Restoring a Wooden Sash Window

What I'd expected to be a painting job turned into a partial rebuild and restoration of a sash window and window sill after discovering that much of it was composed of compost rather than wood. This was a satisfying project that involved rendering, casting concrete, detailed wood work and painting. Photos here show before and after. Not a bad result.

Tools

1. Table saw
2. Router and table
3. Multi-tool
4. Paint brushes
5. Belt sander (optional but makes the job quicker)
6. Detail sander
7. Heat gun and scraper
8. Grinder with wire brush cup (optional but make the job quicker)
9. Wood saw (or mitre saw if available)
10. Hammer
11. Hammer drill / masonry bits
12. Clamps (sash clamps ideally)

Supplies

1. Silicone sealant (I used Dowsil 781)
2. Wood primer (I used Leyland trade undercoat, oil based)
3. Gloss paint (I used Leyland trade gloss, oil based)
4. Wood filler (I used Toupret powder wood filler)
5. Cement and sand
6. SBR
7. Timber (Hardwood preferably however I did use pine for the the large run as it's all I hand available)
8. Sandpaper (p80, p120 and p240)
9. Expanding foam
10. Headless nails
11. Large screws with masonry plugs

I was hoping this would be a simple prep and paint job but life is never simple when your own a house that was built in 1820. After getting up close and personal, it turned out that a good few sections of the sill, box frame and window had significant rot that would require new timbers.

It appears that the bottom section of the left hand side of the box frame had previously been replaced but unfortunately this was rotten again. Additionally the entire edge that meets the wall was also soft so I decided to remove the entire run and start from scratch.

There's no real finesse to this operation. Just get in there with a crowbar and claw hammer. You'll also likely need to remove some of the render to allow for a new piece to be fitted.

You can see in the photos that I've also given everything a quick once over with a twisted wire brush cup attached to a grinder to clean up the flaky paint, this helps to get a better idea of the underlying condition of the existing timber and is much quick than using a sander or scraper.

Using the old timber for reference and a combination square, rip the new timber to the correct size using a table saw. Finally cut to length either by hand or using a mitre saw.

The new section can then be installed using nails to secure it to the existing frame long the right hand side. The left hand side will be a bit wobbly at this point but we'll be securing this to the wall by replacing the render which will sure everything up nicely.

Again, given how rotten the existing sill was and how rusty the screws I just got in there with a crowbar and it came away pretty easily.

I've recently been forgoing a wood sill and instead casting a concrete one in place instead. They seem to have a tendency to rot and so going with concrete will save you future maintenance, and it doesn't take much longer to do. With this particular job, I decided to salvage part of the existing sill which allowed me to use it a former for the concrete and meant I had an existing drainage channel, shape, angle and width - making the concrete job much simpler.

Steps

1. Cut out rot from back of sill using a table saw and hand saw.
2. Secure salvaged section back into place using large screws and rawl plugs.
3. Mix up 3 parts sand, 1 part cement and some SBR.
4. Pour to fill missing section to back of sill. Use a drill or multitool to give it a good vibrate to shake the cement down into all the cracks and crevasses. Smooth. You'll need to leave it at least 3 days to proper go off.
5. I also rerender the wall where it had been damaged when installing the new box frame timber.

At this point I noticed that there was more rot to bottom of the frame. I removed all the soft wood using a multitool and chisel and then cut new sections of timber to splice in. You don't have to be super tidy with this step as we'll use wood filler to blend in the new sections to the old. New timbers were secured using wood glue and a screw or two.

I used a heat gun and scraper, followed by sand paper to prep the window frame for painting.....unfortunately during this process I hit a slight snag, the window fell apart. I completed the prep work regardless but some woodwork was now required!

Luckily the top sash was in much better condition.

After the set back above I had a poke around and discovered there was significant rot to the bottom rail (bottom edge) and part of one stile (the bit that goes up the side). This timbers are a little tricky to replace given the number of features they have but there was little choice. The mouldings holding the glass into the frames were also I a bad state so I removed most of them.

The stile was fixed by cutting out the rotten section and creating a replacement timber. This section includes a rebate that was cut using a router and table with a 1/2" straight bit. The splice was installed using wood glue and clamps.

The bottom rail was replaced by replicating the existing rail using a table saw. The bottom of this rail is angled so that it sits flush with the sill when in place. This made the job a bit of a head scratcher at times, as the angle means you lose a reference for the fence on the saw but I got there in the end. The replacement rail was then secured into the existing frame using mortise and tenon joints which were again glued and clamped. A sash clamp or two is very handy at this point.

The mouldings were replaced for new. I made these on the router table which is a bit tricky but I had an able helper to assist. Buying new ones is an option if this is daunting. They are secure using nails and a bead of silicone to ensure a good seal.

I gave everything a good final sand and then I realised I'd made my life difficult but installing the new mouldings before painting them. I'd also used silicone to secure them in place and provide a weather seal, so I opted to paint them in place, which was fiddly and time consuming. A much better idea to paint them first and them install them.

I opted for an oil based wood primer/undercoat as I find the water based stuff to be very prone to drips and runs and I also feel that for outdoor stuff that oil gives more protection. The only downside is how long it takes to dry, at least 24 hours, whilst you have a big hole in your bedroom wall where the window should be! Luckily the weather was nice and my wife was mostly understanding.

Similar to the windows, a heat gun, scraper and sander were used to prep the box frames for painting. At this stage I also used a wood filler to blend/feather in the spliced timbers.

An angled brush really helps when cutting in here. I also painted the new render (the photo was taken when this paint was still wet, hence the colour difference).

After waiting for the undercoat to dry, I gave everything a very light sand using 240 grit and then glossed everything, again using an oil based paint. After waiting for everything to dry (even longer this time, 48 hours, and even then it wasn't properly hard), I reinstalled the sashes, took some photos and had more than one celebratory beer.

I'm pretty happy with the result given I'm a rank amateur and given the number of nasty surprises along the way but I got there in the end.