Can I fit 150mm projecting uPVC window cills directly onto my damp-proofed concrete porch floor?

[Me109bf]

Hi everyone,

I'm currently rebuilding my front porch and would appreciate some advice from experienced builders or window installers.

I've already purchased Kestrel Heritage 2800 windows and a composite front door, using the Deceuninck 3338 projecting PVC cill, which is 150mm deep and approximately 30mm high.

My porch has a solid, flat concrete floor extending across the openings, with existing damp-proofing. The external ground will be 150mm below the DPC.

The windows were originally manufactured to fit beneath the existing concrete lintels, with the cills sitting at porch-floor level.

However, my builder has installed two courses of bricks beneath the window openings because he's concerned rainwater could get underneath the PVC cills and track backwards into the porch.

Unfortunately, this would mean raising the existing concrete lintels and potentially disturbing the porch roof, creating substantial additional work.

I've attached photos of the current construction and the manufacturer's actual cill cross-section.

My questions are:

1. Can these particular projecting PVC cills be safely installed at the existing concrete floor/DPC level?
2. Does the cill design and drip edge provide sufficient protection against water tracking underneath?
3. Would correct sill bedding and sealing be adequate, or should there be an additional DPC tray or waterproof membrane?
4. Are the two courses of brickwork actually necessary?

Ideally, I'd like to remove the two courses and retain the existing lintels, provided the windows can be installed correctly and remain watertight.

Any advice on the proper installation detail, particularly with regard to Building Regulations Part C, would be greatly appreciated.

 

[Wayners]

Building control involved?

 

[ronniecabers]

Not sure, with the external floor 150mm below dpc why the builder wants two extra courses of bricks? Surely sitting the cill on those bricks still runs the same risks if on your existing slab?
Taking all that into account , your builder is actually making the entrance to your property worse by adding an increased height step . I would contact local building control, as i have a suspicion that this would be a building regulation fail under Approved Document M.

Personally , providing cill is bedded down sufficiently on either silicone or mortar, I see no reason why it should ever leak water, providing water isn't pooling below the door. To allow enough ' drip ' for water to run off the cill and not track back the minimum overhang of the front edge of the cill i would go at 30mm from finished render/brick

 

[Me109bf]

Thanks for your advice. That's exactly what I was thinking. The concrete slab already has DPC installed and is 150mm above external ground level, so I can't see the benefit of adding two courses of bricks.

I was considering using CT1 Anthracite sealant beneath and around the sills to prevent water ingress. Would that be sufficient, or would you recommend anything else? I'm also planning to clad the exterior with stone, so I'll ensure the sills have a suitable overhang and drip.

 

[ronniecabers]

Yes CT1 should be flexible enough to deal with any contraction/expansion due to changes in the weather. Noting you are using Anthracite , I'm going to assume the door and cill are anthracite? In which case I would ' vent ' the underneath of the cill with a few drill holes through the first layer of pvc. Anthracite will get hot in the sun unvented cills run a risk of bowing in the heat , also if Anthracite door , the door will swell in the heat, so to prevent potential for warping always engage the locking points when ypu close it , do t need to actually lock it with key , just pull the handle up ( assuming its a multipoint lever handle lol) . This will keep the door from being able to move top and bottom in the heat , youd be surprised how much they flex without the locks engaged

 

[Me109bf]

Thanks, really appreciate the advice. Yes, the door is composite and both the door and 150mm uPVC sills are anthracite grey. I'll make sure to engage the multipoint locks.

Interesting point about ventilating the sills. I'll check with Kestrel windows , whether they're already ventilated or need additional holes before installing them. Thanks again!