Will I need a cill for my alu bifold?

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Hi, I’m having a few alterations done done to my house and this includes adding 3.6m aluminium bifold doors . I’m trying to get the threshold as flush with internal floor as possible.

To achieve a completely flush threshold, I would need to take the brick level 20mm below the DPC. I’m not happy to do this.

instead, can I not fit a 25mm cill and put the frame on the DPC? This will give me a nice 5mm internal lip on the threshold. I can’t raise the internal floor height.

if it helps, externally I will be having a linear drainage gully along the house. The height of this will be flush with the external side of the threshold.

thanks in advance,
 
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Bit lost with what you are asking... on one hand you are saying that you need to remove 20mm of brick, below DPC to get the door to fit, but you also say can you fit a cill? SO frame and cill would still be 20mm below Existing DPC? I gues you could drop the frame below and then make a lead/DPC ' tray ' for the doors to sit on, bring it up the wall to the existing height of the DPC. My biggest concern with this is potential water ingress, even with a Linear drainage in driving rain conditions you could still get water in the property. What threshold have you gone for on the doors?
 
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Sorry, I’m not the best explainer. Hopefully this makes more sense.

I can achieve a flush threshold by fitted frame and cill below the current DPC
or
Not use a cill and fit the frame onto the DPC.

are either of these options acceptable and what is preferred?
 
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Well, yes , you could sit it directly on DPC ...BUT you need the water to be able to drain over the edge of the brickwork and out of the building , which I very much doubt you could achieve. The use of a big enough cill would alleviate this, its just making sure DPC etc below the cill and bring partly up the wall to keep the integrity of the DPC . Is the doorset on the side of the property that gets the weather ? A completely flush threshold WILL let water through, as no Bifolds are severe weather rated with flush thresholds
 
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The doors will be facing SW.

I was thinking…
Part of my extension will mean I will be removing a down pipe. I’d like to install a drain against the back of the house, including under the bifold. This drain will run to the down pipe.

As the frame will be set back approx 30mm from the brick face, I was hoping to fill the gap under with some of the patio slab we will be having.

here’s a birds eye view of what I’m imagining.
 

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