Timber Frame Construction- when to fit windows

[benben5555]

We are about to start building our timber frame house and am interested in thoughts on this. I've seen some where the windows are fitted in the frame before the brickwork goes up on the outside. Is this recommended/ best? What is the fixing detail? What about sealing to the brickwork, as we want a tape seal rather than silicone. We have some brick and some timber weatherboard. Thanks

[Thorfun]

I was told by our window company to ensure the roof was finished before the windows were installed to ensure the roof was fully loaded. So I did that. Our timber cladding came much later. I don’t know with bricks though. 

[TerryE]

We fitted our windows and doors immediately following the last day of the frame erection, but we had ours standing out into the 50mm airgap with the skin lapping over the front of the windows.  We stapled heavy duty polythene sheeting over the windows to protect them when the building we going on and carefully cut this out as one of the finishing off jobs once the scaffolding was down.  See my blog for profiles and more details.

[benben5555]

10 hours ago, TerryE said:

We fitted our windows and doors immediately following the last day of the frame erection, but we had ours standing out into the 50mm airgap with the skin lapping over the front of the windows.  We stapled heavy duty polythene sheeting over the windows to protect them when the building we going on and carefully cut this out as one of the finishing off jobs once the scaffolding was down.  See my blog for profiles and more details.

Why did you choose to do that though? Just for speed of construction?

[benben5555]

10 hours ago, Thorfun said:

I was told by our window company to ensure the roof was finished before the windows were installed to ensure the roof was fully loaded. So I did that. Our timber cladding came much later. I don’t know with bricks though. 

Did you sit the back edge of the cill on the timber frame? Ours is designed with the window further out so it would have no bearing on the timber frame

[Roundtuit]

Our recommendation, which we followed, was roof on first (or at least loaded-out) to 'settle' the frame, then brickwork, then windows. The windows are set about 40mm back from the face of the brickwork and span the cavity and are fitted using compriband.

[Thorfun]

39 minutes ago, benben5555 said:

Did you sit the back edge of the cill on the timber frame? Ours is designed with the window further out so it would have no bearing on the timber frame

Iirc the windows were 60mm back from the outside of the TF. 

[Gus Potter]

12 hours ago, benben5555 said:

We are about to start building our timber frame house and am interested in thoughts on this. I've seen some where the windows are fitted in the frame before the brickwork goes up on the outside. Is this recommended/ best? What is the fixing detail? What about sealing to the brickwork, as we want a tape seal rather than silicone. We have some brick and some timber weatherboard. Thanks

Now that is a dilemma. Here are two facets.

 

In the past lenders would release funding at certain stages. Underbuilding out of the ground, wind and water tight, first fix, completion say.

 

Their idea of wind and water tight was that the windows / doors had to be in and the roof weather tight but no tiles say. But for self builders windows and doors are a big ticket item so you wanted to buy them as late as you can in terms of the cost curve  / interest paid on the loan. Also windows and doors can get damaged so best left until you have stopped using Sthill saws etc in close proximity.

 

Structurally wise the windows are often fitted to the timber frame which moves up and down and thus so does the window with it and the finishes on the inside. To accommodate this movement we put a compressible material under the cill where it extends over the outside leaf of masonry which for all intents and purposes stays still. Over the head we allow a gap between the top of the window and the timber frame lintel (to account for deflection from roof loads.. tiles etc) and a flexible mastic joint between the outside of the window frame and the outer leaf of masonry.

 

In Scotland we have been doing this for decades so know that it works ok if the drawings are followed... that is what we strive for!

 

Remember that a timber framed house is very much a "living" structure. Yes when the frame is first built it will shrink a fair bit but in autumn after the heating has been off over the summer it will swell a bit. It is always moving.

 

The problem is that if you fix your windows to the outer leaf of masonry is that it is much harder to create the aesthetic movement joints between the timber frame / attached finishes and the inside face of the window frame. The window frame then be still as it is fixed to the masonry, the window cill, wall paper ect will then be moving up and down. Best to keep the big flexible mastic joints outside than on the inside?

 

Would I fix a window to the outer skin of masonry on a standard structural timber frame if it was my own house.. no.

 

 

 

 

 

[saveasteading]

Timber shrinks a lot, even when kiln dried. On a 3 storey timber building we did, the lift supplier forecast a 50mm shrinkage, hence the lift would stop in the wrong places. As advised we did roof and brick walls before the lift went in.

3 storeys with flat roof isn't extreme.

Moral:  leave the windows as late as possible.

[Gus Potter]

34 minutes ago, saveasteading said:

On a 3 storey timber building we did, the lift supplier forecast a 50mm shrinkage,

Now that is "arse" covering" or maybe the lift supplier (like a good few contractors do) was pointing out to the SE that they did not know what they were doing and should stick to designing garden walls?

 

Off the top of my head I would go for 25- 30mm shrinkage on a three storey at concept design stage.. with some good site contol.. big difference.. as any more than that all the wall ties will be out of spec for example.. and they won't sell any lifts these days based on that 50mm caveat. The Architect will have a blue fit if you go back and "give them the good news".. on floor levels and how they may change.

 

I suspect that if you went to an Architect as an SE and said.. this is the best I can do you may not be invited to work with them again.

 

Ok.. what do you do when you have to meet the accessibilty threshold for BC at say 10-15mm.. then a year or so later disabled folk can't get out their flat as they need to wheel up 30mm to get to the communal landing as the flats have shrunk but the main stairwell is as built.. storey for another day.. but is goes along the lines of once the completion certitificate is issued then disabled folk are left to their own devices... which is bad design and bad for everyone.

 

For all.. timber frames shrink and swell a fair bit! But with careful design this can be effectively managed.

 

 

55 minutes ago, saveasteading said:

Moral:  leave the windows as late as possible.

 

Good advice.

 

 

 

 

 

 

 

[saveasteading]

2 hours ago, Gus Potter said:

that is "arse" covering

I may have mis-remembered 30 or 50, but the advice was good. How much it shrinks when sheathed with osb must be much less. So the external and shear walls probably stayed high while the other stud walls shrank.   

Anyway it is a good reason to delay the cost, and to avoid the premature use of   (and damage to)  the lift.

The lift supplier provided their own steel support cage which was not connected to the shaft.

 

Anyway, wood does shrink and twist, and isn't always as dry as it is meant to be.

[eandg]

Advice we got was have your windows fixed to the kit, extending out across the cavity then 10mm beyond to allow for them to be sealed against the brickwork. 

[TerryE]

On 31/10/2022 at 18:47, benben5555 said:

Why did you choose to do that though? Just for speed of construction?

Why not?  The house was secure with a turn of a key from day 1.  It was also weatherproof, so the slab and everything else could slowly dry out.  It was about 9 months before we plaster-boarded and skimmed then another 4 before we decorated.  Five years after moving in, and not a single settlement or other crack in our entire plasterwork.

 

The polythene protection meant that we had no scuffs or other damage to our aluclad windows and doors.

 

Also if you have read the blog then using Continental-style inward opening windows and hiding about 75mm of the frame behind the stone skin gave a visually pleasing thin-look frame that mirrored traditional window furniture rather than contemporary frame which look like they've as much wood framing as glass.  

Edited November 2, 2022 by TerryE