Felt flat roof window upstand replacement

Question

Any help would be appreciated. I am replacing my small flat roof window on a felt roof but will also need to replace the upstand which is very old. The new upstand is a little smaller than the previous one. 
What would be the best way to remove the old upstand as the flashing pieces extend around 180mm onto the deck, should each entire flashing piece be removed from the deck? I can cut around the base of the upstand to remove that easy enough, the felt pieces on the deck seem well stuck down. Thank you

Sonny · Answer

Thank you for your message—happy to help.

In this situation, the best approach would be to remove the existing flashing pieces in full rather than cutting tightly around the base of the old upstand. Although it may seem easier to leave the sections that extend approximately 180mm onto the deck, doing so can compromise the integrity of the new installation.
We would recommend carefully lifting the existing felt back beyond the current flashing area—ideally allowing at least 300–450mm of working space around the opening. This will give you enough room to install new felt and dress it correctly into the new, smaller upstand.

If the existing felt is particularly well adhered, gentle heat can be used to soften the bitumen and assist with removal, taking care not to damage the deck beneath. Once removed, inspect the deck for any signs of deterioration and carry out repairs if required before installing the new upstand and waterproofing system.

Taking the time to fully remove and renew this section will ensure a far more durable and watertight result.
Kind regards sonny ecolegacy

Kenny · Answer

You don’t want to start removing large areas of well-bonded felt, as that usually causes more damage than it’s worth and makes it harder to re-seal the roof properly. The best approach is to cut neatly around the base of the existing upstand and remove the upstand itself, leaving the main field of felt on the deck in place if it’s still sound and firmly bonded.

Since your flashing extends about 180 mm onto the deck and is well stuck down, it’s better to keep it and work into it rather than trying to lift it. Once the old upstand is removed, trim the felt back cleanly and install the new, smaller upstand. Because the new kerb is smaller, you’ll need to fill or pack the gap between the existing felt and the new upstand so you have a smooth transition with no dips.

After that, you should install new felt layers to tie everything together properly. Typically this means applying an underlay patch that extends at least 150–200 mm onto the existing felt, followed by a new cap sheet that runs up the new upstand and overlaps onto the existing roof covering. This effectively creates a new watertight collar around the upstand.

In short, don’t try to remove the full flashing pieces from the deck if they’re well bonded. Remove the upstand, keep the existing felt where it’s sound, and overlay and tie in the new waterproofing properly

Martin · Answer

Felt flat roof window upstand replacement

When replacing an old flat roof window upstand, it is important to remove enough of the existing flashing and felt so the new installation can be fully sealed and waterproof.

If the existing flashing pieces extend around 180mm onto the roof deck, the best approach is usually to carefully remove the old flashing sections back to a suitable point rather than trying to leave old materials underneath the new upstand. Any old felt or flashing that is loose, damaged or poorly bonded should be removed to create a clean surface.

The existing upstand can often be cut away at the base, but care should be taken not to damage the roof deck below. Once removed, the new upstand should be correctly positioned, fixed securely, and the waterproofing rebuilt using compatible roofing materials.

Because the new upstand is smaller than the original, the surrounding area will need careful detailing to avoid creating weak points where water can track underneath. A roofing professional would normally ensure there is sufficient overlap and that all joints are properly sealed according to the roof system being used.

The most important part is not just fitting the new window — it is ensuring the transition between the window, upstand and flat roof covering is completely watertight.

Daniel · Answer

If the new upstand is smaller, I’d remove the old flashing properly rather than trying to work around it. The flashing is there to keep the roof watertight, so it’s better to start with a clean surface.

If the felt is well bonded to the deck, you don’t necessarily have to rip the whole roof up. You can carefully cut back the old felt far enough to install the new upstand and then dress new felt over it, making sure you have a good overlap onto the existing roof. If the old felt is brittle or damaged while you’re removing it, it’s worth cutting back a bit further until you reach sound material.

The main thing is to make sure the new upstand is securely fixed and the new felt is properly bonded and lapped so water can’t track underneath. If the roof is nearing the end of its life anyway, it may be worth re-felting that section around the rooflight rather than trying to patch onto very old felt.

Benjamin · Answer

If you’re replacing the upstand and the new one is smaller, the best approach is usually to remove the old upstand and the associated flashing pieces, then make good the roof covering before fitting the new upstand.

Matiss · Answer

Hi, thanks for your message. Yes, this is definitely something that can be done. In most cases, the best approach is to remove the old upstand completely, including the flashing pieces if they extend around 180mm onto the deck — especially if the new upstand is smaller, as you’ll want a clean, sound surface to work from and ensure the new flashing seals properly.
If the existing felt on the deck is still in good condition and very well bonded, sometimes it can be carefully trimmed back and tied into the new system, but that depends on its condition and how secure it is. We’d usually assess it once exposed to make sure there’s no risk of leaks later on.
If you can send over a few photos of the current rooflight/upstand area, we’d be happy to give more specific advice. Thank you.

Felt flat roof window upstand replacement

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