Pointing external window reveals

[tren]

So few months back, I fabricated and install new Oak windows on our barn conversion. Unfortunately I have now noticed that water is coming through the reveal and I am looking for advice for the best way to remedy.

I repointed the entire building using and around the windows using St Astier NHL 3.5 and 1:2.5 ratio as recommended by Cornish Lime in Bodmin. For the reveals I put some fiber in the mix.

For the reveal I made 12mm trims on either side which was used to make sure I have a nice thick layer of mortar around the windows - see pictures.

As you can the mortar has come off the trim creating a gap (3rd pic) which is most likely where the water is coming through. Probably due to the expansion/contraction of the wood.

What is my best solution here? Hoping I might just to widen the gap a bit and fill it back with something (burnt mastic?)

 

[Kearn]

What happens behind the trim/mortar?

Lime won’t adhere to timber easily without a mechanical key, especially smooth hardwood. NHL also sets very hard and rigid, especially 3.5 and is not flexible like an air lime, so as the oak moves, the NHL will not. Hence the gap. Fibres will help with the lime shrinkage/cracking, but not the window movement.

Ideally, you want a gap that you can point lime into, behind which you’d be twisting and hammering oakum initially to plug the gap, with a lime mortar fillet over. The oakum does the waterproofing, if the lime lets too much moisture through.

Presumably your windows are fixed with wedges and there’s a gap around the perimeter? In which case I’d remove the trim, twist tight and hammer firm, oakum to fill the gap. Then mortar a thick and reasonably deep fillet onto top of the oakum, within the gap itself. Depending in the size of gap, some SS mesh may help, or SS screws protruding from the outer faces of the timber into he void, so the lime has a key in the window itself. Then the trim could go back over the lime, but aside from aesthetics, it may not have much function. Either way, the trick is to fill the gap around the widow, not just lime up to it.

 

[a twig]

At the risk of causing great upset to members of this parish (purists avert your gaze)…

Windows were the one area of the house where I used squirty foam to install, then once the outside was pointed in and the mortar had shrunk back I used clear silicone between the mortar and the windows.

 

[tren]

At the risk of causing great upset to members of this parish (purists avert your gaze)…

Windows were the one area of the house where I used squirty foam to install, then once the outside was pointed in and the mortar had shrunk back I used clear silicone between the mortar and the windows.

This is what I had in mind. I might give this a go at least for the this winter. The weather is so bad here (rain and wind every day) that I don’t think I could do any pointing. If it fails then I will strip it all out and start again. I might give burnt mastic a go this time around.

 

[fernicarry]

I might give burnt mastic a go this time around.

Window reveals are one place where I take a belt, braces and everything else approach. I put a strip of expanding foam tape along the bottom of the wooden sill to give it a flexible seal against the uneven stone sill. After mortaring the reveals I use a junction sealing mastic like 215LM. And finally I put burnt sand mastic around the edge, mostly for cosmetic purposes, but it does also give an initial layer of protection.

 

[paulc]

Windows were the one area of the house where I used squirty foam to install, then once the outside was pointed in and the mortar had shrunk back I used clear silicone between the mortar and the windows.

Use a foam gun for fine control over the quantity being squirted. The plastic tip from the tube of silicone held over the end of the gun nozzle enables one to get the foam in to just about any gap.
If you go for a foam gun, get a couple of cheap ones from Amazon or ebay and treat them as a disposable item.

 

[Cubist]

Two questions; what did you use to pack and seal the boundaries of the new frames as they were installed? And, what coatings, were used on the oak?

 

[tren]

Two questions; what did you use to pack and seal the boundaries of the new frames as they were installed? And, what coatings, were used on the oak?

I used foam to pack the windows and used osmo for the windows. I know foam is not ideal. i really wanted to use traditional method but my stones were so irregular causing the gap around the window to be anything from 2mm to 25mm that foam with a gun was the ‘best’ option. I retrospective I should have straighten/even my window opening with the angle grinder before fitting my windows to give me a bit more flexibility

 

[Cubist]

Presumably you mean an injected expanding foam? Also, what Osmo product did you use and did you treat ALL the surfaces of the frames including those that would be hidden in the wall? Did you intend the wood to blacken and ulimately silver

I'm afraid you may end up having to remove those lime fillets and the wooden trims to enable a proper and effective water proof seal around the full perimeter of the frames including the cill. Expanding foam can provide an effective foundation for other waterproofing materials and obviously great for filling irregular sized cavities but no more than that. The lime fillet, because it is breathable, will absorb water and allow it to penetrate to the interior where it can ultimately escape OR be absorbed into surrounding materials it encounters along the way. If the OSMO product you used is merely water resistant, as most of their products are, there is a good chance that such moisture has also affected the various sides of the oak frames just as it has done on the lower exposed sections outdoors. Using an angle grinder to 'regularise' the surrounding stonework would have been pointless and entirely wasteful. The best materials to seal the perimeter of the frames would have been oakum and caulking cotton hammered into the permitter gaps. These materials can be braided into ropes to of considerable thickness and I have used them extensively around the main timber frame and door/window frames and in gaps in excess of 25mm in some instances. Once firmly hammered into place these can then be coated with a lime mortar, or burnt sand mastic to provide the final closure of the seam.