Removing some of truss Webbing

[UKG2024]

Hi,

Can anyone offer advice?

I'm currently planning to convert my garage into a music room and would like to achieve height in the room.

Please see the truss in the photos. It's a standard 'w' shaped webbing.

What I'd like to do is to attach 6 x 2 rafters to the existing rafter area of the truss. Meeting at the apex. And then run a 6 x 2 collar tie from approx. half way up the webbing where the 'w' meets the rafter part of the truss.

My question is:

Will this be structurally sound enough?

I think that the 6 x 2 rafters will be strong enough to resist the downward force from the roof and the 6 x 2 collar ties situated halfway down will be strong enough to tackle the 'spread' force.

Does anyone have experience of this? Can anyone offer advice?

Thanks
Gavin

 

[Spectric]

Don't, the structural integrity of a finks truss is in it being complete. The diagonal brace that comes from the apex is under tension whilst the other is under compression taking the weight of the roof. They were designed to span wider distances without any interior load bearing walls and the load is transfered only through the outer walls. It is all about the triangulation so start cutting and you could end up with views of the sky.

You could look at using room in roof trusses but that is a roof rebuild.

 

[Jacob]

.......

Can anyone offer advice?
....

Don't do it.
Get an architect or structural engineer to advise or redesign.

 

[Pedronicus]

Don't do it.
Get an architect or structural engineer to advise or redesign.

Seconded. Had a customer who thought he would make it easier to get around in his trussed loft by cutting a few 'lumps of wood' out of the way. Cost him half a new roof!!

 

[peter-harrison]

The previous posters are making me a bit nervous as I did this to my workshop!
The distance between the bottoms of the rafters and the floor was about 2.4m which would have been a huge pain in the ass in a workshop that handles a lot of 8X4 sheets and 3m boards. So I decided on surgery. I cut the bottom trusses out and replaced them about 700mm higher (to allow for insulation etc). I used timber with a larger section than the original, and used sawtooth washers between all the joints which were made with coach bolts. I also doubled up on the thickness of the sloping pieces between the top of the walls and the cross-ties (sorry about the names, I'm a furniture maker not a chippy)!- this was to help if this section was under more stress than designed for.
It's 12 years now and still standing firm.
I should add that the roof is corrugated iron so quite light.

 

[Spectric]

Were the rafters in your workshop made on site during the workhop build or pre made using nail plates and delivered to site ?

These types of truss are only used to keep cost down and avoid using a skilled roofer, once upon a time roofs were cut on site by skilled tradesman but that reduced the dividends paid to the shareholders.

If you look at the calculations involved in the design of these trusses they are only retain strength if handled correctly and are not modified, if you look at them they will look identical but are fitted with every other one turned 180° so that the nail plates on the bottom member do not all line up.

 

[Jameshow]

The previous posters are making me a bit nervous as I did this to my workshop!
The distance between the bottoms of the rafters and the floor was about 2.4m which would have been a huge pain in the ass in a workshop that handles a lot of 8X4 sheets and 3m boards. So I decided on surgery. I cut the bottom trusses out and replaced them about 700mm higher (to allow for insulation etc). I used timber with a larger section than the original, and used sawtooth washers between all the joints which were made with coach bolts. I also doubled up on the thickness of the sloping pieces between the top of the walls and the cross-ties (sorry about the names, I'm a furniture maker not a chippy)!- this was to help if this section was under more stress than designed for.
It's 12 years now and still standing firm.
I should add that the roof is corrugated iron so quite light.

A better way might be putting in some heavy purlins in say 10x3 and doubling up the rafters.

Older early 20c terraced houses were built like that.

But obvs take professional advice.

 

[peter-harrison]

Were the rafters in your workshop made on site during the workhop build or pre made using nail plates and delivered to site ?

These types of truss are only used to keep cost down and avoid using a skilled roofer, once upon a time roofs were cut on site by skilled tradesman but that reduced the dividends paid to the shareholders.

If you look at the calculations involved in the design of these trusses they are only retain strength if handled correctly and are not modified, if you look at them they will look identical but are fitted with every other one turned 180° so that the nail plates on the bottom member do not all line up.

They were gang nailed trusses. The man who built the workshop got them cheap as they had been made the wrong size- He designed the building around them.

 

[Adam W.]

Like all things it can be done. Components will need to be beefed up and the carpentry done properly, but it's not impossible.

The span and the pitch plus the weight of the roof covering will all have to be considered before doing anything.

 

[Henniep]

Hi,

Can anyone offer advice?

I'm currently planning to convert my garage into a music room and would like to achieve height in the room.

Please see the truss in the photos. It's a standard 'w' shaped webbing.

What I'd like to do is to attach 6 x 2 rafters to the existing rafter area of the truss. Meeting at the apex. And then run a 6 x 2 collar tie from approx. half way up the webbing where the 'w' meets the rafter part of the truss.

My question is:

Will this be structurally sound enough?

I think that the 6 x 2 rafters will be strong enough to resist the downward force from the roof and the 6 x 2 collar ties situated halfway down will be strong enough to tackle the 'spread' force.

Does anyone have experience of this? Can anyone offer advice?

Thanks
Gavin

what roof covering do you have. Your trusses look like the type designed for tiles. If you've got e.g. metal sheeting for roofing you would have some leeway with the number of supports needed in the trusses

 

[MarkAW]

Mines the same at 2.4m with added plasterboard ceiling. I don't have much problem with timber. Usually turn it round on the skew. I can just about stand it up leaning slightly.

I have more trouble with sheet materials, but it's not the height with those, it's all the benches/tools that're in the way. I've never flipped a sheet on end, even outside, it's far easier to flip sideways. I usually take sheets outside on a dry day to break down with a track saw.

 

[Lard]

I'm not sure I understand exactly what you want to do but here's my penny worth.

Truss manufacturers will tell you that any alterations to 'their' designs will negate any warrantees etc etc.....which is to be expected, fair enough......however....

Go back to first principles. The weight of a certain WIDTH of roof coverings is taken by the battens within that width, which then pass the same to the relative rafters. If rafters are beefy enough they will transfer the loads directly to the wall plates and so onto the external walls. If they're not beefy enough (purposefully designed so - sometimes for heights or weight reduction or spans etc) then purlins are added at mid or third-spans. These split the span of each rafter and allow any loads to be transferred to the gable walls.

Trusses are slimmer, lighter weight alternatives to rafters but do the same job - ie each one takes its share of a certain width of the roof coverings.

There is nothing wrong with a roof designed to use a combination of both systems - ie some trusses and some rafters - and so if you have an existing roof, the removal of individual trusses will need replacing with suitably section-sized rafters. You'll have to consider things like diagonal bracing, and the ceiling joist tie-effect of course, but it's not an impossible task.

However, I may have misunderstood what you're after.

 

[UKG2024]

Thank you all.

My plan is to attach new much thicker rafters to the existing trusses at building regs measurements - 6 x 2 for the span. My thinking is that this will take the downward load from the roof. (the existing truss wood sizes are 75 x 38 mm which is nowhere near strong enough.)


Then to accommodate for thrust or sideways force pushing the walls out I plan to again use 6 x 2 to create a rafter tie in the bottom 3rd of the ceiling. And again 6 x 2 as a collar tie in the top third of the ceiling.

This will achieve an extra 2 feet in ceiling height which is what I'm looking for.

Only after constructing this and adding back the diagonal bracing etc will I then remove the bottom run of the truss.

My understanding is that I'll be left with a structure that is strong enough and within regs.

Hopefully this makes sense. Again thanks for your replies.

 

[Spectric]

A drawing would explain your intentions much clearer because at the moment the lower chord goes from wall to wall but too low for your needs and you are getting an extra 2 foot height with the modifications which gives some odd looking structures.

 

[TomGW]

I know someone who cut out the entire length from one end of a 1970s bungalow to the other and replaced it with a chipboard 'tunnel' of floor, side walls, sloping ceiling and flat section at the top. This created a rigid box extending through the width of the roof, which was converted into two bedrooms. Today, at least 40 years later it's still intact with no hint of sag in the ridge etc. However it did have internal walls supporting the 'box' which undoubtedly is a critical element of its continued existence.

 

[UKG2024]

Hi,

this image shows the existing truss in green and the proposed plan in red below.

Existing truss 38 x 75

Proposed framing 44 x 145

 

[HOJ]

I'm in the process of gearing up to do a replacement roof and extension to a 1920's house with an 8m wide, 37 degree pitch, hip end roof, which has raised tie trusses, the BC wont accept us upping the timber sizes by using the rafter tables and as such we need to get SE calcs done, the existing rafters and ties are all 4" X 2" @ 16" centers.




This the type of roof we are doing, but bigger so the timber sizes aren't suitable.




I had to get an SE in to check on an existing roof, the same type, a couple of years ago, in that case we just needed to beef up the connections to the rafters and ties and put a couple of binders in, these are part of the calcs for that:




The problem with modifying a "gang nail" type roof is the the way it all works in the triangulation of tension and compression forces, along with the bracing, I would suggest it may be more viable to start from scratch with a completely new roof, rather than compromise one structure and then adding another.

 

[Spectric]

When having a new roof have you considered having room in roof truuses / attic trusses even if at this point in time you don't actually make it into a room ?

 

[TomGW]

Hi,

this image shows the existing truss in green and the proposed plan in red below.

Existing truss 38 x 75

Proposed framing 44 x 145

What you are considering is actually building a new roof structure inside the roof, whilst retaining & attaching the rafter elements of the existing, with the battens, tiles etc in place.
I would check if your proposed design would be structurally adequate (as stand alone) for a new build, and disregard the fact that you are retaining any part of the existing.

 

[paulm]

Buildings insurance may be invalidated unless notified and supported with structural engineer report and sign off ?