DPM? I don't want it lapped up, that'll be silly

You can keep the DPM under the slab and tie it into the wall DPC, with the slab edge protected appropriately. The key is maintaining continuity without creating a moisture bridge.

^woody^ · 20 Aug 2026

Lots of new builds have this very same situation - two courses of brick between the slab and the DPC.

In real terms it's not an issue as you should expect humidity and condensation issues from the cold surfaces regardless, and would need to deal with these via ventilation.

If you are concerned, use solid red or blue engineering bricks and a good 3:1 mortar to prevent excess rising dampness. Or coat the section of wall with a durable liquid DPM.

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Stupid garage DPM/DPC question

VDubDan

noseall

valuablealter

^woody^

DIYnot Local

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